Hygrometers and humidity measurement instruments test for absolute humidity, relative humidity, or dew point in air. The range humidity measurement instruments operate in is typically from 0 to 100% humidity. They are sometimes combined with other sensing devices such as temperature sensors.

Measurement Capabilities

Humidity measurement instruments and hygrometers can sense a number of different factors.

Absolute humidity, expressed as grams of water vapor per cubic meter volume of air, is a measure of the actual amount of water vapor or moisture in the air, regardless of the air's temperature.

Relative humidity (RH), expressed as a percent, also measures water vapor, but relative to the temperature of the air.

The dew point temperature, which provides a measure of the actual amount of water vapor in the air, is the temperature to which the air must be cooled in order for that air to be saturated and dew to form.

Because of the intertwining of atmospheric measurements, humidity measurement instruments are sometimes equipped with pressure and temperature sensors as well. Three main applications for humidity measurement instruments are judging moisture in gases or air, bulk solids or powders, or else if fuels or other liquids.

Technology

There are many technologies for humidity measurement instruments.

• Capacitive or dielectric instruments have a material that absorbs moisture, which changes its dielectric properties and enhances its capacitance.
• Chilled mirror technology uses a mirror that is chilled to the point that moisture starts to condense on it. This temperature is the dew point.
• With electrolytic technology, moisture is proportional to the current needed to electrolyze it from a desiccant.
• For resistivity or impedance style sensors, a material absorbs moisture, which changes its resistivity or impedance.
• In strain gage instruments, a material absorbs water, expands and is measured with a strain gage.
• Psychrometers, often called wet/dry bulbs, measure relative humidity by gauging the temperature difference between two thermometers, one wet and one dry.

Range

One critical specification for these devices is the humidity or moisture range to be measured or else the dew point range. Humidity and moisture accuracy is expressed in terms of percentage of measurement. The dew point accuracy, since this is a temperature reading, is expressed as a variance in temperature output.

Output

Outputs from hygrometers can be:

• Analog current
• Voltage
• Frequency
• Digital, including computer signals
• Switch or alarm

They can have analog, digital or video type displays and can have a number of different form factors. They can be PCB-mount devices, standard sensors or transducers, or a simple gauge or indicator. They can also be various types of instruments, whether handheld, benchtop or mounted.

Features

Humidity measurement instruments can have a number of features to make them more useful or easier to use. These can include:

• Pressure and temperature compensation
• Data logging
• Event triggering
• Self-testing
• Self-calibration
• Battery power

Standards

Humidity measurement instruments can be designed, produced, used, and tested according to various standards. Example standards include:

ASTM D4230 - Standard method of humidity measurement with dew-point hygrometer

BS ISO 6327 - Determination of water dew-point of natural gas with cooled surface hygrometers

Image credits:

Buck Research Instruments, LLC | Duncan Instruments Canada, Ltd.