Page:Journal of the American Society of Mechanical Engineers, Volume 33.pdf/676

79The log of these tests shows that the error in the depression of the wet blub in the sling psychrometer for various velocities is as shown in Table 4 and Fig. 10. This error has been taken as directly proportional to the depression. More accurately it is proportional to the difference of the fourth powers of the respective absolute temperatures, except for the stem correction. However, where the depression is the usual small percentage of the absolute temperature, the error in assuming direct proportionality is insignificant.

80The sling psychrometer, however, is subject to another error, heretofore seemingly overlooked. As shown in experiment No. 1, there is a rise in temperature due to the impact of the air upon the bulb, which, in the case of the dry bulb is 66 per cent of the theoretical, or $0.25p$, and in the case of the wet

TABLE 4 AVERAGE RESULTS OF TEST FOR RADIATION ERROR IN WET BULB OF SLING PSYCHROMETER
1	Velocity pressure, in. water	0	.08	0	.16	0	.25	0	.55	0	.75	1	.00
2	Velocity, ft. per min	1160		1640		2050		3040		3550		4100
3	Room temperature No. 2	85	.0	82	.0	85	.1	85	.2	87	.2	87	.25
4	Dry-bulb temperature No. 1	87	.125	83	.93	83	.88	83	.975	88	.125	88	.59
5	Calorimeter temperature No. 3	68	.935	69	.725	71	.158	71	.740	68	.494	68	.830
6	Wet-bulb temperature No. 4	59	.21	70	.01	71	.353	71	.892	68	.654	68	.965
7	Calorimeter depression (difference between items 4 and 5)	18	.19	14	.205	12	.722	12	.285	19	.681	19	.760
8	Difference between wet-bulb and calorimeter temperature	0	.295	0	.285	0	.195	0	.152	0	.160	0	.136
9	Ratio of wet bulb minus calorimeter temperature to calorimeter depression (item 8 ÷ 7)	0	.016	0	.020	0	.015	0	.012	0	.008	0	.007
10	Item 9 corrected for difference in impact	0	.01606	0	.02018	0	.01527	0	.01239	0	.00853	0	.00771
	${\text{Temperature difference due to impact}}=0.3\times 0.047\times {\text{velocity pressure}}$
	${\frac {dt'}{dt}}=0.3{\text{at}}t'=69{\text{to}}70{\text{deg. fahr.}}$
	$0.047={\text{temperature difference (thermometer No. 3 - thermometer No. 4)with dry-bulb }}$${\text{thermometers due to difference in impact at 1 in. velocity pressure.}}$

bulb, $0.25p{\frac {dt'}{dt}}$. This, however, would have no effect upon the calculated absolute humidity, but only on the temperature and consequent relative humidity.

81The type of psychrometer which lends itself to the most accurate determinations is the Aszmann aspiration psychrometer^[1] shown in Fig. 14. Here the air is aspirated through two tubes containing the wet and dry-bulb thermometers. The wet-bulb temperature is brought to a minimum by the use of an atomizer. This serves also to moisten the inner surface of the enveloping tube, thus cooling it and preventing radiation. In this type of psychrometer it should be noted that the impact of the air upon the thermometer bulbs largely neutralizes the reduction in pressure, producing the velocity as demonstrated in experiment No. 1.

1. For full description see Zeitschrift für Instrumentenkunde, January 1892.