Document JJkd543a34Qpp8oknzZp1MGj2

1 FOTIDAmEDTALS f IliDUSTRIAL HVGIERE 3M115030 .(-Direct-Reading Gas and Vapor Monitors The concentration of gases and vapors in air can be determined readily by direct-reading instruments. Direct-reading instruments can be defined as those devices in which the sampling and analysis are carried out within the instru ment, and the required information can be read (from a dial or indicator) directly. The ideal direct-reading instrument should be capable of sampling air in the breathing zone of the worker, and should specify the concentration of the substances under investigation -- either as an instantaneous concentration or as a timeweighted average, as required. Alternatively, the reading may be in terms of percentages of an appropriate standard. In most cases, provision for keeping permanent records of the reading would be essential. Direct-reading instruments for vapors and gases include the following types: Colorimetric-type devices. Solid. A known volume of air at a fixed rate is passed through a small-diameter glass tube containing porous solid granules impregnated with a reagent that reacts with the vapor or gas contaminant and then changes color -- the degree or shade of color or length of color stain is related to the vapor or gas concentration. Paper tape samplers. A known volume of air is passed through paper impregnated with a reagent that reacts with vapor or gas and then changes color -- the degree of color indicates vapor or gas concentration. Liquid. A known volume of air is bubbled through a liquid reagent that reacts with the vapor or gas; an indicator that changes color when reagent is consumed by a definite quantity of vapor or gas indicates the vapor or gas concentration in the sample. Thermal. Conductivity. The specific heat of conduc tance of vapor or gas is measured to indicate the vapor or gas concentration. Combustion. The change in electrical resist ance of the heated filament in a Wheatstone Bridge, caused by combustion of the vapor or gas, is measured to determine vapor or gas concentration. Others. Includes potentiometry, coulometry, thin film electrochemical cells, infrared analysis, and polargraphy. Gas chromatography. Various vapors and 3M 115031 gases migrate differentially in a porous sorption medium contained in a column. Separated vapors and gases later are desorbed by heat and carried by an inert gas to a detector such as an ionization or electromagnetic device for mea surement Each type will be discussed in turn. Colorimtric-Typ Device* Direct-reading colorimetric devices use the chemical properties of a contaminant for the reaction of that substance with a color producing agent. A detection technique widely used by industrial hygienists, safety engineers, and others has been the colorimetric indicator or detector tube. The simplicity of operation, low initial cost and versatility regarding detection of numerous contaminants makes this a popular instrument for field use. Nevertheless, like all instruments, these devices have limitations regarding applicability, specificity, and accuracy. The user must be familiar with these limitations if he is to make proper judgments. Detector tube system* Basically, the colorimetric detector tube system is composed of two elements: the pump and the colorimetric indicator tubes (see Figure 19-1). The bellows and piston-type pumps are designed to draw a fixed volume of air with each full stroke. The indicator tube is a hermetically sealed glass tube containing solid granular material such as silica gel, alumina, or pumice that has been impregnated with an agent that reacts when air containing a specific contaminant or group of contaminants is drawn through the tube (see Figures 19-2 and -3), To conduct a test, the two sealed ends of the indicator tube are broken off, the tube is inserted into the tube holder and the specified volume of air is drawn through the tube. A specified period of time is permitted for each pump stroke to completely draw its full volume of air. Length-of-stain type. The length of the stain produced or the ratio of stain length to gel length is compared to a chart to determine the percent concentration (see Figure 19-4). With some length of stain tubes, the stain front may not be sharp, so that the exact length of stain cannot be readily determined. One can make an approxi mate estimate by rolling the tube in the fingers to determine the variation of the stain front. The decision to use the visual average, the maximum Figure 19-3. -- Hand operated bellows pulling contaminated air through a detector tube showing stain along about one-half the length of the indicator granules in the tube. Courtesy National Draeger, Inc, 3M 115032 Direct-R ading Gas and Vapor M nit rs Figure 19-4. -- Examples of length of stain tubes intended for sampling periods of several hours. Courtesy National Draeger, Inc. stain length, or some other means of determining the stain end, will depend upon the experience of the user. Results of the calibration test on known concentrations performed before using the tubes in the field would be very helpful. Color-change type. Colorimetric indicators of the color change type are very similar to the stainlength-type tubes. The primary difference is that the chemical reaction of the contaminant gas or vapor with the indicating reagent produces a realtively uniform color for the short length of reactive section. A color comparison must be made between the indicator tube and a standard color chart provided with each container of detector tubes of this type. It is also important to recognize that some color stains fade or change with time, and thus, the readings should be made as promptly as possible or as recommended by the manufacturer. The visual judgment for length of stain or color change detector tubes depends strongly upon the color perception of the observer and the lighting conditions. The exposed tubes should be exam ined in an area with daylight or incandescent illumination. Fluorescent lighting may not give a good match for some color systems. Mercury vapor lamps should generally be avoided because the color change may not be visible and the color stain end is difficult to determine. Flow rate The flow rates of the pumps used in color- change and length-of-stain instruments must be checked regularly. Airborne dusts and lint will quickly clog the orifices of these instruments, causing low flow rates and generate inaccurate measurements. Flow rates for color-change and length-of-stain devices must be maintained in accordance with the manufacturer's operating instructions. Proper flow rate ensures adequate "residence" type of the sample in the device, and provides sufficient time for the contaminant to react with the chemicals in the detector tube. For meaningful test results, residence time must be the same as those that were used to develop the color chart or length-of-stain chart supplied by the manufacturer. Specificity Each detector tube is designed to measure a specific gas such as hydrogen sulfide, chlorine, mercury vapor, nitrogen dioxide, carbon diox ide, or hydrocarbons. Because no device is completely specific for the substances of interest, care must be taken that interferences do not invalidate the sampling results. Many common gases and vapors react with the same chemicals, or have similar physical properties, so that the instrument may give falsely high or low readings for the substance being sampled. Specificity has been one of the primary considerations in selecting these reactions. However, only a few tubes are limited to the detection of a single gas or vapor. A precondi tioning section is employed in a number of these systems to; 1. Remove potentially contaminating substances 2. React with the gas or vapor to convert the compound to a more suitable reacting com pound 3. React with the gas or vapor with the release of a new gas or vapor that can be measured by the second section. Interference from other substances in the sample air should always be given consideration. In most cases, the manufacturer has identified interfering substances and conditions, and included this information in the instruction sheet for the tubes. Chemical reactions occurring in the detector tubes are temperature dependent. Ideally, the