Material moisture
The material moisture (also solid moisture ) indicates the amount of free water contained in a solid .
General
The uptake of water is referred to as absorption or adsorption and the release as desorption . The two processes always occur simultaneously, the totality of the process is called sorption . Each material has very characteristic properties in this regard; so-called sorption isotherms are used to represent these .
Types of binding of the water in the solid
The water can be bound in the solid in different ways :
- Chemically bound water (also crystal water ) - water built into the structure of the substance (e.g. gypsum )
- Adsorption water - water attached to the phase interfaces
- Adhesion water - water adhering to the surface of polar macromolecules
- Capillary water - water in capillaries, cracks, wedges, pores
- Interstitial capillary water - cell water
- Dripping water
The chemically bound water is generally not of interest for the material moisture, as it does not count as free water . If chemically bound water is expelled from the material, this is associated with the destruction of the material.
Chemically bound water or the water resulting from the oxidation of hydrogen atoms is of interest if the material is used for combustion and the resulting water vapor affects the calorific value .
Humidity values
If the anhydrous mass is used as a reference value, the humidity can be more than 100% (for example in the case of wood moisture ), if the moist mass is used as a reference value, the humidity is less than 100%. This can lead to confusion when making comparisons.
The material moisture can be represented by various parameters. The most important are listed below:
- Moisture content or water content
- is the ratio of the mass of the water contained in the substance to the mass of the anhydrous substance
Formula:
- Moisture content or water content
- is the ratio of the mass of the water contained in the substance to the total mass of the substance
Formula:
- Volume-related moisture content
- is the ratio of the volume of the water contained in the substance to the volume of the anhydrous substance
Formula:
- Volume-related moisture content
- is the ratio of the volume of the water contained in the substance to the total volume of the substance
Formula:
- Dry matter content
- is the ratio of dry matter to total weight
Formula:
- Absolute dryness
With
- - mass (physics) of water
- - Mass (physics) of the anhydrous substance
- - Total mass (physics) of the sample
- - volume of water
- - Volume of the anhydrous substance
- - total volume of the sample
- - density of the sample
- - Density of the anhydrous sample
Measurement method
The material moisture can be determined with many moisture measuring methods. The most important are listed below:
Direct measurement methods:
- Gravimetric method (Darr method absolute moisture analyzer )
- Calcium carbide method (analytical)
- Karl Fischer method
Indirect measurement methods:
- Capacitive sensor
- Conductivity measurement method
- Microwave measuring method
- Infrared reflection / absorption
- Humidity compensation process
- Time domain reflectometry (wave propagation speed)
- Tensiometer (soil moisture) (capillary forces in the soil)
See also
swell
- Klaus Kupfer: Material moisture measurement - basics, measuring methods, applications, standards. Renningen-Malmsheim: expert-Verlag, 1997, ISBN 3-8169-1359-8 , ( contact & study 513).