Page:Journal of the American Society of Mechanical Engineers, Volume 33.pdf/675

conditions for 1 hour previous to the test. Consecutive readings were taken of marked uniformity, and the average results are given in Table 3.

76The actual calculated drop in temperature due to 1-in. air blast under the above conditions is

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$D=(82.7+459.6)\left[1-\left({\frac {397}{398}}\right)^{0.29}\right]=0.38{\text{deg.}}+$

77It will be noted that thermometer No.3 read 0.047 lower than No. 4 at 1 in pressure. However, at a wet-bulb temperature of 70 deg., 1 deg. in the temperature of the dry-bulb produces only 0.3 deg. increase in wet-bulb temperature; i.e., ${\frac {dt'}{dt}}=0.30$. Therefore at 1 in. pressure the error would be 0.014 and at ¼ in. pressure 0.0035. Hence, in any case, the correction would be negligible.

TABLE 3 EFFECT OF AIR. BLAST OF KNOWN INTENSITY ON READINGS OF THERMOMETERS NO. 3 AND NO. 4

Thermometers				Temperature Differences
No. 2	No. 1	No. 4	No. 3	1 and 3	1 and 4	3 and 4
. . . . . .	82.797	. . . . . .	82.653	0.084	. . . . . .	. . . . . .
. . . . . .	. . . . . .	82.566	82.613	. . . . . .	. . . . . .	0.047
. . . . . .	. . . . . .	. . . . . .	. . . . . .	. . . . . .	0.031	. . . . . .
Barometer pressure = 29.3 in. mercury = 397 in. water.

The temperature increase $E$ produced by an air blast equivelent to $p$ in. of water may be expressed by the equation

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$E=\left({\frac {0.38-0.13}{1}}\right)p=0.25p$

or ⁠0.25/0.38⁠ = 66 per cent of the theoretical temperature.

78Experiment No. 2. This was for the purpose of determining approximately the per cent of error due to radiation and stem correction in the depression of the wet bulb of the sling psychrometer, and was accomplished by comparing wet-bulb thermometer No. 4, with wet-bulb thermometer No. 3, which was protected from practically all radiation by surrounding it with insulated wet surfaces at precisely the same temperature, and by protecting the stem with a wet cloth. This arrangement is shown in Fig. 11, and may be termed a wet-bulb or evaporation calorimeter. The protection for the wet bulb consisted in an annular vacuum tube, having the exterior surface covered with a wet cloth, and the interior with a tube of wet blotting paper. This was placed in an open tube leading from the air duct so that there is the same circulation of air over the wet surfaces as over the wet-bulb thermometer placed within. Thermometer No. 4 was rotated while being subjected to the blast, so that the condition in the sling psychrometer would be exactly reproduced.