tubes should be used as close to room tempera ture as possible. Any hot or cold extremes will tend to alter the reliability of the detector tube 3M 115033 Figure 19-5. -- Battery operated pump used for prolonged sampling with detector tube in a holder that attaches to worker's shirt. Courtesy Mine Safety Appliances Ca. and false results could be reported. Also, volume of air going through the detector tube is temperature dependent. Interchanging tubes of various manufacturers will lead to erroneous results. The reason for the error can be ascribed to the fact that the sampling rate of the various pumps is not the same, nor is the reaction rate of the chemical reagents in the indicator tubes. Each manufacturer produces, calibrates, and sells his equipment as an integral system and never advocates interchanging the tubes or pump with those of other manufactur ers. Shelf life The shelf life of detector tubes is a critical consideration because they may not be used with sufficient frequency to be exhausted within the manufacturer's indicated expiration date. Fre quently, the tube life can be extended by storage under refrigeration. Freezing temperatures should not adversely affect tube shelf life; however, the tubes must be warmed to room temperature before use. The detector tubes should be stored at temperatures below 86 F (30 C) and never in direct sunlight. Each tube manufacturer provides specific instructions concerning the operating principles of the sampling kit and conversion tables to help interpret the concentration of contaminant. Some brands of detector tubes are calibrated in terms of milligrams per liter (mg/1) by the manufacturer. Conversion from milligrams per liter to parts per million at 77 F (25 C> and 760 mmHg can be done by using the following equation: ppm = milligrams per liter , , ------------------1----------- X 2-* o molecular weight 3M115034 ((P--Dir ct-Reading Gas and Vap r Monitors Long-term detector* Long-term chemical detector tubes are a comparatively recent development and should correct an important omission in the use of detector tubes, i.e., the need to test conformity to the 8 hour time-weighted average. Because the long term tubes are intended to function continuously, the hand pump is unsuitable and a miniature air mover is needed to sample at a rate of 10-20 cmVmin (see Figure 19-5). The tubes shown in Figure 19-2 are intended for brief sampling periods using hand aspirators to sample the air. Those in Figure 19-4 are intended for prolonged sampling using battery operated pumps over periods of several hours (see Figure 19-6). Obviously, making reliable tests with indicat ing tubes requires thorough knowledge of their limitations and care in their use. Experience has shown that the following measures help to minimize some errors: 1. Test each batch of tubes with a known concentration of the air contaminant that is to be measured. 2. Read the length of stain in a well-lighted area. 3. Read the longest length of stain if stain development is not sharp or even. 4. Observe the manufacturer's expiration date closely, and discard outdated tubes. 5. Store detector tubes in accordance with the manufacturer's recommendations. 6. Refer to the manufacturer's data for a list of interfering materials. Certification of chamiea! detector tube system* The National Institute for Occupational Safety and Health (NIOSH) of the U.S. Department of Health, Education, and Welfare has issued regulations for the certification of detector tube 6< 'h Tape width t Ficure 19-7 shows a section of the double-track, impregnated paper tape, with only the top half exposed to the contaminant. The bottom half, or reference track, is kept unexposed. Courtesy MDA Scientific, Inc, units. These regulations appeared in the Federal Registeron May 8, 1973, (38 FR 11458) and were incorporated in the Code of Federal Regulations as Title 42 CFR Part 84. Performance specifications. The certification program is designed to ensure the compliance of commercial detector tube units with the estab lished performance specifications. One produc tion batch of detector tubes submitted by a manufacturer is tested for each particular contaminant of interest to ascertain the com pliance of the tubes with the performance specifications. At the time of submission of the test batch, the applicant for certification must also submit a quality control plan whereby he commits himself to routinely inspect each batch of tubes produced for compliance with NIOSH performance specifications. If the batch of tubes submitted to NIOSH for testing is found to meet the performance requirements, and if the manufacturer's quality control plan is acceptable, then NIOSH issues a certificate to the manufacturer covering the tubes for that particular gas or vapor. Thereafter, the manufacturer must affix the NIOSH certifi cation seal to each box of tubes which is to be marked for the measurement of that particular contaminant. Quality control. The quality control of the manufacturer of the detector tube is a critical limitation of the use of the tube. Improper preparation of the reagents and solid support can, obviously, cause incorrect results. Improper packing within the tube can either be too tight to cause restriction of airflow or can be too loose to allow the material to shift while being handled. Variations in the particle size of the detecting material in the tube can lead to striations in the stain and cause a very indefinite stain line. Calibration. The calibration by the manufac turer is critical as well as the preparation of the standard charts. Most manufacturers have com prehensive quality control programs taking into account the above factors but it is the responsibil ity of the user of the tube to know that the entire detector tube system is reliable. Accuracy requirements. NIOSH certifies a manufacturer to produce a gas detector tube unit to meet the minimum requirements set forth in the regulations (basically 35 percent accuracy at one-half the exposure limit and 25 percent at 1 to 5 times the exposure limit). The quality of future production lots is evidenced by a quality assurance plan which NIOSH approves as part of the certification. Adherence to the quality assurance plan is verified by periodic plant inspections and testing samples purchased on the open market. Colorimetric Tape Samplers The instrumental technique of a chemically impregnated paper tape has been in use since the early 1950's. The first units were developed for the detection of hydrogen sulfide, using a lead acetate reagent system impregnated into filter paper that produced a dark stain (lead sulfide) when exposed to this substance. The concentra tion of HjS was then determined by measuring the transmission of light energy through the stained paper. Principle of operation In modem devices the chemically treated paper tape is drawn at a constant rate over the sampling orifice and the contaminant reacts with the chemical to produce a stain. The intensity of the stain can be measured by reflectometer and the result can be displayed as a function of concentration. Figure 19-7 shows a section of the double track, impregnated paper tape. Only the top 3M 115036 Is--Dir ct-R ading Gas and Vap r Monit rs SERIES 7000 MONITOR SCHEMATIC i__________ I Figure 19-8 is a schematic of the Series 7000 monitor, showing the use of reflected light and its measurement. The capstan-driven cassette provides constant movement of the impregnated paper tape. Courtesy MDA Scientific, Inc, i track is exposed to the contaminant; the bottom, or reference track, is kept unexposed. Therefore, only the top track becomes stained. By directing light of equal intensity (from a common source through match fiber optics) to both the top and bottom track, and mounting a set of matched photoelectric detectors at an angle of 45 degrees, the difference in reflected light can be measured. The system thereby compensates for slight tape variations. This is illustrated in Figure 19-8, which schematically illustrates the general principle of operation. This diagram also snows the capstan driven cassette, providing constant movement of the impregnated paper tape past the exposure orifice and readout section of the optical block/gate assembly. While the tape moves, a constant flow of the sample air is aspirated through the porous impregnated tape, controlled by means of the self-contained pump and flow controller. A typical monitor in the series is shown in Figure 19-9, together with the strip chart re corder. Field calibrations can be accomplished conve niently at any time, by use of a test strip provided with each monitor. The strip has a calibrated stain, equivalent to the stain that would be produced by a known concentration of the contaminant. Electrical Direct-Reading Instruments Dangerous concentrations of flammable gases and vapors can occur in many operations. In some cases the complete elimination of the hazard is possible, but in many working environ ments the presence of flammables may be unavoidable. In these cases it is essential to be able to determine whether the concentration of the flammable gas or vapor is such that it becomes an explosion hazard. Instruments for both the detection and measurement of combust ible gases and vapors are described in this section. 