F.A.Q.

Capacitive moisture measurement indirectly determines the water content of bulk solids based on their electrical properties. It is particularly suitable for continuous online measurement during ongoing processes.

The measurement method is based on a capacitor. If the moisture content of the material in front of the sensor changes, its relative permittivity also changes. With a value of around 80, water has a significantly higher permittivity than dry bulk solids, which typically have values between 2 and 10. This difference is directly reflected in the measured capacitance.

The measurement is carried out using a stray field capacitor behind a robust ceramic measuring surface. After material-specific calibration, the sensor delivers a stable moisture signal in real time. The method is suitable for many non-conductive bulk solids. Highly conductive or very wet materials, on the other hand, can only be measured to a limited extent.

You can find more detailed information in our blog article: How a capacitive moisture measurement works

Capacitive online moisture measurement is suitable for many non-conductive bulk solids and is primarily used where moisture needs to be continuously monitored during the ongoing process. Typical applications include biomass, sawdust, mineral raw materials, gypsum, clay, salt, animal feed, foodstuffs, powders, granulates and numerous recycled materials.
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The method is particularly reliable with homogeneous material structure, uniform grain size and stable density. Constant conveying and compaction conditions at the sensor are equally important, as these have a direct influence on the measurement signal.

Restrictions arise primarily with very coarse or highly inhomogeneous materials and with highly conductive substances. High process temperatures above approximately 100 °C and highly variable compaction conditions can also impair measurement stability and must be taken into account when designing the measuring system.

DMMS, BMMS and MMS are all based on the capacitive measuring principle, but differ in terms of construction, range of functions and type of signal output.

DMMS is a modular and highly flexible system with a central evaluation unit to which several sensors (up to 16) can be connected and managed. It offers a wide range of options for transmitting measured values.

BMMS is compact, operates without an external evaluation unit and delivers the analogue signal directly from the sensor. It is ideal for individual measuring points.

MMS is a purely analogue system in which calibration is performed via potentiometers. It is suitable for simple applications with fixed process conditions.

For precise and reproducible moisture measurement, the conditions under which the bulk material passes the measuring point are crucial. A uniform bulk height, stable compaction and the most homogeneous grain size possible form the basis for reliable measured values.

The selected installation location has a decisive influence on these conditions. Uneven conveying, highly variable grain distributions, material jams or adhesions lead to fluctuations that cannot be compensated for by calibration. Many measurement problems therefore arise not from the measurement system itself, but from unstable process conditions.

The sensor should therefore be installed at a physically sensible and representative location where the material flow is as consistent as possible – not necessarily at the most obvious position in the plant.