DETERMINING RELATIVE HUMIDITY

WITH A SLING PSYCHROMETER

GENERAL

This method describes how to determine the relative humidity with a sling psychrometer. A psychrometer measures the ambient air temperature (dry bulb temperature) and the wet bulb temperature. From these temperature measurements, the percent relative humidity is determined.

APPARATUS

1.   Sling psychrometer

PROCEDURE

1. Before using, take the end cap off the psychrometer and fill the reservoir with water. Be sure that the wick is saturated with water and covers the mercury reservoir on the wet bulb thermometer. Also, be sure that the mercury reservoir on the dry bulb thermometer is dry.

2. Pull the tube clear of the body so that the body can swivel.

3. Hold the tube and whirl the body two to three revolutions per second.

4. Continue whirling until the temperatures stabilize (should take 1-2 minutes).

5. Stop whirling and immediately read temperature of the wet bulb thermometer and then the dry thermometer.

6. Figure the temperature difference between the two thermometers.

7. Use the attached table to determine the relative humidity.

EXAMPLE

After following the above procedure, a dry bulb temperature of 86°F and a wet bulb temperature of 71°F were obtained. Thus, the temperature difference is 15°F.

From the “Relative Humidity” table, locate the column at the top headed with 15. Follow this column down to the row headed with dry bulb temperature of 86°F. The intersection of the column and the row gives the number 47. This number is the percent relative humidity.

REPORTING RESULTS

Record the time, date, dry bulb temperature, wet bulb temperature, and relative humidity.