3M115037 t ble gas indicator. Instruments of this type were designed to detect the presence of explosive or combustible gases in the air. Combuttion theory Combustion processes are usually explained in terms of the fire pyramid shown in Figure 19-10, Combustion is pictured as the combination of oxygen and fuel with the release of heat. The combustion of methane (for example) in oxygen with the formation of carbon dioxide, water, and heat may be shown by the chemical equation: CH* + 2 Or - CO2 + 2 H2O + A fuel + oxygen = carbon dioxide + water + heat Reaction mechanism. The mechanism of the combustion reaction involves a number of intermediate steps. The free radicals, or interme diate combustion products, react with each other and with CH, and O, until they are ultimately Figure 19-9 depicts a monitor, together with the strip chart recorder and an alarm. Courtesy MDA Scientific, Inc. Introduction Portable direct-reading air sampling instru ments have eliminated much of the guesswork in detecting the presence of flammable gases or vapors. Before the development of these sam pling devices, it was necessary to collect a sample of the suspected atmosphere and return it to a laboratory for analysis. This required the services of experienced technicians and a considerable investment in laboratory equipment. By the time the analysis was completed, the concentration of the suspect ed atmosphere could have changed considera bly. Direct-reading instruments enable the opera tor to obtain immediate indications of gas or vapor concentration by reading a meter dial. This does not mean, however, that the mere reading of a meter implies a valid test. On the contrary, the operator must be thoroughly familiar with the use and limitations of the instruments and devices. One of the most useful instruments of the direct-reading type is the hot wire, or combusti Figure 19-10. - The "fire pyramid." Oxygen, heat, fuel, and chain reactions are necessary components of a fire. Speed up the process and an explosion results. 3M 115038 Dir ct-R acting Gas and Vapor Monit rs VOLUME PERCENT IN AIR TEMPERATURE F converted to C02 and water, releasing heat in the process. As the combustion of methane proceeds, the rate of energy release is balanced by energy dissipation, such that a limiting rate of reaction is reached. The example of the combustion of methane in air to produce carbon dioxide and water shows that when methane is burned, the combustion products expand away from the combustion zone taking with them kinetic energy in the form of heat. A limit is placed on the temperature and pressure of the system, and the rate of reaction reaches an equilibrium value. Explosions. If the methane and oxygen are placed in a closed container and the reaction is initiated, an explosion results. In this case, the products of combustion and the heat produced are not removed from the reaction zone. The temperature of this system would continue to rise. Since the system is confined, the pressure rises also. Both effects operate in the direction of increasing the rate of reaction. There is no mechanism for dissipation of energy; therefore, the rate of reaction increases more rapidly until all of the reactants are consumed. Rapid oxidations such as those encountered with gasoline-air mixtures are explosive if enough air is premixed with the gasoline vapor in a confined space to permit an essentially complete reaction. The reaction becomes explosive be cause the oxygen atoms in air are in close contact with the carbon and hydrogen atoms and can therefore react instantaneously. In other circum stances, gasoline simply bums because the rate of the reaction is controlled by the diffusion of oxygen to the combustion zone. Many chemical reactions can be potentially explosive if the heat buildup proceeds too rapidly, and the reaction rate escalates. Flash point. The minimum temperature at which the vapor concentration above a liquid is high enough to propagate a flame front when a source of ignition is present is defined as the flash point. Each material also has a characteristic lower explosive limit which is the minimum volume percent of the material in air which can 3M 115039 FLASHBACK ARRESTORS ASPIRATOR BULB Figure 19-12 shows a schematic diagram of a typical hot-wire sampling device. Courtesy Mine Safety Appliances Co. be ignited. Explosiva or flammable range Each material also has an upper flammable limit which is the maximum volume percent of the material in air which can be ignited. For gasoline, this value is 7.6 percent. The signifi cance of the upper flammable limit is not always as obvious as that of the lower flammable limit. If the upper flammable limit is exceeded, the mixture cannot be ignited and sustain combus tion. The balanced chemical equation for the combustion of gasoline represented by octane is shown as: 2 CHhi + 21 O'- 16 CO: + 10 H:0 + heat The percentage of gasoline in air under stiochiometric combustion conditions is 1/21 of the total volume or 4.7 volume percent. If the proportion of gasoline molecules is increased greatly, insufficient oxygen is present to support combustion. Similarly, if the propor tion of gasoline molecules is decreased, insuffi cient gasoline is present to sustain combustion. Stable combustion of gasoline is sustained only within a narrow range of concentration limits. The sequence of events that occur with any fuel-air mixture are the same as those described for the gasoline-air mixture. Fuel-oil-air mixtures are basically the same except for the fact that the flash point of kerosene, for example, is approxi mately 100 F (38 C). The lower flammable limit is 0.7 volume percent and the upper flammable limit is 5.0 volume percent. A comparison of the similarities and differen ces of the interrelationship between physical properties and combustion behavior for No. 1 fuel oil and gasoline is summarized in Figure 19- " Lower flammable (explosive) limit (LEL), When certain proportions of combustible vapor are mixed with air and a source of ignition is present, an explosion can occur. The range of concentrations over which this will occur is called the flammable (or explosive) range. It includes all concentrations in which a flash will occur or a flame will travel if the mixture is ignited. The lowest percentage at which this occurs is the lower flammable (or explosive) limit (LEL), and the highest percentage is the upper flammable (or explosive) limit (UEL). Mixtures below the LEL are too lean to ignite, and mixtures above the UEL are too rich. On the simplest type of instrument (an explosimeter) only one scale is provided, usually with readings from 0 to 100 percent LEL. However, the detectable changes produced by combustion are too small to be measured accurately in the presence of the low concentra tions of contaminants usually encountered in evaluating potential health hazards. For exam ple, the LEL of even the most explosive gas is of the order of 1 percent, or 10,000 ppm, which is well in excess of the threshold limit value for any gas. Instrument design Several manufacturers make explosimeters or combustible gas indicators. Although they differ somewhat in design and operating features, their operation is based on the phenomenon that a measurable amount of heat is