Moisture meters measure the moisture content in bulk solids, liquids and gases. Some devices use infrared (IR), radio frequency (RF), or microwave techniques to evaporate water from a sample. Others determine moisture content by measuring a material’s conductance, resistance, or capacitance and calculating a corresponding moisture value. Variables measured include relative humidity, absolute humidity, specific humidity, and dew point. Relative humidity is a measure of the amount of water in the air compared with the amount of water the air can hold at a measured temperature. By contrast, absolute humidity is the mass of water vapor in a given volume of air. Specific humidity is the absolute humidity divided by the total mass of the given volume of air. Dew point is a measure of the temperature to which air needs to be cooled in order for saturation to occur. Moisture meters that measure pressure and temperature are commonly available. These specialized devices often include special circuitry or additional probes. 

Form Factors and Other Things to Consider

   
There are several form factors for moisture meters. Many require a secondary device or instrument for data acquisition or the transmission of humidity information. Integrated circuit (IC) chips mount on printed circuit boards (PCB). Benchtop or floor-standing moisture meters include a full casing or cabinet and an integral interface. Handheld units include an integral probe and are designed to be operated while held in one hand. Gauges mount on walls, well, racks, or DIN rails and include an integral display. Both analog and digital devices are available; however, simple light emitting diode (LED) power indicators are not considered to be a display. Video display terminal (VDT) styles include cathode ray tube (CRT) monitors and flat panel display (FPD). Mounted transmitters do not include a display, but provide an integral sensor or probe.  

Video credit: Flir Commercial Systems / CC BY-SA 4.0

Specifications

  
Selecting moisture meters requires an analysis of performance specifications and output options. Humidity range and dew point range are usually expressed as linear outputs. Humidity accuracy and dew point accuracy measure the closeness of a measured or computed value to its true value. Analog current outputs include variable levels such as 0 – 20 mA or 4 – 2 mA. Common analog voltage outputs are 0 – 10 V and ±5 V. Moisture meters with frequency or pulse outputs use amplitude modulation (AM), frequency modulation (FM) or pulse width modulation (PWM). With switch-type outputs, contacts are open or closed depending on the state of the variable being monitored.

Related Standards

• ASTM D4444 - STANDARD TEST METHOD FOR LABORATORY STANDARDIZATION AND CALIBRATION OF HAND-HELD MOISTURE METERS
• BS ISO 24115 - GREEN COFFEE - PROCEDURE FOR CALIBRATION OF MOISTURE METERS - ROUTINE METHOD
• GOST 29027 - MOISTURE METERS FOR SOLID AND DRY SUBSTANCES. SPECIFICATIONS AND TESTING METHODS