Page:Journal of the American Society of Mechanical Engineers, Volume 33.pdf/656

Rh 36At the time of this writing, the author is not prepared to give any deﬁnite data with regard to such determinations, but will present a method employing the evaporation calorimeter which apparently affords great accuracy and upon which greater reliance can be placed than upon previous methods, in all of which the air or other gas must be measured with precision. This measurement, when dealing with air quantities sufficient to give accurate determinations, seems to present the chief difficulty.

37In the present method, on the contrary, no air measurement, other than the determination of its density through temperature and barometric pressure, is required. It is, indeed, in this respect, closely allied to the throttling calorimeter method of determining the speciﬁc heat of steam. In other respects, however, it has a great advantage over that method in that it requires no subtractive calculations sensitive to error, but equates the known latent heat of water vapor, directly to the unknown speciﬁc heat of air, the weight of the water vapor having a known ratio to that of the air.

38This method consists, ﬁrst, in bringing a continuous supply of air close to saturation, where its moisture content can be determined with great accuracy by means of a wet and dry-bulb thermometer and applying the rational psychrometric formula [3] assuming an approximate value for the speciﬁc heat; second, in heating this current of air of known moisture content to any desired amount andtaking the wet and dry-bulb readings as in experiment No. 2, Appendix No. 2. By applying the rational psychrometic formula derived in Pars. 58-67, we have

where


 * $$C_{pa}$$ = mean speciﬁc heat of air of constant pressure between temperatures $$t$$ and $$t'$$
 * $$C_{ps}$$ = mean speciﬁc heat of water vapor between temperatures $$t$$ and $$t'$$
 * $$t$$ = temperature of the dry bulb
 * $$t'$$ = temperature of the wet bulb
 * $$r'$$ = latent heat of water vapor corresponding to $$t'$$
 * $$W$$ = weight of water vapor actually contained in 1 lb. of dry air; i.e., it is the ratio of the weight of water vapor to the weight of air in the mixture