released when a combustible gas or vapor is burned. Most meters contain a battery-operated electrical circuit, known as a Wheatstone Bridge, which is balanced by means of controls on the outside of the instrument. A schematic illustration of the basic flow system and wiring diagram are shown in Figure 19-12. Wheatstone Bridge circuit. In one part of the I [jf ! i 3M 115040 (^(_Dir ct-R ading Gas and Vap r Monit rs RESPONSE TO METHANE LEL 5.3% UEL 14.0% Figure 19-14, -- Meter readings ai various methane-in* air concentrations. Courtesy Mine Safety Appliances Co Ficcre 19-13. -- A flammable gas detector with a dualscale meter. One scale indicates 0 to 100 percent of the LEL and the other 0 to 1000 ppm. Courtesy Bat h/l:arh Instrument Co, bridge, the air being sampled is passed over filaments that have been brought to a high temperature. If the air contains a combustible gas or vapor, the heated filaments cause combustion and additional heat is released, increasing the electrical resistance of the filaments. Another part of the bridge contains sealed similar filaments which are heated in identical fashion but not in the air stream; these sealed filaments cancel out all changes in electrical current and resistance due to temperature variations in the wire or to characteristics of the instrument itself. The net effect is that the change in resistance to the flow of electrical current in the filaments in the air stream is only due to the presence of combustible gases. These changes in the electrical current are registered as "percent LEL" on the instrument's meter. The manufacturer's instructions for operating a combustible gas indicator should be carefully reviewed before the device is used. In general, all explosimeters require a brief initial warm-up period so that the batteries can heat the filaments. Most combustible-gas indicators are equipped with a length of sampling tubing with a metal probe at the end. The probe is held at the sampling point, and, a few seconds later, the response can be read on the meter (see Figure 1913). Generally, the air is drawn through the probe and meter by means of a hand-operated rubber squeeze bulb. In some instances, however, a small electrically operated pump in the instru ment case is used for this purpose, fn most work areas, the concentration of combustible gas or vapor fluctuates constantly, and it is necessary to observe the instrument carefully and to make a judgment concerning average and peak readings. Zero adjustment. The zero adjustment must be made by taking the instrument to a source of air that does not contain combustible gases or vapors, or by passing air into it through an activated carbon filter which will remove combustible vapors and gases except methane. Since methane is not removed by activated charcoal filters, extra caution is required if the presence of methane is suspected. In addition, the filter should be changed periodically because it becomes saturated during prolonged use and will no longer remove many of the combustible gases and vapors. If the zero adjustments are made in fresh air, care must be taken that no combustible gas or vapor is present in an amount which would influence the instrument's response. Interpretation ot meter readings The user of any intrument should be tho roughly familiar with precautions to be taken in its operation; users of combustible-gas indicators 3M t HOT WIRE GAS DETECTOR RESPONSE TO METHANE I Figure 19-15 shows relationship between meter reading and combustible gas concentration. Courtesy Mine Safety Appliances Co. must also be aware of interfering gases and vapors that could create major aberrations in instrument response. One such precaution is that the 0-to-100 percent scale should be used first, to determine whether an explosive atmosphere exists and to prevent overloading the O-to-IO percent LEL scale. The typical meter responses to methane gas are shown in Figure 19-14 at the LEL, in the explosive range, and above the UEL. If the pointer of the meter travels into the red portion of the scale and remains there, an explosive concentration is present. However, if the pointer climbs rapidly to the red area and then falls back to zero, there is either a concentration above the UEL or a gas mixture which lacks sufficient oxygen to support combustion (see Figure 19-15). Zero reading. Sometimes the probe of a combustible-gas indicator or explosimeter is placed in a manhole (or other space not normally occupied by people) to determine if there is a potentially explosive or dangerous concentration of gas present. Under this condition, the- instru ment may show a zero response for several different reasons. Assuming that the batteries are charged and the instrument is functional, the absence of a continued meter response can mean either that there is little or no combustible gas in the space being tested, or that the concentration is so high that it is above the UEL and combustion cannot occur because of insufficient oxygen. A very high concentration can be identified by carefully watching the needle as the probe is moved into and withdrawn from the space being tested. At some point during entry and with drawal, the probe must pass through the LEL concentration and enter the flammable range. At this point, the needle will jump briefly, then settle back to zero. This needle jump is a clear indication that a high concentration is present. Thermal conductivity. Atmospheres contain ing no oxygen or combustible gas or vapor but containing pure argon would produce an upscale reading. The reason is that argon has lower thermal conductivity than air, which is composed chiefly of nitrogen and oxygen. When the instrument is zeroed on the air, it is balanced for a given wire temperature -- a function of the electrical heat input on the filament and of the cooling effect of air. If the air sampling medium is replaced with argon, there will be less dissipation of heat from the filament, causing a temperature rise of the filament and a resulting increase in resistance and an upscale 3M115042 i--Dir ct-Reading Gas and Vapor M nitors Figure 19-10. -- Flammable gas detector using the hot wire system to measure flammable gas and vapor levels. One scale measures 0 to 100 percent of the LEL, the other 0 to 10 percent of the LEL. Courtesy Mine Safety Appliances Co. meter reading. High-flash point solvents. Although it is relatively easy to operate a combustible gas indicator to detect a flammable gas or vapor, there are some limitations in using the instru ment. The instrument will respond only to those combustibles drawn through the sampling system. If the flash point of a material is higher than the normal ambient room temperature, a relatively low concentration will be indicated. Should a closed vessel holding such a contami nant later be heated, as by welding or cutting, the vapor concentrations will increase, and the atmosphere of the container, which originally showed a low concentration of vapor, may then increase and be explosive. In the testing of atmospheres in drying ovens or other places where the temperature is unusually high, there may be some difficulty in measuring solvents (such as naphthas) that have a relatively high boiling point. The vapors in such samples may condense in the sampling line which is at temperatures below those of the oven, thus giving a false indication of safety. In some instances, condensation can be prevented by heating the sampling line and instrument to a temperature equivalent to or above that of the space to be tested. Combustible-gas indicators test only flamma ble gases and vapors in air. They are, therefore, not applicable for measuring combustibles in steam or inert atmospheres, becuase of the absence of oxygen necessary to support combus tion on the filament unit. Dual-teal* instrument* To measure high concentrations of combusti bles (above 100 percent of the LEL), it is necessary to increase the basic range of the instrument. Until recently, that range increase was accomplished by using a dilution tube, a dilution valve, or a range multiplier -- all of which essentially caused dilution of the sample with air in a ratio of 10:1 to 20:1, producing a new mixture that could be indicated on the scale of the indicator. Industrial hygienists and safety professionals