At one end of the spectrum of the moisture world is the classic Speedy Moisture Tester where you use the reaction of water with calcium carbide to cause a mini explosion resulting in a moisture measurement reading. This method is used frequently for soils, concrete, and other like materials.
Then there are instruments that use absorption of electromagnetic frequencies to detect and quantify moisture. These work using frequencies in the near infrared microwave and radio spectra. Pretty sophisticated algorithms are needed to get the moisture measurement job done with some of these. The instruments that do this are above the $12,000 price range.
There is what I consider the "other side of the moisture measurement world". Here we measure how moisture travels between the environment and a material and how it travels between the material and the environment. I call this the "World of Sorption". In fact, there is a water sorption group on LinkedIn devoted to this type of moisture measurement.
The "sorption universe" is involved in measuring how much moisture is yielded into a controlled environment (sometimes referred to as water activity) and how much water is adsorbed from an environment of controlled humidity.
Devices called water activity meters are used to determine this moisture yield characteristic. This is done by putting a sample in a closed chamber and measuring the level of relative humidity created by the moisture leaving the material. Lots of information on the Water Activity Resource Page.
When water activity is generated for varying environmental conditions and at different water content levels, the relationship of moisture content and water activity can be developed and presented as Sorption Isotherms.
To measure the other direction (adsorption of water instead of shedding water), a technique known as Dynamic Vapor Sorption can be used. To give a simplified picture of this technique, imagine that you pass a gas with known moisture content over a sample in a closed environment. The amount of moisture adsorbed by the sample can be determined by weighing the sample at the start and then again when it has become saturated. The difference of before and after gives a measure of the absorbed moisture. This is kind of the opposite of loss on drying (LOD). I've named it Gain on Wetting or (GOW).
Hope this little ramble contained some useful insight. Please share it with associates who you think might enjoy it.
I remain mystified, as usual, by the consequences of looking into what seems like a simple question.
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P.P.S. If you like more information on instruments that measure, check the info on this Moisture Page
By Art Gatenby