From ISA Handbook of Measurement Equations and Tables, 2nd Edition

Introduction

Popular devices for humidity measurement include the hygrometer, a device which measures only relative humidity; thermohygrometer, a device which measures both temperature and humidity; psychometer, which measures humidity and dew point through water evaporation rate interpretation; and dew point meter, which measures the temperature at which moisture will form in the sampled environment. In process control, moisture and temperature often need to be measured in combination.

Thermohygrometers are available in wall mount, dial meter type units that do not require electrical power and digital models. Battery operated digital units are popular. Dial meter type thermohygrometers generally use a "cellulose" sponge type sensor for humidity and a spring or glass bulb thermometer for temperature. As moisture increases, the sponge expands and the lever mechanism moves the indicating needle. Accuracies are typically in the +/- 3% range, and response time is slow. Electronic thermohygrometers generally use either a capacitance or resistance sensor. As the humidity rises, the circuit resistance or capacitance changes a digital display reading.

When portability is needed, a psychometer is often used. It typically has two thermometers-a normal "dry" bulb thermometer, plus another called the "wet" bulb, featuring a wick moistened with water. As air passes over the two thermometers, two temperatures (wet and dry bulb) are generated. Using a table, the humidity can be calculated.

Principles of Humidity and Moisture Measurement

Measurement Factor

Measurement Description

Units of Measure

Wet Bulb Thermometer

The temperature of a wetted thermometer in a stream of air.

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Products & Services
Dial Thermometers
Dial thermometers are bimetal or bi-metallic, liquid or gas-filled, and vapor-tension-based. These devices display temperatures in Fahrenheit, Celsius, or both.
Humidity Transmitters
Humidity transmitters provide electrical outputs that are proportional to humidity inputs. They measure variables such as relative humidity, absolute humidity, specific humidity, moisture content, dew point, temperature, and pressure.
Industrial Thermometers
Industrial thermometers are basic bulb style thermometers designed for use in many industrial applications. Traditional liquid-in-glass design is typically used due to low cost and ease of use.
Moisture Meters
Moisture meters are used to measure the moisture content in bulk solids, liquids and gases.

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