use another instrument to measure toxic as well as fire hazards. Because the threshold limit value (TLV) of many gases and vapors represents a very small fraction of the LEL concentration (one percent of any combustible in air is equivalent to 10,000 parts per million), it is necessary to use a highly sensitive combustible gas indicator. Models are equipped with a dual scale meter graduated from zero to 100 percent and zero to 10 percent of the lower explosive limit. To determine the explosive concentration of a variety of different combustibles, calibration curves are furnished for correlating meter readings for individual gases. Readings are taken on the zero to 10 percent LEL range to measure toxic concentrations of the gases and vapors that may be flammable (see Figure 19-16). The selector switch on the panel of these instruments changes the metering circuit from one calibration to another, so that percent LEL measurements can be made directly, eliminating the need for calibration curves or charts. Several types of combustible-gas indicators have been designed to be calibrated to measure specific combustibles. One variation of the instrument has adjustable calibration controls and can measure five different gases or vapors in the zero to 100 percent LEL range for each. Another type has a dual scale multiple calibration of zero to 10 percent and zero to 100 percent of the lower explosive limit. Natural gaa and p*trol*um vapors A special instrument has been designed to 3M L Figure 19-17. -- A selection switch enables this instrument to distinguish between various vapors and gases. Courtesy Mine Safety Appliances Co. differentiate between natural gas and petroleum vapors in manholes, sewers, and other under ground openings. The two gases can be distin guished electrically by using a selector switch that operates the filament at two temperatures, above and below the temperature at which natural gas will ignite. The different readings provide a measure of the petroleum vapors and the natural gas present (see Figure 19-17). Leaded gasoline. When a hot-wire-type combustible-gas indicator is used to test vapors of leaded gasoline, a combustion product of the tetraethyl lead is deposited on the filament unit, reducing the catalytic activity of the filament. To circumvent this poisoning effect, a special version of the standard instrument is available for gasoline service. In those instruments, the voltage is boosted across the detector to maintain a sufficiently high filament temperature to prevent contamination. Inhibitor filter. An additional means to prevent lead poisoning is an "inhibitor filter" that is inserted in the filter cavity of the standard instru ment. This filter produces a chemical reaction with the tetraethyl lead vapors to produce a more volatile lead product of combustion, thereby preventing contamination of the catalytic plati num filament. In all hot-wire instruments, flashback arrestors are mounted in the flow system at the inlet and outlet ends of the detector housing. This prevents propagation of a flame outside the filament chamber if there is a flammable mixture of gas or vapor surrounding the detector. Poisoning tho catalyst Minute concentrations of silicone vapors -- even one or two parts per million -- can rapidly poison the catalytic activity of a platinum filament. A hot-wire combustible-gas indicator should not be used where silicone vapors are present. Effective performance requires that the operators know the correct use of instruments to detect explosive and toxic concentrations oi combustibles. Various refinements and design improvments have been incorporated in the conventional gas indicator to meet occupational safety requirements. Interferences. Interfering gases and vapors can seriously affect instrument response, and an experienced tester recognizes the indications of their presence. The manufacturer's instructions should be thoroughly understood because high concentrations of chlorinated hydrocarbons (for example, trichloroethylene) or of an acid gas (sulfur dioxide) may cause depressed meter readings where high concentrations of combusti bles are also present. Trace amounts of these interferences may not affect the readings directly, but can corrode the sensitive detector elements. High-molecular weight alcohols in the atmosphere may burn out the filaments, making the instrument inoperative If such limitations are understood, the tester can obtain sufficiently valid results. Portable combustible gas alarm systems A portable combustible gas alarm is available to monitor industrial atmospheres for concentra tions of flammable gases. The instrument is sensitive enough to detect the presence of a broad range of flammable gases and vapors before they reach their lower explosive limit in air. An illuminated meter indicates combustible 3M 115044 '((_Dir ct-Reading Gas and Vap r M nitors i* Figure 19-18 shows a portable continuous combustible gas alarm instrument. Courtesy Mine Safety Appliances Co gas or vapor in concentrations of zero to 100 percent LEL. When concentration reaches the preset limit, a red alarm light and a loud horn inside the unit are activated, providing both visual and audio warnings of a dangerous concentration. The horn may then be switched off; in which case, the pilot light will blink until the unit is reset and the combustible gas concentration is below the set point. The red alarm light will remain on, even if the concentration falls below the preset danger point, until the reset button is pressed. Figure 1918 shows an example of a portable combustible gas detector/alarm. Combination oxygen and combustible gas monitors Another version is a portable combustible-gas monitor and oxygen indicator designed to simplify the job of inspecting areas for combusti ble gases or vapors or an oxygen deficiency by combining two instruments in a single unit. A battery-powered pump draws the sample across two sensors. The results of the tests are registered almost instantaneously on the meters. This instrument's gas detector operates on the sample principle as the combustible gas alarm described. However, oxygen is sensed directly by a unique galvanic cell inside the oxygen analyzer. The cell, containing one gold and one lead electrode in an electrolyte, is encapsulated in inert plastic. Oxygen diffusing through the fluorocarbon polymer face of the cell initiates redox reactions, which generate a minute electrical current that is directly proportional to the oxygen partial pressure. A temperature- compensated electronic circuit converts the current to a proportional voltage displayed on the analyzer's meter as oxygen concentration. Oxygen Monitors Although oxygen does not have a specific TLV, its level in industrial air must often be measured, particularly in enclosed areas where combustion or other processes may use up the available oxygen. Excess oxygen, which may result from oxy-acetylene or oxy-hydrogen operation, should also be monitored so it does not contribute to fire hazard. Air normally contains about 21 percent oxygen by volume. Sixteen percent is considered the minimum to support life. In some cases, howev er, air with less than 19.5 percent may be considered deficient, as in applications at high altitudes where atmospheric pressures are lower. In many locations, however -- such as mines, manholes, tunnels, or other confined spaces -- it is possible for the oxygen content to be sufficient ly low that it is hazardous to life. In such situations, it is necessary to determine the oxygen content and, in addition, by taking a sample, to determine whether combustible gases are pres ent in dangerous concentrations. Both direct- and indirect-reading instruments are available to sample breathing air for oxygen content. Direct-reading samplers are, for the most part, small, lightweight, and relatively easy to use. Among the direct-reading instruments are those based on the coulometric principle, and colorimetric and paramagnetic analysis. Galvanic tensing calls One portable oxygen indicator uses a small sealed galvanic sensing cell to measure oxygen on a 0 to 25 percent scale (see Figure 19-19). A minute electrical current is generated in the cell in proportion to the oxygen content of the atmos phere being sampled. Although