Dew Point transmitter. Humidity/Temperature transmitter. Relative humidity transmitter.

Image Credit: Clark Solutions | Kobold Instruments | Setra Systems

Humidity transmitters provide electrical outputs that are proportional to humidity inputs.

Variables Measured

They measure variables such as relative humidity, absolute humidity, specific humidity, moisture content, dew point, temperature, and pressure.

• Relative humidity is a measure of the amount of water in the air compared with the amount of water that the air can hold at the measured temperature.
• Absolute humidity is the mass of water vapor in a given volume of air.
• Specific humidity is absolute humidity divided by the total mass of the given volume of air.
• Moisture content or water content is the ratio of water mass to total mass.
• Dew point is the temperature to which air must be cooled in order for saturation to occur.
• Pressure and temperature measurements come from devices that include additional probes.  

Humidity Transmitter Technologies

Humidity transmitters use several technologies.

• Capacitive devices contain a dielectric material whose properties change as humidity is detected and capacitance increases.
• Resistive devices exhibit changes in impedance in response to humidity.
• Humidity transmitters that use chilled mirrors are cooled to a specific temperature, the dew point, at which moisture begins to condense on the surface.
• Electrolytic devices measure moisture proportional to the current needed to electrolyze it from a desiccant.
• Infrared (IR) instruments use the infrared spectrum to measure the absorption of moisture.
• With microwave-based devices, the amplitude and phase of a microwave signal is affected directly by the amount of moisture in a material.
• Psychrometers determine relative humidity by measuring the temperature difference between two thermometers, one wet and one dry.
• Strain gauges or piezoresistive elements measure the expansion of materials that absorb water.
• Nuclear humidity transmitters measure how neutrons are slowed by the presence of moisture in a material.

Specifications

Selecting humidity transmitters requires an analysis of performance specifications and features.

Image Credit: Care Processing Instruments

Performance Specifications

Performance specifications include:

• Operating temperature is the temperature range in which devices are designed to operate.
• Relative humidity range and dew point range are ambient ranges that can be measured with a linear response.
• Relative humidity accuracy and dew point accuracy are percentages that indicate the closeness of a computed amount to its true value.

Compensation

Some humidity transmitters are temperature-compensated and maintain constant output readings across a range of temperatures. Other devices are pressure-compensated.

Features

Features for humidity transmitters include:

• Self-calibrating devices will recalibrate themselves between uses.  
• Self-testing and diagnostic devices notify users when an internal fault condition is detected.
• Data logging devices are capable of logging and storing data.  
• Event-triggered humidity transmitters can be configured to capture or log data when an event occurs.
• Battery-powered devices use batteries for full operation, not just backup.

Output and Displays

Humidity transmitters differ in terms of display type, electrical outputs, and digital outputs.

There are three basic types of displays.

• Analog meters or gauges use needles or light emitting diode (LED) indicators.
• Digital readouts are numerical or application-specific displays.
• Video display terminals (VDT) include cathode ray tubes (CRT) and flat panel displays (FPD).

Outputs

Electrical outputs for humidity transmitters include analog currents, analog voltages, frequencies or pulses, and switches or relays. Digital outputs include Ethernet, modems, parallel interfaces, serial interfaces, small computer systems interface (SCSI), transistor-transistor logic (TTL), and universal serial bus (USB).

Standards and Specification for humidity transmitters can be found at the IHS Standards Store.