We will be happy to advise you: +43 7245 / 22 001 13 |
Material moisture measurement in drying processes: requirements, sensor types, installation practice
Comparison of measurement methods: from NIR/SWIR to microwave and capacitive measurement.
Drying processes are among the most energy-intensive process steps in many industries. If drying is too long or too intensive, energy consumption increases unnecessarily; if drying is too insufficient, residual moisture can lead to quality problems. Inline moisture measurement provides the data basis for continuously and precisely controlling drying processes – instead of relying on random samples or fixed drying times.
This article provides an overview of common measurement methods for inline moisture measurement, assigns them to the most important dryer types, and shows typical application areas.
Table of Contents
Why is inline moisture measurement so important in drying processes?
Continuous drying processes are rarely completely stable: fluctuations in initial moisture content, material density, throughput, or ambient temperature constantly change how much drying energy is actually required.
Without continuous measurement, drying is usually conservatively oversized to reliably avoid excessive residual moisture – this costs energy and reduces throughput. Inline moisture measurement makes the actual residual moisture visible at the dryer outlet or at intermediate points and enables automated, demand-based control of temperature, residence time, or energy input.
Measurement methods for inline moisture measurement
NIR/SWIR measurement
How it works:
Evaluation of the material-specific absorption of near-infrared or short-wave infrared light, particularly pronounced in water in the range around 1,5 µm. (Explained in detail in our article "What is an NIR sensor?".)
Advantages: Non-contact, fast response time, well suited for near-surface measurement on moving material.
Disadvantages: Only suitable for near-surface measurements, sensitive to surface texture and heavy dust formation.
Lorem ipsum dolor sit amet, consectetur adipiscing elit. Ut elit elit, luctus nec ullamcorper mattis, dapibus leo pulvinar.
Microwave measurement
How it works:
Electromagnetic waves penetrate the material; changes in transit time or attenuation provide moisture information across the entire cross-section of the material.
Advantages: Detects moisture throughout the entire cross-section of the material instead of just on the surface, therefore well suited for bulk materials, largely independent of color and surface structure, robust against dust.
Disadvantages: Lower spatial resolution, more complex calibration in the case of highly fluctuating bulk density.
Capacitive measurement
How it works:
Moisture is measured by changes in the dielectric properties between electrodes, usually as a built-in probe directly in the material flow.
Advantages: Compact design, low maintenance, well suited for spot measurements at narrow points (e.g. discharge chute).
Disadvantages: Limited range, sensitive to material inhomogeneity and installation distance.
Resistance/conductivity measurement
How it works:
Measurement of the electrical conductivity of the material, which correlates with the moisture content – usually via contact electrodes.
Advantages: Simple, cost-effective technology for many bulk materials.
Disadvantages: Direct material contact is required, dependent on material composition and ion content, less suitable for highly variable material chemistry.
Traversing measuring beams (scanning systems)
How it works:
A measuring beam traversing the width of the material guides several sensor heads (usually NIR and/or microwave) across the web-shaped material and creates a complete cross-section.
Advantages: Provides a complete moisture profile across the entire width instead of just a single point value, establishing a basis for automated profile control.
Disadvantages: Significantly higher investment and integration costs, only makes sense for web-shaped materials, usually only economical at one location.
Which method is suitable for which type of dryer?
Dryer type | Typical stuff | Suitable measurement methods | Why |
Belt dryers | Bulk goods, fibers, food on conveyor belt | NIR, microwave | Accessible from above, uniform layer of material on the belt |
Drum/rotary dryer | Granules, bulk materials | Microwave, capacitive at the discharge | Material moves inside a closed drum; measurement is usually taken at the outlet. |
Fluidized bed dryer | Fine granules, powder | Microwave, NIR at the discharge stream | Strong material movement within the dryer itself makes measurement inside difficult. |
Contact cylinder dryer (e.g. paper machine) | Sheet-shaped material | NIR spot measurement, microwave, measuring bar | The linear material allows for point or traversing measurement. |
Tunnel dryer | General cargo, sheets, molded parts on conveyor belt | NIR, camera-based additional control | Consistent positioning on the conveyor system enables targeted measuring points. |
Spray dryer | Fine powder at the discharge point | Capacitive, NIR at the discharge chute | Measurement within the spray chamber itself is hardly practical; therefore, measurement at the material exit point is necessary. |
Microwave dryer | Various bulk materials | NIR or capacitive (not microwave, due to interference from the dryer field) | The dryer's own microwave energy would interfere with a microwave measurement. |
Typical applications
- Paper, cardboard and tissue manufacturing: Continuous moisture control in the drying section
- Food and feed production: Drying of grains, snacks, flakes or extrudates
- Wood and biomass processing: Drying of shavings, pellets or sawn timber
- Chemical and pharmaceutical industry: Drying of granules and powders
- Building materials industry: Drying of sand, gypsum or cement precursors
- Textile industry: Drying after dyeing or coating processes
- Battery cell manufacturing: Drying of coated electrode foils
Selection criteria for suitable measurement technology
- Material form: In web, granular or powder form, or as individual items
- Accessibility inside the dryer: Is measurement possible inside the dryer itself, or is it only useful at the inlet/outlet?
- Required penetration depth: Surface moisture or moisture throughout the entire material cross-section
- Environmental conditions: Dust, temperature, and possible interference fields from microwave drying
- Required location resolution: Single measuring point or complete cross-section
- Regulatory objective: Simple limit value monitoring or automated, continuous process control
Frequently asked questions about inline moisture measurement in drying processes
Can the moisture be measured directly inside the dryer, or only at the inlet/outlet?
This depends on the type of dryer. With belt dryers, measurement directly above the material is possible, while with drum, fluidized bed, or spray dryers, measurement is usually only practical at the material outlet, as the dryer interior is inaccessible or the material movement is too strong.
Why is microwave measurement unsuitable for microwave dryers?
The dryer's own microwave energy would interfere with or distort the measurement. Methods like NIR or capacitive measurement, which are based on a different physical principle, are a better choice here.
Is a single measuring point sufficient, or is a cross-sectional profile needed?
For uniformly distributed material, a single measuring point is often sufficient. For web-like material with potential variations across its width, a traversing measuring bar system provides additional, valuable information.
How quickly does an inline moisture sensor react to changes?
Depending on the measuring principle and installation location, the response time is in the range of a few seconds, which is necessary for effective automated control of the dryer performance.
Inline moisture measurement from Sensorik Austria
Depending on the dryer type and material, Sensorik Austria uses different sensor systems: SensoWeb “moist” , the complete package in heady-duty design for point-based inline moisture measurement in paper machines, and tri²dent “moist” for material moisture measurement in drying processes away from paper machines.