the galvanic cell is always on, meter readings are not given unless the instrument is activated by a small pushbutton. Coulomatric oxygen detectors One instrument to measure oxygen content, to determine oxygen-deficient atmospheres, areas of excess oxygen, and the oxygen concentrations in storage tanks and compartments of vessels is the coulometric detector. Detection is accomp lished with a primary galvanic cell consisting of a zinc and hollow carbon electrode in a special 3M Figure 19-19. -- Portable oxygen indicator measures oxygen on a zero to 25 percent scale. Courtesy Mme Safety Appliances Co. electrolyte. A gas mixture flows through the interior of the carbon electrode and diffuses through the carbon to the interface with the electrolyte. The oxygen combines with hydrogen that has been brought to the electrode as hydrogen ions by an electric current generated by the cell itself, causing polarization. That action is counteracted by the oxygen, which depolarizes the cell and changes the terminal voltage and meter reading according to the amount of oxygen in the gas being tested. The instrument has a range scale of zero to 25 percent oxygen by volume, Coulometric instruments are suitable for a wide variety of process applications where oxygen must be measured in gaseous samples, or dissolved oxygen must be determined in aqueous or nonaqueous solutions. It can be used to make gaseous measurements of percent oxygen and millimeters of partial pressure and liquid mea surement of percent saturation and ppm for a wide variety of process applications where oxygen must be measured in gaseous samples such as flue gas, oxygen in hazard environments, and purity of oxygen in respiration studies. Colorimetric analysis There are three types of direct-reading colorimetric indicators: liquid reagents, chemi cally treated papers, and glass indicating tubes containing solid chemicals. For oxygen analysis, however, the most important type is the glass indicating tube. The glass indicating tube procedure is popular because it is an easy convenient method of indicating the concentration or dangerous lack of oxygen. To use a detector tube the sealed ends are broken, the tube is placed in a holder, which is fitted with a calibrated piston pump, and the recommended air volume is drawn through the tube. The observer then evaluates the concentra tion of the suspected substance in the air by examining the exposed tube. Concentrations of a gaseous substance are indicated by a color change or length of stain produced in the chemical material in the tube. The tint of color or length of stain is compared with a standard furnished by the manufacturer. Oxygen detector tubes indicate concentrations of two to 30 percent, and the substances in the tube turns brown (from the original white). All indicator tubes have a limited shelf life, and manufacturer's instructions should be followed. Paramagnetic analyaia Paramagnetic oxygen analyzers are available in typical models that will measure the oxygen content of an atmosphere. Most units operate on the principle that magnetic lines of flux pass through oxygen more easily than through other gases. Oxygen is paramagnetic (attracted into a magnetic field). The paramagnetic property of oxygen is caused by its atomic and molecular structure, and is inversely proportional to its absolute temperature. When oxygen is heated, it loses its paramagnetic property and becomes diamagnetic (repelled out of a magnetic field). The device provides an accurate, easy-to-use instrument for reliable oxygen measurements. Hand-held units are available in models powered by flashlight batteries. In the operation of a typical unit, a sample of air passes through the oxygen analysis cell over an electrically heated resistor. Oxygen is attracted to the magnetic field of the active resistor, where it is heated and loses its magnetic property. The cooling effect on the active resistor is in proportion to the oxygen content of the gas. The nonactive resistor acts as a reference. The difference in resistance produces a voltage proportional to the amount of oxygen present in M1150$ -Direct-Reading Gas and Vapor Monitors the sample. Other units operate simply by passing a gas sample into a test chamber where a suspended magnetized object will rotate in proportion to the oxygen content of the sample. Indirect-reading inatruments Basically, in an indirect-reading oxygen analyzer, the oxygen in a gas sample is absorbed by a special solution. This creates a partial vacuum inside the analyzer. The solution being absorbed rises in a tube, and the height of the solution in the tube indicates the oxygen content in the sample. Users of such indirect-reading devices are cautioned that the absorbing solutions used are corrosive, and, when a solution has to be replaced, trial runs should be made in a laboratory where malfunctions can be corrected without spillage of the solution. Other technique* Oxygen has the ability to reduce the intensity of light emitted by fluorescent materials. A device that uses an ultraviolet light source to "excite" a fluorescent chemical has been deve loped to determine oxygen content of a given environment. The fluorescent chemical is ab sorbed by a porous glass disk, and the degree of fluorescence is monitored by a photoconductive cell. As the oxygen content of the atmosphere in contact with the disk is reduced, the fluorescence increases. Carbon dioxide monitors, such as those used in breweries, indirectly function as an oxygen deficiency indicator. Such detectors contain a regenerable cartridge of soda lime through which an electrically driven pump draws atmos pheric air. Any carbon dioxide in the sample is absorbed by the cartridge and the difference in the volume of gas sampled and the volume after passage through the cartridge is registered by the levels of liquid in graduated glass tubes on the front of the instrument. Many of these instruments feature alarms that sound when the COj concentration becomes hazardous. Polarographic detectors operate on a current that is generated between two electrodes immersed in a reagent electrolyte contained in a special cell when gas diffuses through a porous membrane separating the electrolyte liquid from the atmosphere. By varying the electrode materials and electrolyte, such cells can be made to react to the presence of specific inorganic gases that undergo self-ionization in aqueous Ficure 19-20. -- Example of direct-reading carbon monoxide monitors. Courtesy Bullard, solution. Such devices can be used to monitor the depletion of oxygen in respirable atmospheres. Carbon Monoxide Monitors One of the most insidious toxic gas hazards in an industrial atmosphere is carbon monoxide (CO). Odorless, tasteless, and colorless, CO can be deadly even in small concentrations. Carbon monoxide can occur in many areas, including gas and utility properties, garages, bus terminals, sewers, vaults, blast and open-hearth furnaces, and mines. There are a number of instruments available for measuring carbon monoxide (see Figure 1920). Some portable CO monitors use catalytic combustion as the principle of operation. When CO comes into contact with the catalyst (usually Hopcalite, a mixture of manganese dioxide with other oxides for removing and for detecting CO), it oxidizes and forms carbon dioxide. During the oxidation process, heat is generated in proportion to the amount of CO present and is measured by a sensitive resistor called a thermistor, which reacts quickly to heat changes. The thermistor is connected to a Wheatstone Bridge (described earlier in this chapter). Research and development has produced other types of CO detecting instruments (see Figures 19-21 and 19-22). One of these combines solid electrolyte technology with thin-film deposition processes. The sensor is an electro chemical polargraphic cell. This cell electrooxidizes CO to COj in proportion to the partial 3M 115047 Figure 19-21. -- This carbon monoxide monitorrecorder is the system recorder and analyzer designed for and now being used by OSHAdministration for measuring compliance standards for CO levels in industry. The nine-pound unit is either AC or battery operated, and it uses principles of electrochemical oxidation to gather permanent data on carbon monox ide