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

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

[3]

$C_{pa}={\frac {r'(W'-W)-C_{ps}W}{(t-t')}}$

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