Document 0OK2O54LEzKjGD7YJZjv4EBR

Heating Ventilating Air Conditioning Guide 1939 The instrument is essentially an alcohol thermometer with a bulb approximately 54 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 obtain 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. Alt traces of. moisture must be removed from the dry Kata, before timing to eli- 'rtiinate error, introduced by evaporation. : V 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 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 arid 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 theNwick must still be moist. Standard psychrometric tables should be used8. 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. Psychrometric Tables for Vapor Pressure; Relative Humidity and Temperatures of the. Dew-Point; J. S. Department of Agriculture, Weather Bureau. Washington, D. Cl . 830 Chapter 44. Test Methods and. Instruments 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 pf the duct, with two bulbs extendiftg; into the duct, Will be found very convenient. Owing to the velocity of the1 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 teinperatures, and others recording relative humidity directly. Any form of wetr 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, A minimum velocity of 900 fpni is usually recommended but Velocities from 300 to 1000 fpm have been found suitable urider favorable conditions9. For .extremely low humidities, or for hurriidity rneasurements above 212 F, a thermal conductivity method is available10, . CARBON DIOXIDE DETERMINATION11 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 Stafidard Methods for Examination of Air, American Public Health'Asso ciation, Vol. 7, No. 1; American Journal of-Public Health, Jan., 1917.) A "thermal cpridiictivity method may also be used to measure carbon dioxide in air over a range of 0 to 1.5 per cent12. ; ,/ Where field conditions are such that this apparatus may riot be con veniently used, as in street cars, air samples, may, be collected in clean bottles having mercury-sealed rubber stoppers; and these may be sub- jected to laboratory analysis. , , ,. ; DUST DETERMINATION Many laboratory methods have been developed to measure the dust-in the air. These involve the collection of dust on sticky plates, on filter paper, in water, on'porous crucibles, or by electric precipitation,.and the subsequent determination of the amount of dust by microscopic counting, weighing, or titration. While there is no standard method, the. Hill __________ . pp. 528-30). ----------- ---- W. VOl. 0, 1NO. O, AUgUSt, lyiW, r / .. . lsGas Analysis by Measurement of Thermal Conductivity, by H. A. Dayned (CtAridgc. Press, 1933). nA.S.H.V.E. Research Report No. 959--Indices of Air Changes and'Air Distribution, by F..C. Hough* ten and J. L. Biackshaw (A.S.H.V.E. Transactions, Vol. 39, 1933, p; 261). Loc. Cit. Note i0. .. 831