levels. The attached recorder, battery-powered, records on pressure-sensitive paper and requires no ink Supply. Courtesy Energetics Science, Inc. Figure 19-23. -- Battery powered, pocket-size device that provides an accurate, economical and easy means of measuring carbon monoxide concentrations in ambient air. It can be used wherever workers are exposed to conditions requiring continuing checks for CO; it can be carried in the pocket, on the belt, or simply held in the hand. Courtesy Mine Safety Appliances Co. Ficure 19-22. -- Carbon monoxide monitor small enough to be worn by the user. Courtesy Energetics Science, Inc. Figure 19-24. -- A battery operated carbon monoxide detection instrument using an electrochemical cell. Courtesy Mine Safety Appliances Co. 3M 115048 8-Direct-Reading Gas and Vapor Monitors pressure in the sample area and the resulting electrochemical signal is amplified and tempera* ture compensated to drive the meter. Samples are introduced to the sensor by diffusion through a gas-porous tetrafluoroethylene membrane (see Figure 19-23). The portable type CO detector features both visual and audible alarms when the danger level is reached. The battery powered instrument can measure CO in the atmosphere in the range of 1 to 500 parts per million by volume (see Figure 1924). Other Types of Equipment Infrared analyzers Many gases and vapors, inorganic and organic, have characteristic infrared spectra that absorb infrared radiation over a spectrum of wave lengths in a manner which can be converted into characteristic graphs. These can be used to detect the presence of air contaminants and also to determine their concentration in air (see Figure 19-25). Basically, there is an infrared generator in the analyzer that emits the full frequency range. It is an established fact that various gases and chemicals absorb infrared energy at specific frequencies (bands). Also, the window material in the cell limits the frequencies that can be used. The combination of window materials, etc., and the absorption of energy by gases at specific frequencies, determines how the instrument is made selective to a specific chemical or gas. The instrument's actual operation is really quite simple. At one end there is a source, which emits the infrared energy through two cell paths simultaneously. At the opposite end is a detector which is looking at the energy being transmitted through the two cells. One of the cells is the sample cell in which a gas is injected to be analyzed, which the other cell is sealed with a special mixture inside. If the sample injected into the sample cell contains a gas which absorbs energy at the frequency that was selected, then the detector will see less energy coming through the sample cell than the comparison cell. This unbalance is converted into an electrical signal by the detector. Where a single contaminant is present, identifi cation and measurement is achieved with ease but where a number of absorbing contaminants are present separation is not possible. In such Figure 19-25. -- An infrared analyzer can be used to monitor gas and vapor exposures. Courtesy Wilks Scientific Corp. cases, the total concentration (that is, of all absorbing contaminants) can be determined and supporting methods, such as mass spectrometry and gas chromatography, are required. Gn chromatography In gas chromatography, the ccomponents of a mixture migrate differentially in a porous sorptive medium. Chromatography is primarily a method of resolving complex mixtures, and this depends upon the differential migration of the components through the porous medium. This differential migration is carried out so that each component separates as a discrete substance. The separated substances appear in a carrier gas as a function of time as the carrier gas passes through the absorption column. Detection of the separat ed components takes place as the carrier gas emerges from the column. Analysis for a specific component requires a method (either specific or nonspecific) for the detection and identification of the isolated components of a mixture. The use of particular reference substances and the sorption time sequence technique are suitable methods as well as the relative migration of carrier gas and components under standardized conditions. The readout system includes a thermal desorp tion unit for removal of the collected sample from the charcoal tubes -- a gas chromatograph with column backflush capability, and suitable data readout and recording equipment. The 3M 115049 Cl Figure 19-27. -- Oxidant monitor provides continuous measuring of oxidant concentrations. Sensor reactions are electrochemical and occur at the polarized electrodes immersed in a flowing film of sensing solution. The current produced is directly proportional to the mass per unit time of oxidant entering the sensor. Courtesy Mast Development Co. Figure 19-26. -- The fully portable organic vapor analyzer has been certified by Factory Mutual for use in Class I, Division I, Croups A, B, C, and D atmospheres. The 10-pound unit can then be used to track down the exact source or cause of any vinyl chloride monomer buildup, when used with an optional gas chromato graph system. Courtesy Century Systems Corp. readout system can be portable or fixed (see Figure 19-26). Instrument reliability, ease of operation, and ease of calibration are key considerations in defining the minimum required technical skill levels for the operators. Many advantages are obtained by on-site readout systems, including cost saving, rapid data collection, increased flexibility, and greatly reduced sample handling and storage problems. Other types of monitoring equipment are shown in Figures 19-27 and -28. Calibration All instruments for sampling gases or vapors must be calibrated before use and their limita- Ficure 19-28. -- Direct-reading H,S monitor. Courtesy Control Instruments. 3M 115050 Dir ct-Reading Gas and Vap r Monitors 3M115051 TABLE 19-A. A TYPICAL CALIBRATION CHART (MSA) L tx. - % by Volune jU.SB-M Bulletin *503] PARTS PER Million {in squares I METER READING fm squares I L.V < Threshold Liiihj Values |A C.G-I H 196-4) 10* L.E L Switch Setung * L E.L. Switch Selling (P.PM.) O S. . OFF SCALE Mete Reading No. Name LEI T.L.V n 3X 5* 5* 10* 20* 50* 75* 2 3 4 6 6 7 6 9 10 1 L V. TLV 1 L.V L E L L.E.L. L.t.L L.E.L. L.E.L. No. Formula 1 Acetone 2.6 1000 338 676 1026 1378 1716 2056 2416 2756 3133 3432 7 It 16 5 9 19 48 72 1 CHjCOCHj r Acetaldehyde i Acrylonitrile 4.1 200 32B 697 h3 0 20 240 480 1025 1394 1722 2091 2419 2786 3116 3414 720 960 1200 1410 1650 1860 2070 2280 2 5 4.5 6 15 30 75 OS 2 CMjCHO 8 16 32 78 OS 3 4 Ally! Alcohol 2.5 2 2251 450 650 875 1100 1325 1525 1750 1975 2175 8 15 28 61 83 4 C3H5QH 5 Amyl Alcohol 6 Seiuene 1 2 100 180 216 1.4 25 140 294 504 660 604 948 1092 1224 1344 1452 434 574 728 868 1006 1162 1302 1442 4 5 10 16 37 50 6 C5H11OH 6 12 24 58 83 6 CjH* 7 Butyl Acetate B Butyl Alcohol 17 200 170 ' 340 527 697 884 1071 1275 1496 1720 1955 1 5 4 2.5 5 T1 28 42 7 CK3COOC4H9 1.4 100 210 406 602 798 980 1146 1316 1456 1568 1666 1 2 3 4 6 15 33 46 6 C4H90H 9 Carbon Oisollide 1 25 20 275 625 1000 1375 1750 2.5 5 TO 24 35 9 CS; 10 Carbon Mono*Je U5 100 750 1500 2260 3000 3750 4500 5250 6000 6750 7500 10 22 42 100 OS 10 CO 11 Cyclohexane 1.3 400 143 286 429 572 715 645 988 1118 1261 1404 7 10 18 6 11 23 57 63 11 CjHij 12 Dimafine (UOMHJ 13 Ethyl Acetate 2.5 05 450 2.5 400 325 675 725 100Q 1325 1675 2025 2375 2750 3100 10(25 1400 1775 2175 2625 3050 3450 4000 3 5 5.5 11 21 53 77 12 ICH3I3NJH2 B 5 10 20 50 75 13 CH3COOC2H5 14 Ethyl Alcohol 4.3 1000 215 473 774 1075 1419 1763 2107 2494 2871 3311 85 12 18 9 17 31 70 101 14 C2H50H 15 ErhyJ Ether 1.9 400 114 247 399 570 741 931 1021 1311 1520 1729 5 75 12.5 6 12 23 56 62 15 CjHsOC;M5 16 Ethylene Chloride 6.2 50 496 1054 1590 2140 2666 3224 3720 4250 4774 5270 7 14 28 68 98 16 CH30CH3CI 1? Ethylene Oxrde 3.0 50 405 780 1155 1530 1B90 2265 2640 3000 3345 3705 5.5 11 21 545 90 17 CjH0 IS Gasohne (leaded - with inhibitor) 1 4 600 140 294 44B 608 770 924 1085 1246 1400 1580 B 12 20 5 11 22 55 82 18 VS Heptane U 500 132 270 402 540 672 KM 936 1068 1200 1332 9 135 21 6 11 21 50 73 19 CjH16 20 Hexane 1.2 600 144 298 444 600 750 900 1056 1212 1368 1512 8 11.5 195 5 9.5 18.5 47 69 5 20 C6HU 21 