Document 5b4VDEe5d2v1dg34MVJDVoYqJ

American Society of Heating and Ventilating Engineers Guide, 1936 in which V = average indicated velocity obtained by the anemometer traverse. A = gross area of grille, square feet. K - coefficient determined by experiment. For average use, with exhaust grilles, i K may be taken as 0.8 for all usual velocities. This formula is of advantage, especially with ornamental grilles, in that the free area need not be measured. The flow of air through registers and grilles is of considerable impor tance, being frequently the only convenient method of measuring the volume of supply air to a room. While duct measurements, if available, are more dependable, grille measurements provide a fairly accurate method, if care is taken in the technique of using the anemometer. Kata-Thermometer The Kata-thermometer can be used to determine air velocities pro vided the walls and surrounding objects are at or near the room tem perature. Especially at low velocities it constitutes a useful instrument for readily detecting drafts. The instrument is essentially an alcohol thermometer with a bulb approximately % in. in diameter and x/i in. long with a stem 8 in. long reading from 100 F to 95 F, graduated to tenths of a degree. To take readings the bulb is heated in water until the alcohol expands and rises into a top reservoir. The time in seconds required for the liquid to fall from 100 F to 95 F is recorded with a stop watch and this time is a measure of the rate of cooling. The dry Kata loses its heat by radiation and by convection so for constant velocities the time of cooling is a function of the dry-bulb tem perature of the surrounding air. The wet Kata; which has a cloth covering fitted snugly around its bulb, loses heat by radiation, convection, and evaporation, and for constant velocities its rate of cooling is a function of the wet-bulb temperature of the air irrespective of the dry-bulb- tem perature or relative humidity. It does not follow, however, that the difference in rate of cooling of the dry and the wet Kata is caused by evaporation. A change in the wet-bulb temperature produces a change in the surface temperature of the wet Kata which in turn affects the heat lost by radiation and by convection. Several precautions should be taken to bbtain the best results with this instrument: 1. To obtain velocity readings use the dry Kata since the error in timing is reduced. 2. The instrument should be heated and allowed to cool two or three times before recording the final time of cooling. The first reading is not reliable. 3. All traces of moisture must be removed from the dry Kata before timing to eli minate error introduced by evaporation. 4. Use only the formula applying to a particular instrument. Each Kata receives an individual calibration. HUMIDITY MEASUREMENT The sling psychrometer is the recognized standard instrument for determining humidities. In order to obtain accurate readings considerable 766 Chapter 43--Test Methods and Instruments skill is required on the part of the operator. The wicking and water must be clean and the temperature of the water should be slightly above the wet-bulb temperature of the surrounding air. The psychrometer should be swung rapidly and several and frequent observations should be made to see that the wet-bulb temperature has become stationary before the final reading is noted. Care should be taken that the wet-bulb has reached a minimum temperature, but the wick must still be moist. Standard psychrometric tables should be used4. In making wet-bulb measurements below 32 F the same procedure is followed as above 32 F. The water is liquid at the start, but as the sling is operated it will freeze rapidly enough so that in quickly giving up the latent heat of fusion, the indicated wet-bulb temperature may drop below the actual wet-bulb temperature. After the liquid on the bulb has become thoroughly frozen the wet-bulb temperature will rise to normal. A very thin film of ice is more desirable than a thick film. Care must be taken to read the temperatures in the region below 32 F accurately because the spread between the wet- and dry-bulb is small. In taking humidity readings in ducts it is usually impracticable to use a sling psychrometer. For this work the stationary hygrodeik arranged for bolting on to the side of the duct, with two bulbs extending into the duct, will be found very convenient. Owing to the velocity of the air. passing over the bulbs within the duct an accurate reading will be secured, corresponding to that given by the sling psychrometer. Various forms of humidity recorders are available, some merely re cording wet- and dry-bulb temperatures, and others recording relative humidity directly. Any form of wet- and dry-bulb device must have sufficient air velocity over the thermometer bulbs to insure accurate readings; this velocity should be secured by a fan if the air is not itself in motion, as in a duct. For extremely low humidities, or for humidity measurements above 212 F, a thermal conductivity method is available6. CARBON DIOXIDE DETERMINATION6 At ordinary concentrations carbon dioxide is not harmful. The amount of carbon dioxide in the air is a convenient index of the rate of air supply, and of the distribution of the air within rooms. Unequal carbon dioxide concentrations in parts of a room indicate improper air distribution. The Petterson-Palmquist apparatus has been generally accepted as the standard device for the determination of carbon dioxide in air investiga tions. The principle involved is the measurement of a given volume of air, the absorption of the contained carbon dioxide in a caustic potash solution, and the remeasurement of the volume of air at the original pressure in a finely graduated capillary tube, the difference in volume representing the absorbed carbon dioxide. (See Report of Committee on Standard Methods for Examination of Air, American Public Health Asso ciation, Vol. 7, No. 1; American Journal of Public Health, fan., 1917.) . .aUiu ,U, ncasuie, tveiauve ntimidity ana temperatures of the Dew Point; U. S. Department of Agriculture, Weather Bureau, Washington, D. C. Gas Analysis by Measurement of Thermal Conductivity, H. A. Daynes, Cambridge Press, 1933. Indices of Air Changes and Air Distribution, by F. C. Houghten and J. L. Blackshaw (A.S.H.V.E. Transactions, Vol. 39, 1933). 767 /