Document g2L0rw8qLEbbgO7n25ra20G5e

HEATINC VENTILATING AIR CONDITIONING GUIDE 1941 obtain accurate determinations. Since occupants also give off moisture, the increase in humidity may also be used as an index of ventilation within a space. Humidity determinations are much simpler to make, but the accuracy may be affected slightly by absorption of moisture by hygroscopic materials such as fabrics and wood within the space. Meas ured amounts of either carbon dioxide or water vapor may be added for test purposes. However, neither method is used at the present time, and more direct methods of measuring air supply and air distribution are in favor. MEASUREMENT OF RELATIVE HUMIDITY Wet- and dry-bulb mercurial thermometers are usually used to deter mine relative humidity. The sling psychrometer is a common mounting of the thermometers to permit swinging. The wet-bulb wick and water for wetting it must be clean, and the temperature of the water should preferably be slightly above the wet-bulb temperature. An air stream velocity of 900 fpm is recommended, although velocities from 300 fpm to 1000 fpm have been found satisfactory for passage over the wet-bulb wick. The velocity may be obtained by whirling the thermometer or by aspirating air over the wet-bulb. In ducts, the air flow itself gives the proper evaporating conditions. Several observations should be made until the minimum temperature is reached. Relative humidity may be obtained from tables or psychrometric charts19. Although it is common practice to use the charts which are based on a barometric pressure of 29.92 in. mercury, a correction for barometric pressure is necessary for extreme accuracy. This correction is made by multiplying the relative humidity as determined from the chart by the ratio of the observed barometric pressure and the standard barometric pressure. For temperatures below 32 F, the water on the wick is allowed to freeze, during which time the temperature will drop below the true wetbulb. A thin film of ice is more desirable than a thick one, and it is satisfactory to remove the wick and freeze a thin film directly on the bulb. Care must be taken to read the temperatures accurately due to the slight wet-bulb depressions. Tables for ice conditions must be used29. The dew-point apparatus for humidity measurements consists of a polished plated container cooled by the evaporation of a volatile liquid within. The temperature at which the first slight water vapor forms is the dew-point. If the temperature is below 32 F, the deposit will appear as frost. Another method of determining humidity is by chemical means in which the water vapor is removed by a drying agent and weighed on a chemical balance. A thermal conductivity method is available for temperatures above 212 F or for extremely low humidities21. DUST DETERMINATION The measurement of dust is complicated by the many kinds involved. Some of the collecting methods are impingement on viscous surfaces, "Psychrometric Tables for Vapor Pressure, Relative Humidity and Temperatures of the Dew-Point, (U. S. Department of Agriculture, Weather Bureau. Washington, D. C.). "A. Review of Existing Psychrometric Data in Relation to Practical Engineering Problems, by W. H. Carner and C. O. Mackey (A.S.M.E. Transactions, January, 1937, p. 33: Discussion. A.S M E. Transaettons, August, 1937. p. 528). ,lGas Analysis by Measurement of Thermal Conductivity, by H. A. Daynes (Cambridge Press. 1933). 636 CHAPTER 34. INSTRUMENTS AND TEST METHODS impingement at high velocity under water, collection on pofous crucibles through which air passes, and electric precipitation. Determination may be by direct weighing of samples or by microscopic counting. The most commonly used methods are the modified Hill dust counter using micro scopic count, the Smith-Greenburg impinger which collects samples in water and which are counted under a microscope in a Sedgwick cell22, and the Lewis sampling tube with the analytical determination of the increase in weight of a porous crucible. All reports should state the method of sampling and counting. The A.S.H.V.E. Code for Testing and Rating Air Cleaning Devices Used in General Ventilation Work specifies the porous crucible method23. HEAT TRANSFER THROUGH BUILDING MATERIALS The A.S.H.V.E. Standard Test Code for Heat Transmission Through Walls specifies the use of the guarded hot plate for tests on homogeneous materials. It further states that the mean temperature shall be at 60 F and the materials dried. This code describes the construction and use of the guarded hot box for determining overall heat transmission coefficients of built-up sections. The standard temperature range through the test section is specified as 80 F and the mean temperature of the wall as 40 F. The Nicholls heat meter is very useful for determining the heat flow through walls of buildings24. MEASUREMENT OF HEAT EXCHANGE FOR COMFORT CONDITIONS Several instruments have been devised to measure the effect of various factors as they relate to the comfort of the body. The principle ones are the Kata-thermometer, Dufton's eupatheoscope, Vernon's globe ther mometer, Winslow and Greenburg's thermo-integrator, and Yaglou's heated globe26,26. These instruments are attempts to stimulate and measure the heat exchanges between the human body and its environ ment. In order to stimulate conditions of hard physical labor, the entire surface of the device is covered with a wet cloth. At present special attention is being given the thermo-integrator as a means of measuring radiant effects of environmental conditions. COMBUSTION ANALYSIS The analysis of flue gases to determine completeness and efficiency of combustion is usually made chemically with the Orsat apparatus. This consists of a measuring burette, a leveling bottle, and three pipettes. Carbon dioxide is absorbed in the first pipette by potassium hydroxide, !3Public Health Bulletin, No. 144, 1925, (U. S. Public Health Service). 2*Testing and Rating of Air Cleaning Devices Used for General Ventilation Work, by S. R. twis (A.S.H.V.E. Transactions, Vol. 39. 1933. p. 277). *4A.S.H.V.E. Research Report No. 6S5--Measuring Heat Transmission in Building Structures and a Heat Transmission Meter, by P. Nichoils (A.S.H.V.E. Transactions.^]. 30, 1924, p. 65). ^Instruments and Methods for Recording Thermal Factors Affecting Human Comfort, by C. P. Yaglou, A. P. Kratz and C.-E. A. Winslow (Year Book, American Journal Public Health, 36-37). *The Thermo Integrator--A New Instrument for the Observation of Thermal Interchanges, by C.-E. A. Winslow and Leonard Greenburg (A.S.H.V.E. Transactions. Vol. 41, 1935, p. 149). 637