Hydrogen Sulfide 4.3 10 516 1011 1466 1935 2387 2838 3290 3741 4171 4622 6 12 24 56 79 21 MjS 22 Iso-Propyl Acetate 23 Iso-Propyl Alcohol IT Iso-Butyl Methyl Ketone IMlBKl 1 a 200 252 504 20 400 260 550 1 4 100 152 360 738 990 1224 1476 1728 1962 2214 2463 1 5 810 1100 1390 1660 1950 2230 2510 2790 35 512 665 626 960 1141 1288 1442 1596 1.4 25 5 21 5 45 9.5 16 5 47 8-5 45 6.5 17.5 45 i 6 5 10 22 50 705 22 (CH3J2CHOOCCH3 67 ii (CHjljCHOH 73 }4 CH3COC4H9 25 Methyl Acetate 26 Methyl Alcohol j.t ~ _20G 465 930 6.7 200 402A 83B 1395 1991 2372 2853 3317 3613 4278 4743 1273 1740 2210 2700 3200 3680 4180 4690 2 45 d5 u & 2b CHjCOtiCHj 2 3 9 18.5 365 88 OS 26 CH3OH 27 Mtlhyl Elhrl Kelone (WKJ ~1 1 6 200 1B0 376 565 637 1060 1360 1630 1908 2160 2448 2 3 5 5 10 5 21 47.5 71 27 CH3COC2H5 2fi Octane 29 Pentane 30 Styrene 31 Jetrahydtofutan 32 Toluene 33 Jlylene 53 Chtorobenrene 55 1 4 Oiorane 1.0 600 130 255 1.4 1000 140 294 1.1 too 66 143 20 200 200 400 1 4 200 112 231 1 0 200 ho 220 1.3 76 130 267 20 100 290 560 380 510 448*1 60S 242 341 630 840 350 476 330 450 409 546 830 1100 640 770 915 1060 1210 1365 95 770 924 1085 1246 1400 1560 16 457 583 715 647 990 1133 2 1050 1260 1480 1700 [molrm 2 602 720 846 973 1099 1218 35 560 68o1 800 920 1055 1200 5 682 822 962 1105 1248 1391 1 1370 1640 1910 2180 2450 2720 1 14 24 3 4 55 75 2 1.5 23 5 95 185 44 64 28 CjHio 39 5 11 22 55 82 29 C5M12 4 5 5 10 5 21 525 75 30 CgHjCHCH^ 6 5 12 25 60 86 31 c<HaQ 9 65 13 25 60 07 5 32 c6h5ch3 12 5 6 12 23 42 52 33 CfiH4lCH3)j 25 4 5 8 5 16 5 39 55 5 53 CgH^CI 2 5 5 10 20 50 75 55 C^OCjHiO Copyright 1973 by United States Steel Corporation. Used with permission. Ficure 19-29. -- An example of a calibration test kit for an explosimeter. Courtesy Mine Safety Appliances Co. tions and possible sources of error must be fully understood. It is very important to establish that an instrument responds properly to the substance it is designed to sample. This is generally done by calibration procedures with standard concentra tions of the substance of interest. There are two generally accepted methods for calibrating direct reading, air sampling instru ments: (a) the static method which involves introducing a known volume of gas into the instrument and sampling for a limited period of time, and (b) the dynamic method whereby a known concentration of the contaminant is prepared in a test chamber and the instrument is used to monitor that concentration. The static method is by far the easier, more efficient technique for checking the response of portable gas-detection instruments. A wide range of static-type calibration kits are in general use today. Some of these consist of a container filled with a known concentration of a specified gas in air, a regulating valve with a pressure gage for measuring the pressure in the container, and a hose adapted that connects the cylinder to the instrument to be checked. Once the calibration kit is attached to the instrument, a sample of the gas-air mixture from the container is permitted to flow into the device. The meter reading of the instrument is then compared with the known concentration of the sample to determine the correctness of the response (see Table I9-A). Calibration of combustible-gas detection instruments is accomplished by similar methods. Combustible-gas detection instrument calibra tion kits generally consist of a cylinder containing a known concentration of methane-in-air and a rubber bladder into which the gas is forced (see 3M --Dir ct-Reading Gas and Vap r M nitors TYPICAL CALIBRATION CURVES 1.0 .9 .8 Z 7' 5 6Q A " .5 ae -4 S .3 10 20 25 30 40 50 60 70 80 90 100 % OF LOWER EXPLOSIVE LIMIT Figure 19-30. -- Calibration curves for different materials. Figure 19-29). With these kits, the instrument to be checked is first calibrated to zero and the rubber bladder assembly is connected to the device. Then the needle valve on the calibration cylinder is inserted into the rubber bladder which, in turn, is pressurized. The calibration gas is drawn from the bladder into the instrument, providing an upscale reading on the instrument's meter. This reading is compared with the response curves furnished with the instrument to determine whether or not the device is calibrated correctly. In contrast, the dynamic method of calibrating gas-detection instruments is somewhat more difficult and requires facilities not always available to the average industrial plant. In dynamic calibration, a specific concentra tion of the contaminant is prepared in a laboratory situation "and placed in a special test chamber along with the instrument. Then the instrument is activated and a meter reading is taken to determine the correctness of the re sponse. The rate of airflow and the rate of addition of the contaminant to the sample stream must be carefully controlled under dynamic calibration methods to produce a known dilution ratio. Dynamic systems offer a continuous supply of contaminant, allow for rapid and predictable concentration changes, and minimize the effect of wall losses as test substance comes into equilibrium with the interior surfaces of the system. As discussed earler, combustible-gas detec tors are generally calibrated to indicate percent LEL of the gas or vapor being tested. However, interpreting the result of tests with these instru ments requires careful evaluation of all factors involved. For instance, some instruments do not respond in the same manner to different flammable contaminants. If the meter reading of an instrument is plotted on a vertical axis and the percent LEL is plotted on the horizontal axis, different curves will be established for such materials as natural gas, acetylene, gasoline, and 3M 115053 f carbon disulfide (see Figure 19-30). For this reason, it is important for the manufacturer to supply calibration curves for the air contaminant being monitored as well as to understand the limitations of the instrument. Summary The ultimate goal of the hazard evaluation process is to determine the exact amount of vapor or gaseous contaminants present in the work environment. Proper operation of various instru ments used in hazard analysis is essential to make sure that the information obtained in air quality tests is accurate enough to give an interpretation that is useful. Faulty operation of air-sampling instruments can result in low readings; these can falsely indicate that no hazard is present when, in fact, dangerous conditions might exist. Likewise, high instrument readings and the resulting implemen tation of a hazard-control procedure may be instituted where none is needed. A permanent record should be maintained of all calibration procedures, data, and results. The type of information to be kept for this record includes instrument identification, temperature, humidity, trial run results, and final results. It is important that the operator have a thorough understanding of the operation of the instrument, as well as knowing the instrument's intended use and the calibration procedures recommended by the manufacturer. References American Conference of Governmental Industrial Hygienists. Air Sampling Instruments for Evaluation of Atmospheric Contaminants, 5th ed. Cincinnati: ACGIH, 1978. American Industrial Hygiene Association. Direct Reading Colorimetric Indicator Tubes Manual Akron. Ohio. AIHA, 1976. Environmental Health Monitoring Manual. Environmental Health Services, United States Steel, 1973. National Institute for Occupational Safety and Health. The Industrial Environment -- Its Evaluation and Control Washington, D.C.: Superintendent of Documents, U.S. Government Printing Office, 1973. Patty, F.A.. ed. Industrial Hygiene and Toxicology. 3rd rev. ed. "Volume I, General Principles," New York, N.Y.: Interscience Publishers, Inc., 1978. Ruch, W.E. Quantitative Analysis of Caseous Pollutants. Ann Arbor, Mich.; Ann Arbor -- Humphrey Science Publishers, Inc., 1970. Ruch, W.E,, ed. Chemical Detection of Caseous Pollutants, 4th ed. Ann Arbor, Mich.: Ann Arbor -- Humphrey Science Publishers, Inc., 1970. From Fundamentals of Industrial Hygiene. 2nd edition. Copyright 1979 by National Safety Council, Chicago, IL 60611. Used with permission. r 3M 115054 c ;> 4 * AA5A Mine Safety Appliances Company 600 Penn Center Boulevard Pittsburgh, Pennsylvania 15235 3M 115055