Document O30ynL42xm3RvoENvE8Lx9z9X

256 CHAPTER 16 1965'Guide And Data<Bookt The Weaver-type hygrometer is particularly useful for determining the humidity of air or other gas in pipes or . _ closed vessels at various pressures and temperatures. -It ~ consists of a threaded * plug carrying a central electrode . insulated from the plug, except for a gelatinous electrolytic film. The film is exposed to, the air or gas from tbie "pipe or . vessel, and provision is made for determining its electrical resistance. The; hygrometer, or detector, is mounted in a manifold equipped' with valves whereby the film can be 7. . alternately exposed to the test gas, and to a standard gas / having a known absolute-humidity.. The pressure of the standard gas is' varied until contact with it establishes the - . same resistance-in the film as the test gas, and the humidity ; 3 of the test gas is then determined by computation based on "the gas laws. . , . Chemical Hygrometry Rg. 15Schematic Diagram of Variable {Area Row Meter The humidity of an atmosphere can be measured directly JV'byiextracting and weighing the water vapor from a known J* sample. For'precise'laboratory work,- powerful' desiccants such as a sulfuric acid and phosphorus pentoxide are used for the extraction process)-while for some purposes, calcium chloride, lithium-chloride, or silica- gel are satisfactory. linkage, causes a pointer to move across an indicating dial, moving a pen across a recording chart, or actuating a pneumatic, or electric control mechanism. Common organic. materials employed are . human5 hair/: animal membrane, animal horn, wodd, and'paper. Unfor-' Freezing the water vapor out of a measured stream of air or gas with solid carbon dioxide, and weighing the resulting ice, is a similar operation. A thermal conductivity method for -gas -analyais om be .used for. temperature above ;212 F, or for very low humidities.41 . tuhately, although, the field has been well explored, ' ho organic material.has been found which can"be relied upon1 MEASUREMENT OF MEAN RADIANT to consistently reproduce its action over, an extended'period'.* of'time.* Such 'devices require initial calibration and' frequent' recalibratioh or setting, especially'if* they are exposed to: extremes of either high or low humidity! In^spite- of; this; they, are highly useful because they read directly in .terms of humidity, they are ample and inexpensive by compar ison with, most other types, and the inconvenience of reP calibration is often less than that of' providing distilled ` water and servicing the wicks of wet- and dry-bulb psy\; chrqmetere. - .-->* . - TEMPERATURE . , Because'of its effect' oh.human comfort,* it is sometimes' desirable to measure the radiation exchange betodeh a per son and his surroundings. The radiant environment of a space is usually expressed in terms of ita;mean-* raiiiant temperature (MRT), which is defined, in .Chapter 67. The .Vernon globe thermometer-4* is commonly- used ,to measure, mean- radiant temperature. -This instrument con sists.of a .6-in,, diameter-.hollow copper sphere coated.frith flat black paint and having a thermocouple or. thermometer Electrolytic Hygrometers'. bulb at its center. The temperature aRstimeH by the globe at equilibrium is. the result of a balance between the heat The dampness, and therefore the electrical resistance of; a;salt film,- varies with the humidity of the* atmosphere to:-* gained or lost by' radiation and the loss or gain by conven tion. In terms,of heat-transfer relationships: i! which' the film is exposed, and-at least two types of instru- j moits-based on this fact have been developed.; The-Dun--; '- .n4 ".3V + o;i63xii6yv?(i;-ty'i: v,? <1^ more hygrometer was originally designedLfor-use in radio- , tohere sondes orsmali balloons,'and means'were devised-whereby" the device'transmits humidity data back to earth in.the'i form* of. a*, radio signal.1 in the-more usual form^this:hy-.-' grometer consists of a dual winding* of' small wire on `a nonconducting tube.* -The whole is> coated - with an lelec- trolytic film,* Usually containing a salt such as lithium chlo-; Tm = mean radiant temperature,.' Fahrenheit- absolute * (black-body equivalent)! -1 ' T, => globe temperature, Fahrenheit absolute. F air velocity, feet per minute.' ." 4, 4 - globe ambient air temperature, Fahrenheit* ` ,f ride, which forms an electric connection-between the.wind- ; It will.be noted from; the above equation jthat this air mgs.. Means are provided for determining-' the - electrical ' resistance of the film,- which is an indication of- the humid-' temperature.;and the air velocity around the globe must alsq be determined. It is the velocity measurement, which ity. Id the* radiosonde, variations in the resistance of!the: film affect the frequency of an oscillating: circuit. These- presents the greatest problem in the use of the globe,ther mometer. ... hygrometers are usually calibrated by comparison .with a. . A, too-ephere- radiometer4* has recently been developed wet- and dry-bulb psychrometer. For'caUfiration fdf'some' which,:may..be used, for the measurement of. MRT. This purposes, particularly' for use at -subzero-temperatures, instrument utilizes two spheres-approximately. 