Moisture Analyser / Moisture Analyzer

Values for the moisture or water content can be given either in absolute terms in grams or litres or in relative terms as a percentage of the mass or volume. In the case of the relative values, it is important to note whether the initial value for the percentage is the moist or the dry substance. The higher the water content is, the greater the figures for different reference values deviate from each other.

Examples of absolute and relative moisture data

If the moisture is determined by drying samples, both the absolute moisture and the relative moisture data can be determined using the formulae shown in blue in the table. With some drying balances used as moisture analysers, it is not selectable whether the value of the relative humidity is related to the dry or to the wet condition. However, both values can easily be calculated from the wet and dry mass.

Special feature for gaseous substances
For the gaseous substances, the amount of water vapour that can be absorbed depends on the temperature and also on the pressure. For gases, the relative humidity is therefore not given in relation to the mass or the volume, but to the saturation quantity of water vapour that can be achieved at the current temperature and pressure conditions. If the temperature and pressure values are known, the absolute humidity can also be determined from the relative humidity of gases or gas mixtures.

Many materials contain water under normal ambient conditions and can also absorb certain amounts of moisture and release it back into the environment more or less quickly. Since the water content plays an important role in many physical, chemical and biological processes and thus determines the shelf life and processability of the substances, it is often monitored with the help of the electronic moisture analyzer. Compared to the kiln-drying method, the electronic moisture analyzers have the advantage that they can be used for on-site, very fast and often non-destructive or low-destructive measurements. The various types differ, among other things, in terms of the measuring principle. In most cases, the measuring principle used has a decisive influence on the suitability for certain applications.

Because of the importance of the material moisture for the usability and durability of materials, a variety of measuring instruments have been developed for different tasks and conditions of use. Before deciding on the purchase of a certain type of device, it should be determined which requirements the moisture analyzer must fulfil. This includes, for example, clarifying whether the device must be calibrated for the intended application.

Here is a list of important criteria that should be considered:

- Verified or unverified
- Accuracy for the intended materials
- Adjustment possibility
- Measuring principle and its limitations
- Measuring time per reading
- Measurement on site or in the laboratory
- Examination of samples or on undisturbed material
- Non - destructive/low-destructive measurement
- Repeatability of the measurement
- Continuous measurement on moving material
- Adaptation to specific materials and environmental conditions
- Calculation functions
- Additional measurement functions
- Handling of the instrument
- Storage of the measurement data
- Interfaces for the data transmission
- Available accessories

For the use in changing locations, for different types of materials or for materials whose exact composition is not known, it is often advisable to use several moisture analyzers with different measuring principles or a combined device with several measuring principles. For example, in the case of moisture damage to buildings or stored goods, the spread of moisture penetration can be determined with the device working according to the resistance method. Finally, the samples can be taken from representative locations and their moisture content can be determined with the moisture analyzer balances.

In the following sections, the advantages and limitations of the most commonly used methods of the moisture analysis are briefly explained in order to show their suitability and also their limitations for certain applications.

Moisture balances
Moisture balances simplify the performance of the kiln-drying method, but are often limited to very small sample quantities. These moisture analyzers combine precision balances and a drying chamber in one device. The samples of the material to be tested are dried until there is no more weight loss. The difference in weight corresponds to the proportion of the evaporated water for most materials and if carried out correctly.

Since the sample quantity in moisture balances is usually very small, the drying time is shortened in most cases compared to the drying in a kiln. In addition, the weighing is automatic and various drying programmes can be selected that run automatically after starting. The devices usually not only determine the initial and final weight, they also calculate the moisture content of the sample under study.

Capacitive or dielectric moisture analysers
Materials that can absorb moisture change their properties in the electric field depending on the moisture content. Moisture analyzers that determine the capacitance work quickly and non-destructively. The measurements can be repeated as often as required at the same or neighbouring point. Measurement depth and results are influenced by both the type and structure of the material. They can be altered by foreign materials as well as by salts embedded in the measuring area. For known materials and defined environmental conditions, reliable measurement results are possible. For other conditions, these devices are good for quick determination and isolation of the areas with increased moisture.

Moisture analysers with resistance measurement
As the moisture content increases or decreases, so does the electrical resistance or conductivity. However, since these electrical values are also influenced by temperature and material composition, some important points must be taken into account when using this method. The influence due to temperature differences can be compensated for some models via sensors, special components or manual settings. Stored reference lines for materials or material groups allow a higher accuracy for the selectable materials. However, only a few moisture analyzers with resistance measurement offer the option of storing their own reference lines.

Moisture analyzers with resistance or conductivity measurement offer fast results during the non-destructive measurement. The values allow the moisture deviations to be detected and their extent to be narrowed down. Partially insulated probes are suitable for determining the values at different depths. This allows conclusions to be drawn as to whether there is more moisture inside than outside. This facilitates the assessment of the drying out behaviour and the causes of the moisture.

Corresponding humidity (KRL method) for moisture determination
The moisture content of many materials depends on the temperature and humidity of the environment. If the material can absorb moisture from the air and release it back into the air, a certain material moisture will be established after some time as a compensating moisture for a certain ambient climate. This means that moist materials release moisture in a dry environment and, conversely, dry materials absorb moisture in a moist environment. With the measurements according to the KRL method, it can be determined whether moist materials have already reached their compensation moisture and whether materials or components show increased moisture values due to the influences other than the ambient climate. The measurement is repeatable, so that the statements on the course of drying or moisture absorption are possible.

The determination of the corresponding air humidity can be carried out with the help of material samples, in bulk materials, as well as in boreholes or on the material surface. The samples are placed with the sensor in airtight containers or foil bags. During the measurements in boreholes or on surfaces, air exchange must also be prevented by airtight masking. The German Adhesives Industry Association (Industrieverband Klebstoffe e.V.) describes in TKB Report 2 "The KRL Method for Determining the Moisture in Screeds" the procedure for taking samples of moist screeds analogous to the calcium carbide method. In the case of screeds, it is mandatory to determine the residual moisture before tiling or flooring work to prevent damage to the flooring due to subsequent moisture equalisation.

If the moisture values of the material are above those of the characteristic compensation moisture, the relative humidity of the enclosed air increases until the material no longer releases any moisture into it. If, on the other hand, the material is drier than usual for the prevailing ambient conditions, it absorbs moisture from the enclosed air and its humidity decreases.

Moisture analysers with microwave technology
Microwaves of certain wavelengths are influenced by water molecules and vice versa. They can be used not only to heat food containing water, but also for targeted moisture analysis. The microwaves that hit water molecules are slowed down, damped and partly reflected. These changes can be measured. The water content of electrically non-conductive substances can thus be determined precisely, quickly and non-destructively. Depending on the number of the water molecules in the examined substance, the resonance frequency, transmittance and reflection of the microwave radiation become higher or lower. However, the strength of the change is not only dependent on the water content, but also on other influences. Therefore, the moisture analyzer that works on the basis of the microwave technology must be adjusted to the material and the concrete measuring conditions by the reference measurements.

In general, only low microwave power is required for the measurement, so that the material is hardly heated or influenced in any other way by the measurement. The measurement can be repeated as often as desired at the same position, provides very fast results and also works with moving material. This is why the moisture analysers with microwave technology are often used for inline moisture measurements in manufacturing, drying and storage processes. For sensitive products, very short measuring intervals in milliseconds and accuracies of 0.1 moisture percent are possible. Analogue or switching interfaces can be used to ensure that the countermeasures are taken very quickly in the event of moisture deviations.