Document VKwNvXZz1nwdr7eDmy5qj9dbw

CHAPTER 35 Temperature Measurement, Pressure Measurement, Measure ment of Air Movement, Air Change Measurements, Measure ment ofRelative Humidity, Dust Determination, Heat Transfer Through Building Materials, Measurement of Heat Exchange for Comfort Conditions, Combustion Analysis, Smoke Density Measurements, Carbon Monoxide Measurements THIS chapter presents a description of many test instruments used for heating, ventilating and air conditioning tests and presents a discussion of their use. TEMPERATURE MEASUREMENT Changes in the intensity of heat may be determined by several methods* such as measuring the change in volume of a liquid, the change in internal pressure of a confined gas, the current set up between dissimilar metals joined in a circuit, or the change in resistance of an electrical circuit. Thermometers The most common method used is the change in volume of a liquid such as mercury or alcohol enclosed in glass. Mercurial thermometers may be used for measuring temperatures from -- 40 F to approximately 1000 F. The lower limit, is set by the freezing point of mercury. Since the boiling point of mercury is only about 675 F, the space above the mercury in thermometers designed for higher temperatures must be filled with an inert gas under pressure. Alcohol thermometers may be used for temperatures from -- 94 F to +248 F. The more accurate thermometers are individually calibrated and have divisions etched on the stem. The two most common reference points are the freezing and boiling points of water. On the Fahrenheit scale, which is most commonly used in engineering work, there are 180 divisions between these points. On the Centigrade scale which is used by chemists and physicists,' there are 100 divisions in this range. The, temperature *For a comprehensive treatment of temperature measurement the reader is referred to Temperature, Its Measurement and Control in Science and Industry, a symposium sponsored by the American Institute of Physics and published by Reinhold Publishing Corp. 654 CHAPTER 35. INSTRUMENTS AND TEST METHODS in degrees Fahrenheit equals 9/j of the temperature in degrees Centi grade, plus 32. For permanent installations, glass thermometers are often protected by metal jackets and equipped with metal scales. Due to the heat capacity and heat conductance of the jacket, it is:more difficult to obtain the true temperature at a point with these than with the exposed etched stem type. The latter is usually preferred for test purposes. Where used to measure temperatures in a duct, it may be inserted through a cork or rubber plug. Care must be taken to locate jhe bulb at the point where the temperature is desired and in many cases several must be used to get a correct average. Most mercury thermometers are calibrated for complete'stem immer sion. When incompletely immersed, a stem correction should be made for the most accurate determination. At ordinary atmospheric tem peratures the correction is negligibly small, but it usually is important when measuring high temperatures such as those of steam and flue gas. The emergent stem correction may be calculated by the equation: K = 0.00009 D (<i -- tt) (1) where K = correction to be added, degrees Fahrenheit. D -- number of degrees on the thermometer scale which are hot immersed. h = temperature indicated on the thermometer, degrees Fahrenheit. tt - temperature of the non-immersed mercury column, degrees Fahrenheit. 0.00009 - difference in the coefficient of expansion of the mercury and glass. Since the bulb has considerable area, radiant energy may affect tem perature readings1. In measuring room temperatures, care must be taken to locate thermometers away from hot surfaces such as radiators' or cold surfaces such as walls or windows. Where this is impracticable, shields should be used to screen the bulb from the radiant energy. Thermocouple 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 upon the metals used and the tem perature difference of the two junctions. Often the cold junction is kept at 32 F by immersion in an ice bath. In other instances, a higher tem perature such as that of the atmosphere is used for this junction. By proper selection of metals, any temperature up to 2900 F may be meas ured. Readings are obtained by means of a potentiometer or sensitive galvanometer which may be calibrated directly in degrees. A potentio meter balances the electromotive force against a known electromotive force with no current flowing, hence this method is independent of length and variations in resistance of leads. Calibration of thermocoiiples ior high temperatures may be made against known melting points of metals. Radiation effects may be minimized by using the' smallest size of wires ^Errors in the Measurement of the Temperature of Flue Gases, by P. Nicholls and W. E. Rice. (A.S.H.V.E. Transactions. Vol. 35.1929, p. 473). 655