Page:Journal of the American Society of Mechanical Engineers, Volume 33.pdf/649

may be defined as the minimum temperature to which air of a given moisture content may be cooled without precipitation of moisture.

9Usually it is more convenient to determine the absolute and relative humidities from the temperature-pressure curve by comparing the vapor pressures. The per cent of humidity is ${\frac {D_{2}}{D'_{2}}}$, but it may also be shown to be equal to ${\frac {e_{1}}{e'_{2}}}$; i.e.

[1]

${\text{per cent humidity}}={\frac {e_{1}}{e'_{2}}}={\frac {D_{2}}{D'_{2}}}$

where $e_{1}$, is the pressure of saturated vapor corresponding to the dew point $T_{1}$, and $e'_{2}$ is the vapor pressure at saturation corresponding to temperature $T_{2}$. It also follows that

[2]

$D_{2}=D'_{2}\times {\frac {e_{1}}{e'_{2}}}$

Proof of these relationships is given in Appendix No. 1.

METHODS OF MEASURING ATMOSPHERIC HUMIDITY

10Determinations of atmospheric moisture may be made by four distinct methods:

11Chemical Method. A measured quantity of air is drawn through some de-hydrating solution, such as concentrated sulphuric acid, until the moisture is completely removed and the increase in the weight of the solution noted.

12Hygroscopic Method. This method is chiefly useful in an approximate determination of the relative humidity directly. It is known that nearly all animal and vegetable substances containing albumen or cellulose, and also many mineral salts are very sensitive to changes in atmospheric moisture. The moisture content of such materials at equilibrium is found to bear a direct relation to the existing amount of moisture in the atmosphere.

13The per cent of moisture which they will freely absorb, however, is not exactly the same for the same percentage of humidity for different temperatures. This relationship of moisture content of various textiles to different atmospheric humidities and temperatures has been very thoroughly investigated by Schloessing in France. Fig. 4 exhibits some of the relationships thus determined.

14It is therefore to be seen that the moisture content of the air will be approximately indicated by measuring the increase in weight