Frequently Asked Questions

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.

Yes. Every moisture measurement must be calibrated to the respective material and the specific process conditions. It is not possible to determine the water content directly. Instead, a physical parameter is recorded from which the moisture value is derived.

At ACO, moisture measurement is based on the capacitive measuring principle. The sensor records an electrical parameter that is influenced by the material properties, in particular the permittivity. The moisture content is calculated for each specific material based on this correlation.

Precisely because calibration is so important, stable conditions are crucial: the method works particularly reliably with a homogeneous material structure and uniform grain size. Constant conveying and compaction conditions at the sensor are equally important, as these have a direct influence on the measurement signal.

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.

A DMMS is the right choice for complex plants with multiple measuring points, variable materials and the need for digital communication and a high degree of flexibility.

When suitable:
• For plants with multiple measuring points
• For different materials or changing recipes
• When measurement values need to be transmitted digitally to the plant control system
• When averaging or central data evaluation is required
• In large-scale plants with greater distances between measuring points

Characteristics:
• Modular, digital measurement system
• Central evaluation unit with connection for up to 16 sensors
• Digital interfaces such as Profibus DP or Profinet, optionally analog standard signals
• Material-specific characteristic curves per sensor
• Support for continuous measurement and batch processes
• Online calibration in the installed condition

Options and variants:
• ATEX approval for hazardous areas (Zone 21 or 22)
• Sensor variants for silo and mixer applications
• Sensor housings made of acid-resistant stainless steel
• Extended temperature ranges up to 100 °C

Further information on the DMMS can be found HERE!

A BMMS sensor is the right choice for individual measuring points or compact plants where a robust, space-saving moisture measurement with direct signal output is required.

When suitable:
• For individual measuring points without an external evaluation unit
• In compact plants with limited installation space
• When analog signals need to be transmitted directly to the control system
• Under stable process conditions with manageable material changes
• When simple integration with minimal wiring effort is desired

Characteristics:
• Compact single sensor without an external evaluation unit
• Direct analog signal output (4–20 mA), optional RS485
• Material-specific characteristic curves with switching of up to 8 materials
• Suitable for continuous measurement and batch processes
• In-process calibration

Options and variants:
• Integrated temperature sensor
• Sensor variants for silo or mixer applications
• Sensor housings made of acid-resistant stainless steel
• Different measuring surfaces, e.g. Teflon or rubber

Further information on the BMMS sensor can be found HERE!

An MMS moisture sensor is suitable for simple and robust single-point measurements where stable material conditions are present and a direct analog signal output from the sensor is sufficient.

When suitable:
• For non-critical applications with constant material and process conditions
• For individual measuring points without digital communication
• When analog output signals need to be transmitted directly to the control system
• With manageable requirements for flexibility and accuracy
• In simple plants such as conveyor belts, mixing systems or chute measurements

Characteristics:
• Compact single sensor without an external evaluation unit
• Direct analog signal output (4–20 mA, 0–20 mA or 0–10 V)
• Local calibration at the sensor via rotary potentiometer
• Capacitive measuring principle for stable measurement values under constant conditions

Options and variants:
• Integrated temperature sensor (Pt100)
• Sensor variants for silo or mixer applications
• Different measuring surfaces, e.g. Teflon or rubber
• Sensor housings made of acid-resistant stainless steel

Not suitable:
• For frequent material changes
• For strongly varying installation conditions
• For high requirements regarding measurement accuracy or process flexibility

Further information on the MMS sensor can be found HERE!

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.

Moisture sensors are typically installed in locations where the material is fed in a controlled way and stable process conditions exist.

Typical installation locations include conveyor belts, chutes and hoppers, funnels and storage containers, material discharge points, material bunkers, mixers, silos, pipes and screw conveyors.

Locations with highly fluctuating material quantities, unstable material flow or uncontrolled rearrangements are less suitable, as these can impair measurement stability.

Various installation concepts are available for mechanical integration, such as slide solutions, flange mountings or pipe installations. Depending on the respective installation situation, our installation kits such as SFM1_HACK, SFM4 or PN1 are used, among others.