Page:Journal of the American Society of Mechanical Engineers, Volume 33.pdf/659

$W'$ = weight of water vapor contained in 1 lb. of dry air at saturation at temperature $t'$

[4]

$W'={\frac {Se'}{P-e'}}$

where

$S$ = specific weight of water vapor

$P$ = barometric pressure

$e'$ = vapor pressure at $t'$

39⁠The apparatus for this determination is shown in Figs. 5 and 6. Two fans are required. Fan No. 1 draws the air through moist sponge A. The air in passing through the fan rises in temperature so that it is desirable to saturate it further by passing it through a moist sponge filter B. The wet and dry-bulb temperatures of the air are then taken with thermometers 1 and 2. These readings should be practically identical so that error in calculating the true dew point is negligible. The air is then heated by passing it through an electric heater provided with a rheostat for regulating the temperature. Thence it is blown, still under slight pressure, into fan No. 2, where it is thoroughly mixed and the pressure slightly increased. After the air passes through the second fan, the wet and dry temperatures are taken in the manner previously described, except that the greatest precaution is exercised in the construction of the air passage to avoid radiation from the thermometer bulbs. A differential gage is connected between chambers 2 and 3. Care is taken in regulating the pressures of the fans and in the damping of the discharge to keep the differential gage at zero, so that there will be the same pressure of air on both thermometers. This pressure is measured by a second draft gage D₂, and a third draft gage D₃, is connected to the second fan inlet where it is essential that the pressure be maintained above atmospheric so that any leakage occurring in the apparatus will be outward. Pitot tube and differential gages are used to determine the velocities on the thermometers.

40⁠The accuracy of this method is apparently limited only by the accuracy of the thermometers. Wet-bulb depressions between 20 and 50 deg. may be used to advantage, and the determinations should be accurate to at least 1/10 of 1 per cent.