Page:Journal of the American Society of Mechanical Engineers, Volume 33.pdf/670

62This equation should be used where the true wet-bulb tempera ture is obtained as in the aspiration psychrometer. With the sling psychrometer a correction must be made for the error in depression due to radiation and stem correction. By referring to Fig. 10 showing the per cent of radiation error, it is seen that this is inversely proportional to the velocity. It is also, of course, greatly affected by the conditions of exposure, i.e., whether it is surrounded on all four sides by bodies at the temperature of the dry bulb, or only partly by bodies of that or a different temperature. The effect of radiation out side of an enclosure may be assumed to be approximately one-half of that within an enclosure.

64A sling psychrometer 15 in. in length is ordinarily revolved between 150 and 225 r.p.m. giving a velocity between 1200 and 1800 ft. per min. This will give, according to Fig. 10, a radiation error of 2.6 to 1.75 per cent within an enclosure, and 1.3 to 0.9 per cent with out an enclosure. Hence an average radiation error of 1.6 per cent of the wet-bulb depression may be arbitrarily assumed. The wet bulb depression given by the sling psychrometer may be corrected by this amount to give the true depression, which may be used in the foregoing psychrometric formula, or the formula itself may be modified to allow for this error.

65If this formula is corrected for 1.6 per cent radiation error

[25]

$e=e'-{\frac {(P-e')(t-t')}{2755-1.28t'}}$

for the sling psychrometer.

66Using the true wet-bulb depression in formula [24], letting $e_{2}$ be the vapor pressure corresponding to saturation at the dry-bulb temperature $t$

[26]

$R={\frac {e}{e_{1}}}={\frac {e'}{e_{2}}}={\frac {(P-e')(t-t')}{(2800-1.3t')e_{2}}}$

for the per cent of relative humidity.

67Let $W$ be the grains of moisture per cu. ft. at any vapor pressure $e$, and $W_{2}$, grains per cu. ft. at $e'$; then

[27]

$W=(RW_{2})={\frac {W_{2}}{e_{2}}}\left[e'-{\frac {(P-e')(t-t')}{2800-1.3t'}}\right]$

also

[28]

$W=\left({\frac {460+t'}{460+t}}\right)W_{1}$

where $W$, is the grains of moisture per cu. ft., corresponding to the dew point at vapor pressure $e$.