Document DQ3ZL2a847dRxN44X4M59kNQ

American Society of Heating and Ventilating Engineers Guide, 193^ ------------------------------------------- :--------------------\------------------ straighten out, and a pointer motivated by this straightening indicate, the pressure difference on a suitably graduated scale. ^ High vacuum readings such as are encountered in condenser and steam jet refrigeration practice are commonly obtained by the use of mercun! column vacuum gages. When the readings obtained with the mercurial barometer and those with the mercury vacuum gage have both been corrected to 32 F, the difference in the two readings will give the absolute vacuum in inches of mercury. The following equation may be used to make corrections for temperature: where h -- hi [1 -- 0.000101 (/, - <)] h = height of mercury column corrected to temperature t. hi = actual height of mercury column. <1 = actual temperature of mercury column. t = temperature to which column is to be corrected. m u A gage which indicates pressures slightly above or below atmospheric is known as a draft gage. It is essentially a U tube containing either water kerosene, alcohol, or mercury, with one leg exposed to the air and the other connected to a point where the pressure is to be determined. When j i the pressure being read is equal to atmospheric, the level of the liquid in the legs will be the same, indicating a zero gage pressure. When a pres sure is applied to one leg, one side will fall and the other will rise an equal amount. The difference in height between the two liquid levels indicates the pressure expressed in inches of liquid used in the gage. Various forms of high sensitivity draft gages1 frequently called micro manometers are available for the measurement of small pressure differen tials and may be sensitive to pressures as small as 0.001 in. of water. These gages are often useful where measurements are to be made on pressure differentials less than 0.1 in. of water, although their total range may extend as high as 5 to 10 in. of water. ii '5 { `I Ml TEMPERATURE MEASUREMENT In engineering work, mercurial thermometers are largely employed to measure the intensity of heat. These depend on the uniform expansion of mercury to indicate changes in temperature. An amount of mercury held in a sealed tube with a bulb at one end will rise to one definite level when immersed in melting ice, and to another definite level when immersed in boiling water. These two points are marked, and the space between them is divided into a number of equal portions, each of which is called a degree. In the Fahrenheit scale, there are 180 deg thus obtained, while the centigrade scale has 100 and the Reaumur has 80. Like divisions are marked off on the column above and below these two determined points in order that a greater range of temperature may be read. ` 5 Thermocouples2 may be used to measure any range of temperatures up to 2,900 F. When two dissimilar metals are joined at two points and a Illinois Micromanometer, University of Illinois Engineering Experiment Station Bulletin No. 120, p. 91 Study of the Application of Thermocouples to the Measurement of Wall Surface Temperatures, by A. P. Kratz and E. L. Broderick (A.S.H.V.E. Transactions, Vol. 38, 1932). 778 > Chapter 43--Test Methods and Instruments 0erature difference exists between these junctions, an electromotive free will be developed. Its magnitude depends on the composition of the wires and the difference in temperature between the junctions. A poten tiometer or sensitive galvanometer of high resistance connected to the thermocouple will give a deflection which is a function of the temperature difference between the hot and cold junctions. Thermocouples con nected in series are called thermopiles. Thermocouples for the measure ment of high temperatures are calibrated with the aid of the known melting points of pure metals. Resistance thermometers are suitable for temperature measurements up to 1800 F. These thermometers depend for their operation on the change of resistance with temperature of a platinum, nickel, or copper wire coil, and they are calibrated in the same way as thermocouples. Pyrometers of various types may be used for temperatures above 500 F. The mercurial pyrometer is a thermometer with an inert gas, such as nitrogen or carbon dioxide, above the mercury column to prevent the mercury from boiling. The radiation pyrometer consists of a thermopile upon which the radiation from a hot source is focused by a concave mirror or lens. A sensitive galvanometer or potentiometer with a calibrated temperature scale indicates the thermo-electromotive force created by the heat on the thermopile. The optical pyrometer measures radiant energy by comparing the intensity of a narrow spectral band, usually red light emitted by the object, with that emitted by a standard light source (electric lamp). Thermo-electric pyrometers operate on the same principle as thermocouples. When measuring high temperatures, it is customary to hold the cold junction at room temperature and this may cause some error if the room temperature is above or below the calibration point. For extremely precise temperature measurements, the cold junction is usually immersed in melting ice to fix the cold junction temperature. Various forms of hand-operated and automatic cold junction temperature com pensators are also available. In the measuring of room temperatures care must be exercised to pre vent the results from being affected by the body heat of the observer, by drafts from doors, windows and other openings, or by radiant heat from some local source such as a radiator or wall. All glass thermometers should be mercury thermometers with engraved stems. The total gradua tions of the thermometers should be from 20 to 120 F, in one degree graduations. No ten degrees should occupy a space of less than one-half inch. The accuracy throughout the whole scale must be within one-half degree. The operator should take hold of the top and no part of the body, including the hand, should be nearer than 10 in. to the bulb. The thermometer should not be closer than 5 ft to any door, window, or other opening; should not be closer than 12 in. to any wall; and should be between 3 and 5 ft from the floor. A sling instrument should be used for extreme accuracy. Thermocouples or resistance thermometers may also be used for room temperature measurements, an advantage being that the operator can read temperatures from outside the room if desired, and thus eliminate the errors which might be caused by his-presence close to the temperature measuring device. For measuring duct temperatures a duct thermometer should be used, with the bulb extending into the duct at least 6 in. When the thermo- 779