Standard · Surface preparation and conditions
ASTM E337
Measuring humidity with a psychrometer (wet and dry bulb).
A psychrometer has two thermometers, one with a wet wick. Air moved over the wick cools it by evaporation. The drier the air, the bigger the difference. The dry-bulb and wet-bulb temperatures give the relative humidity and the dew point, from tables or a calculator.
Two methods
- Method A, aspirated psychrometer: a fan moves the air over the bulbs. It is the more accurate method.
- Method B, sling psychrometer: the inspector whirls the instrument by hand.
From the readings to a decision
The RH and the dew point are only half the check. The inspector also measures the steel with a contact surface thermometer and compares it with the dew point. Most specifications ask for the steel at least 3 °C above the dew point (SSPC-PA 1 states 5 °F), and many for an RH of 85 % or less. ISO 8502-4 asks for the lowest steel reading to be used and says not to use a non-contact (infrared) thermometer.
The CIP Level 1 exam guide names ASTM E337 as its one standard for environmental controls. Electronic hygrometers are a different method; ISO 8502-4 expects both kinds to read within ±3 % RH.
Use it: Dew point calculator
3 practice items in the members area mention ASTM E337 (CIP 1: 3).
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This page describes the subject of ASTM E337 in our own words; it does not reproduce the standard. For the full text, buy the current edition from ASTM International. Checked on 2026-10-14; errors go in the corrections log. CoatMentor is an independent study aid. It is not affiliated with ISO, AMPP, SSPC, NACE International, ASTM or FROSIO.