Microwave measuring method

The microwave measuring method is a non-invasive determination of the water content of materials with the help of electromagnetic waves . The method is used in agriculture , the sugar industry , the food, luxury food and pharmaceutical industries, but also in construction, coal , crude oil and natural gas production as well as transport and storage management.

General

In the case of microwave measurement methods, the difference between the high relative permittivity of water (at f = 2.54 GHz, ε _r = 77) is used in contrast to many other materials. The moisture content is determined by analyzing a solid-water mixture in an electromagnetic field. The influence of the permittivity number of water on the sensor capacity is registered. The measurement frequencies are between 0.3 and 20 GHz . The temperature and salt content of the material are decisive for the selection of the measuring frequency . The material grain also influences the measurement result.

functionality

In microwave measurement methods, the differences between transmitted and received waves are measured, which are caused by attenuation of the amplitude and phase shift . The changes in amplitude and phase result from reflection , refraction and scattering of the high-frequency waves at the boundary layers of the dielectric and from absorption in the interior of the test material. The following two methods form the basis for microwave measurement methods.

Transmission method

Microwaves are emitted on one side of the substance to be examined . Depending on the properties of the material, the waves are reflected at the interfaces of the material or completely or partially absorbed when passing through the material . This process is called transmission .

The changed waves are received again on the other side of the fabric. The changes in wave amplitude and phase due to the modification of the initial field distribution are measured.

Reflection method

With this method, the microwaves are only measured on one side of the material sample. The waves are reflected as they pass through the material and recorded by a measuring device. The measured variable is influenced by the permittivity number.

advantages

Microwave measurement methods are particularly characterized by their reliable operation under harsh industrial conditions. Other advantages are one

non-destructive measurement,
Real-time measurement
high sensitivity and
penetrating measurement, in contrast to optical methods that only measure on the surface of the material, such as B. Near Infrared Humidity Measurement
a short settling time .

The influence of the ion conductivity on the measurement results is small due to the high measurement frequencies. Compared to other methods, such as the nuclear moisture measurement method, the measurement times are short and the occupational safety measures are minimal. The measurement results are not influenced by environmental influences such as dust or water vapor . In this way, high accuracies can be achieved in the measurements. Since microwave measuring processes work without contact, they are independent of the surface properties of the material to be examined. With the help of the microwave measurement method, the moisture in the entire volume of the solid-water mixture through which the radiation is exposed can be determined.

disadvantage

The measurement accuracy of the transmission and reflection methods depends on various interfering variables such as thickness, density and grain size of the material to be examined. By using multi-parameter methods , these dependencies can be compensated for or included in the measurement.

Frequency constancy and stable transmission power can only be guaranteed by using high quality measuring devices. Another disadvantage is that microwave measurement methods only determine a moisture content averaged over the test material. This value does not contain any information about the moisture distribution in the material.

literature

A. Göller: MOISTURE MAPPING - Area- and depth-resolved moisture measurement with the MOIST method . In: DGZfP report volume . 69-CD, 1999 ( PDF - free full text).
Klaus Kupfer: Material moisture measurement. Basics, measurement methods, applications, standards . expert-Verlag, Renningen-Malmsheim 1997, ISBN 3-8169-1359-8 , ( Contact & Study 513).
Stefan Völkner: On the influence of spatially limited discontinuities on the time-dependent moisture distribution in exterior walls . Dissertation, Ruhr University Bochum, 2003 ( urn : nbn: de: hbz: 294-7916 ).

Individual evidence

↑ Microwave measurement technology for determining the dry matter content. (PDF; 416 kB) Braunschweigische Maschinenbauanstalt AG, August 2010, accessed on October 22, 2017 .

[1] Microwave measurement technology for determining the dry matter content. (PDF; 416 kB) Braunschweigische Maschinenbauanstalt AG, August 2010, accessed on October 22, 2017 .