2. in. in di; means have been provided for producing atmospheres *of: ameter,. one ,of - which is gold-plated, and the other,,black! knownhumidity.40 Advantages of instruments'of. this-type, for some'scientific-and-industrial purposes are becoming, apparent, and'some forms of them are on.the market. - The two-sphere-are heated electrically to.the same tern-. perature,':thus eliminating differences in .convection.;,The difference in energy required, by. .the two sphere, to'.main-. ^Measurement. and. Instruments 257 Table 6 .... Measurement of Humidity > Ifjl * Mecwreoeoi Metnu AppGcoiioa Wet-bulb thermometer Room of building, outside air, air with dry-bulb thermome- moving in ducts; standard method Jto&0*> f 0 to 500 PnoMn, f Uaitxjliont 0.1 to 1 Should be used ia air stream moving 1,000 to 1,500 fpm or correction made. Unsuitable for recording at sub-freezing sub-zero temp ,2 Dew point apparatus Condensation type . Occasional laboratory work; not widely used -180 to 200 2 Cumbersome, expensive, difficult to na with precision;-unsuitable for remote use Fog type Simple wide range method -100 to +100 2 Senes of readings necessary for de termination Electrical conductivity Accurate measurement . of dew type point; suitable for remote use - 0 to 200 Unusable below --20 F and below 15% rh or for any moderate humid ity at temp, under 0 F 3 Hygrometer (for relative humidity) . pir gyrometer * For direct rh in air where motion is slight Hygroscopic material . . as above--also for high senai- Electrical conductivity type Infra-red radiation For remote use While usable at all temperatures it is uniquely applicable to extremely low temp 5 Standard salt For standards 6 Chemical analysis 7 Absorption and weighing 0 to 100% rh -40 to 150 F 3% rh 0 to 100% rh 2 to 5% -40 to 150 F rh Considerable lag; low sensitivity; adversely affected by temp above 125 F and rh below 20%; frequent calibration required when used at extremes of range Frequent calibration required when used at extremes of range Requires frequent calibration Requires the use -of very costly equipment Cumbersome process fain temperature equilibrium is measured, and from this difference, the MRT of the space may. be calculated.. ; .Several instruments have, been developed, to evaluate -the combined effect of radiation and convection on-human com fort. Among these are the eupatheoscope and the thermal integrator.*-47 HEAT TRANSFER THROUGH BUILDING i MATERIALS . Thermal Conductivity ' The thermal conductivity (k value) of an insulation, as defined in Chapter 22, is a unit heat-transfer factor. Practi cally universally accepted for -the conductivity determina tion . of flat insulation is the guarded hot, plate, which is described in ASTM test method C-177.4* In its simplest form it consists of an electrically heated plate and two water-cooled plates. Two-identical specimens or slabs of a material are required for a test, and one is mounted on each side of the hot plate. A cold plate is then pressed against the outside of each specimen by* a clamp screw. The heated plate is divided into two portions: the central or measuring section, and the outer or guard section. Dur ing tests the two sections are maintained as nearly as possible at the same temperature., The purpose of the guard section is to minimize-errors due to edge effects. The elec tric energy required to heat the measuring section is care fully observed and converted to. Btu per hour. From this heat quantity, the area of the test section, the temperature gradient, and the speciment thickness, the thermal conduc tivity of the material ran be calculated- Hot plate appa ratus accommodating specimens on the order of one foot square and an inch or more thick is-common. The appa ratus at the.National.Bureau of Standards takes specimens 8-in. square, while plates as large as 3-ft square have been used. The thermal conductivity of cylindrical or pipe in sulation (Chapter 22) is determined in a similar manner except that an equivalent thickness must be calculated to account for the cylindrical shape.4* Recently there has been considerable development of transient state conductivity apparatus utilizing a slender probe.**-** These instruments are commercially avail able and have the advantages of rapidity and requiring a nmftll test specimen. While useful as research and develop ment. tools, the probe has not been generally accepted as have the guarded hot plate and. pipe insulation apparatus. Wall Conductances The thermal conductances' (C values) of many walls can ^ be satisfactorily estimated from the conductivities of their components and their dimensions, but some walls are com plicated by the inclusion of metal, for instance, and tests for conductance are required. The apparatus is required to accommodate large specimens representing actual construc tion. The guarded hot box apparatus was developed for this purpose. .Test specimens for the apparatus at the National Bureau of Standards are 5 ft long and 8 ft high, while others require different sizes, some larger, others smaller. . The guarded hot box is described in the ASTM Standard Tat Code for Built-up Sections C-236-MT.*4 The apparatus, consists essentially of three boxes: a cold box, cooled by a refrigerating machine; a hot box, heated electrically; and