Document V3V8gYGqqDddjX59qbK2moVVg

of and 1935American Society Heating Ventilating Engineers Guide, vacuum in inches of mercury. The following equation may be used to make corrections for temperature: where h = Ml - 0.000101 (t,`- ()1 . ` h = height of mercury column corrected to. temperature t. hi = actual height of mercury column. h = actual temperature of mercury column. t = temperature to which column is to be corrected. *' 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. Wh^ri. 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 prpsr 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. 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 degrees 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. Thermocouples1 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 temperature difference exists between these junctions, an electromotive force 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. See A.S.H.V.E. research paper entitled Study of the Application of Thermocouples to the Measurement of Wail Surface Temperatures, by A. P. Kratz and E. L. Broderick (A.S.H.V.E. Transactions, Vol. 38, 1932). 40Chapter --Test Methods and Instruments For temperatures above 500 F various types of pyrometers are employed. 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. A sensitive galvanometer with a calibrated temperature scale indicates the thermorelectromotive 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)T 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 tempera ture 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 compensators 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 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 tharr 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 meter is to be permanently located in the duct, a pipe flange or nipple should be used to receive the threaded portion of the thermometer stem. When the thermometer is not to be permanently located, a cork or rubber stopper may be placed around the stem to prevent errors from air leakage. Readings should be taken at various locations in a duct so due con- sideration may be given to temperature stratification. Other forms of temperature measuring devices may be used, but the active part must be at least 6 inches from the duct wall. Recording thermometers may be used for testing, and for making continuous records of operation. Care should be taken, however, to insure that time lag due to heavy measuring elements is kept to a mini mum, so that the recorders will properly follow temperature fluctuations. Thermocouples made of fine wire will show less time lag than will many mercury bulb thermometers. 677