# ASTM E337

Source: https://coatmentor.com/standards/astm-e337/ · Updated 2026-10-14

Measuring humidity with a psychrometer (wet and dry bulb).

**Publisher**: ASTM International

**Area**: Surface preparation and conditions

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](https://coatmentor.com/calculators/dew-point/)

**3** practice items in the members area mention ASTM E337 (CIP 1: 3).

## Related

- **Concepts:** [Dew point and the 3 °C margin](https://coatmentor.com/learn/dew-point/)
- **Standards:** [ISO 8502](https://coatmentor.com/standards/iso-8502/)

## About this page

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](https://coatmentor.com/method/). CoatMentor is an independent study aid. It is not affiliated with ISO, AMPP, SSPC, NACE International, ASTM or FROSIO.