“How Many Ways are there to Measure Moisture?”
That is a recurrent question in early discussions about moisture measurement problems. Is frequently asked in connection with specific products and applications. This is a basic primer on the main techniques.
Most Used Methods to Measure Moisture Content
There are four core methods. They include the use of heat, chemicals, electrical properties and electrro magnetic phenomena.
Weight Loss on Drying (LOD)
The earliest and still most widely used method is weight loss on drying. In this techniqu
e, the sample weight at the start is recorded. After drying the sample weight is recorded. The difference in these weights represent the moisture lost. People use many combinations of equipment to get at th
is widely performed test. Some testers use a balance and a hot plate, others a balance and an oven. Today technology has made it easy to use an integrated balance and drying method to automate the procedure. The technique is often referred to as Loss-on Drying or LOD.
Correlate Electrical or Dielectric Changes to Moisture Levels
Another system for moisture detection is based on the fact that many materials change electrical or
dielectric characteristics depending the moisture level in the material. Most of these instruments measure changes in resistance , conductivity or capacitance. Because these techniques are measuring an indirect affect of moisture, calibration is necessary. The calibration isaccomplished by comparing the dielectric reading to a known moisture for the sample. A graph is prepared and used to provide the translation of the electrical characteristic to the amount of moisture in the material sample. In automated instruments these tables are processed by a computer to give readings in % moisture.
Karl Fischer Titration
A third method is based on a chemical reaction and is known a Karl Fischer Titration
. This is named after the chemist who developed the chemical/reagent. The reagent reacts with water to eliminate the conductivity
that was developed by the water in the test sample. This test is accomplished by injecting the material into a solvent and then adding the Karl Fischer Reagent. The reagent is added until the reaction has converted all the water into a non conductive chemical. The amount of reagent used is measured and converted into units of water. Karl Fischer techniques are advantageous for measuring small amounts of water, for samples that contain volatiles that degrade the accuracy of weight loss on drying techniques and for releasing bound water in some materials.
Optical and Electro-Magnetic Energy
Application of the concepts of reflection and absorption of electromagnetic energy is becoming widely used, particularly for in-line or on-line measurement. These techniques based on the finding that water (as well as other chemicals) has very specific absorption wavelengths. Near Infrared (NIR) and Microwave technology are the most commonly used. Although the techniques of NIR and microwave are different, they are both based on the concept of absorption of electromagnetic energy. In these tests the absorption of the wave lengths associated with water are compared to a reference wave length, When the moisture of a calibration sample is known the absorption results can be referenced to moisture. The use of lasers for this purpose is a newer related technique.
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There are several other techniques that involve creating a chemical reaction with water to develop pressure (speedy Moisture Tester is a prime example) or super heating the sample and measuring the volume/weight to the water that was driven off.
I hope this brief visit to the world of moisture measurement was useful. Please feel free to share it with associates who have a need to test for moisture.
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P.P.S. Check out one of the premier LOD moisture Balances.