ASTM E337-02 Test Method for Measuring Humidity with a Psychrometer (the Measurement of Wet- and Dry-Bulb Temperature)

Scope:

In this method humidity of atmospheric air is determined by means of wet bulb temperature and dry bulb temperature readings. This test method is applicable for meteorological measurements at the earth’s surface, for the purpose of the testing of materials, and for the determination of the relative humidity of most standard atmospheres and test atmospheres. This test method is also applicable when the temperature of the wet-bulb only is required. The test is carried out using two types method 

Method A—Psychrometer Ventilated by Aspiration

Method B—Psychrometer Ventilated by Whirling (Sling Psychrometer):

Test procedure:

The wet-bulb temperature depression, the dry-bulb temperature, and the ambient pressure provide the basis for deriving the relative humidity.

Method A—Aspirated Psychrometer:

Establish the airflow and maintain it until a minimum wet-bulb temperature is attained. (With mercury-in-glass thermometers, about 2-min ventilation time is usually necessary.) Read the thermometers with the necessary precision, obtaining the dry-bulb temperature with an overall uncertainty of 60.2ºC or better, and the temperature depression with an overall uncertainty of 60.2ºC or better for uncertainty in the relative humidity of 63 % RH. For an uncertainty in the relative humidity of 62 % RH, obtain the dry-bulb temperature with an overall uncertainty of 60.2°C or better and the temperature depression with an overall uncertainty of 60.1°C or better.

Method B—Sling Psychrometer

Holding the instrument well away from the body, and for outdoor measurements to windward and in the shade, whirl it at such a rate as to achieve the specified airspeed at the wet and dry bulbs. Read the thermometers with the necessary precision, obtaining the dry-bulb temperature with an overall uncertainty of 60.6°C or better, and the temperature depression with an overall uncertainty of 60.3°C or better for uncertainty in the relative humidity of 65 % RH.

Test Specimen:

Thermometers are used for measuring the Wet bulb and dry bulb temperatures.

Data:

Relative humidity from the psychrometric equation,

Where,

e = the partial pressure of water vapor in the atmosphere, Pa,

e_w (T_w) = the saturation pressure of water vapor at the wet-bulb temperature t_w, Pa, 

T = the dry-bulb temperature in °C,

T_w = the wet-bulb temperature in °C,

P = the total (atmospheric) pressure, Pa, 

A = the psychrometer coefficient in K−1