ASTM E337 is a standard test method for measuring humidity with a Psychrometer. This test method outlines guidelines for constructing a Psychrometer (hygrometer) and the techniques needed to accurately measure atmospheric humidity using wet- and dry-bulb temperature readings.
ASTM E337 Measuring Humidity with a Psychrometer (the Measurement of Wet- and Dry-Bulb Temperatures)
What is ASTM E337?
What is the Scope of ASTM E337?
ASTM E337 provides a test procedure for meteorological measurements in the Earth’s atmosphere, including relative humidity studies under standard and specified test atmospheres. This can also consist of a wet-bulb thermometer only. Relative humidity has no unit of its own; instead, it compares two conditions. In ASTM E337, relative humidity is expressed in the form of RH ± rh %. So the measured values lie somewhere between the upper bound (RH + rh) % and lower bound (RH – r) %, keeping uncertainties to around ~ 95% CL.
ASTM E337 Sample Preparation and Equipment
Here is the step-by-step procedure before performing the test-
- The analyst prepares the Hygrometer and inspects the whirling hygrometer before use.
- The analyst verifies that the wet-bulb thermometer is completely covered with a clean wick.
- The dry-bulb thermometer is kept completely dry.
- The Wet-Bulb Wick is saturated with water, and the analyst fills the instrument’s water reservoir as required.
- The analyst thoroughly saturates the wet-bulb wick with water, then allows several minutes for complete saturation. One should not touch the wick with bare fingers, as oils or dirt can contaminate it.
ASTM E337 Testing Procedure and Requirements
There are two methods in ASTM E337: (a) Psychrometer ventilated by aspiration – A consistent airflow is established and maintained until it attains a minimum wet-bulb temperature. The readings on the thermometer are noted, keeping in mind the relative precision. This test method in ASTM E337 is accurate and much more advantageous than the other method. It involves shielding of thermometer bulbs from accidental damage and extraneous radiation, and is convenient; (b) Psychrometer ventilated by whirling (Sling Psychrometer) – The instrument in this test method is instructed to be kept away from the body, kept in the direction of wind or shade such that a consistent rate of airspeed is attained at the dry and wet bulbs.
The two methods in ASTM E337 do not differ much in their test-atmosphere temperature ranges. While in the aspiration method the temperature range can have a wider domain of 5-80 °C and in the whirling method this reduces to about 5-50 °C. In both methods, the wet-bulb temperature should not be lower than 1°C, and atmospheric pressure ought not to be more than 30 %.
ASTM E337 Result Analysis and Interpretation
The procedure provides three important atmospheric parameters:
Air Temperature → Relative Humidity → Dew Point
Only the values expressed in SI units are considered to be standard. The ASTM E337 test procedure does not include any other units.
Updated on September 15, 2026
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