Table 1 – Relative Humidity, % Difference Between Readings of Wet & Dry Bulbs in °F
Dry Bulb °F	1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	16	17	18	19	20	21	22	23	24	25	26	27	28	29	30	Dry Bulb °F
40 41 42 43 44	92 92 92 92 93	83 84 85 85 85	75 76 77 77 78	68 69 69 70 71	60 61 62 63 63	52 54 55 55 56	45 46 47 48 49	37 39 40 42 43	29 31 33 35 36	22 24 26 28 30	15 17 19 21 23	7 10 12 14 16	0   3   5   8 10																		40 41 42 43 44
45 46 47 48 49	93 93 93 93 93	86 86 86 86 86	78 79 79 79 80	71 72 72 73 73	64 65 66 66 67	57 58 59 60 61	51 52 53 54 54	44 45 46 47 48	38 39 40 41 42	31 32 34 35 36	25 26 28 29 31	18 20 22 23 25	12 14 16 18 19	6   8 10 12 14	2   5   7   9	1   3															45 46 47 48 49
50 51 52 53 54	93 94 94 94 94	87 87 87 87 88	80 81 81 81 82	74 75 75 75 76	67 68 69 69 70	61 62 63 63 64	55 56 57 58 59	49 50 51 52 53	43 45 46 47 48	38 39 40 41 42	32 34 35 36 37	27 28 29 31 32	21 23 24 26 27	16 17 19 20 22	10 12 14 16 17	5   7   9 10 12	0   2   4   6   8	1   3													50 51 52 53 54
55 56 57 58 59	94 94 94 94 94	88 88 88 88 89	82 82 82 83 83	76 76 77 77 78	70 71 71 72 72	65 65 66 66 67	59 60 61 61 62	54 55 55 56 57	49 50 50 51 52	43 44 45 46 47	38 39 40 41 42	33 34 35 37 38	28 30 31 32 33	23 25 26 27 29	19 20 22 23 24	14 16 17 18 20	9 11 13 14 16	5   7   8 10 11	0   2   4   6   7	0   1   3											55 56 57 58 59
60 61 62 63 64	94 94 94 95 95	89 89 89 89 90	83 84 84 84 84	78 78 79 79 79	73 73 74 74 74	68 68 69 69 70	63 63 64 64 65	58 58 59 60 60	53 54 54 55 56	48 49 50 50 51	43 44 45 46 47	39 40 41 42 43	34 35 36 37 38	30 31 32 33 34	26 27 28 29 30	21 22 24 25 26	17 18 20 21 22	13 14 16 17 18	9 10 12 13 15	5   7   8 10 11	1   3   4   6   7	1   2   4	0								60 61 62 63 61
65 66 67 68 69	95 95 95 95 95	90 90 90 90 90	85 85 85 85 85	80 80 80 80 81	75 75 75 76 76	70 71 71 71 72	66 66 66 67 67	61 61 62 62 63	56 57 58 58 59	52 53 53 54 55	48 48 49 50 51	44 44 45 46 47	39 40 41 42 43	35 36 37 38 39	31 32 33 34 35	27 29 30 31 32	24 25 26 27 28	20 21 22 23 24	16 17 19 20 21	12 14 15 16 18	9 10 12 13 14	5   7   8 10 11	2   3   5   6   8	0   2   3   5	1						65 66 67 68 69
70 71 72 73 74	95 95 95 95 95	90 90 91 91 91	86 86 86 86 86	81 81 82 82 82	77 77 77 78 78	72 72 73 73 74	68 68 69 69 69	64 64 65 65 65	59 60 61 61 61	55 56 57 57 58	51 52 53 53 54	48 48 49 50 50	44 45 45 46 47	40 41 42 42 43	36 37 38 39 39	33 33 34 35 36	29 30 31 32 33	25 27 28 29 29	22 23 24 25 26	19 20 21 22 23	15 17 18 19 20	12 13 15 16 17	9 10 12 13 14	6   7   9 10 11	3   4   6   7   8	1   3   4   5	1   3				70 71 72 73 74
75 76 77 78 79	96 96 96 96 96	91 91 91 91 91	86 87 87 87 87	82 82 83 83 83	78 78 79 79 79	74 74 74 75 75	70 70 71 71 71	66 66 67 67 68	62 62 63 63 64	58 59 59 60 60	54 55 56 56 57	51 51 52 53 53	47 48 48 49 50	44 44 45 46 46	40 41 42 43 43	37 38 39 39 40	34 34 35 36 37	30 31 32 33 34	27 28 29 30 31	24 25 26 27 28	21 22 23 24 25	18 19 20 21 22	15 16 17 18 19	12 13 14 16 17	9 11 12 13 14	7   8   9 10 11	4   5   6   8   9	1   3   4   5   6			75 76 77 78 79
80 82 84 86 88	96 96 96 96 96	91 92 92 92 92	87 88 88 88 88	83 84 84 84 85	79 80 80 81 81	75 76 76 77 77	72 72 73 73 74	68 69 69 70 70	64 65 66 66 67	61 61 62 63 64	57 58 59 60 61	54 55 56 57 57	50 51 52 53 54	47 48 49 50 51	44 45 46 47 48	41 42 43 44 46	38 39 40 42 43	35 36 37 39 40	32 33 35 36 37	29 30 32 33 35	26 28 29 31 32	23 25 26 28 30	20 22 24 26 27	18 20 21 23 25	15 17 19 21 22	12 14 16 18 20	10 12 14 16 18	7 10 12 14 15	5   7   9 11 13	3   5   7   9 11	80 82 84 86 88
90 92 94 96 98	96 96 96 96 96	92 92 93 93 93	89 89 89 89 89	85 85 85 86 86	81 82 82 82 83	78 78 79 79 79	74 75 75 76 76	71 72 72 73 73	68 68 69 69 70	65 65 66 66 67	61 62 63 63 64	58 59 60 61 61	55 56 57 58 58	52 53 54 55 56	49 50 51 52 53	47 48 49 50 50	44 45 46 47 48	41 42 43 44 45	39 40 41 42 43	36 37 38 39 40	34 35 36 37 38	31 32 33 35 36	29 30 31 32 34	26 28 29 30 32	24 25 27 28 29	22 23 24 26 27	19 21 22 24 25	17 19 20 22 23	15 17 18 20 21	13 15 16 18 19	90 92 94 96 98
100 102 104 106 108	96 96 97 97 97	93 93 93 93 93	89 90 90 90 90	86 86 87 87 87	83 83 83 84 84	80 80 80 81 81	77 77 77 78 78	73 74 74 75 75	70 71 71 72 72	68 68 69 69 70	65 65 66 66 67	62 62 63 64 64	59 60 60 61 62	56 57 58 58 59	54 55 55 56 57	51 52 53 53 54	49 49 50 51 52	46 47 48 49 49	44 45 46 46 47	41 42 43 44 45	39 40 41 42 43	37 38 39 40 41	35 36 37 38 39	33 34 35 36 37	30 32 33 34 35	28 30 31 32 33	26 28 29 30 31	24 26 27 28 29	22 24 25 26 27	21 22 23 24 25	100 102 104 106 108
110 112 114 116 118	97 97 97 97 97	93 94 94 94 94	90 90 91 91 91	87 87 88 88 88	84 84 85 85 85	81 81 82 82 82	78 79 79 79 79	75 76 76 76 77	73 73 74 74 74	70 70 71 71 72	67 68 68 69 69	65 65 66 66 67	62 63 63 64 64	60 60 61 61 62	57 58 58 59 59	55 55 56 57 57	52 53 54 54 55	50 51 52 52 53	48 49 49 50 51	46 47 47 48 49	44 44 45 46 47	42 42 43 44 45	40 40 41 42 43	38 38 39 40 41	36 36 37 38 39	34 35 35 36 37	32 33 34 34 35	30 31 32 33 34	28 29 30 31 32	26 27 28 29 30	110 112 114 116 118
120	97	94	91	88	85	82	80	77	74	72	69	67	65	62	60	58	55	53	51	49	47	45	43	41	40	38	36	34	33	31	120