Time Domain Refractometry (TDR) is a method used to measure the dielectric constant (water content) of a material.

Time Domain Refractometry (TDR) is a method used to measure the dielectric constant (water content) of a material. Metal conductors or rods are used as wave guides for the transmission of the TDR-signal. The device generates a high-frequency-pulse (up to 1GHz) which propagates along the wave guides at the speed near to light, generating an electromagnetic field around the probe. At the end of the wave guides, the pulse is reflected to its source. The resulting transit time from picoseconds to nanoseconds and dielectric constant provide an accurate measurement of moisture content. The moisture content is calculated inside the device and is available at standard analogue outputs.

IMKO, a German company, which part of the Endress+Hauser Group has developed a patented measuring method called TRIME® TDR method. With this technology it is possible to measure the transit time with a resolution of 3 picoseconds. The TRIME® TDR method has been successfully used more than 30 years in the process industry.

The TRIME® TDR -technique has an ideal measuring frequency between 600MHz and 1.2GHz and the measuring field penetrates deep into the material. The electromagnetic pulses generated in the TRIME device are high enough and constant for good resolution and accuracy of measurement. In addition, disturbing influences such as electrical conductivity have nearly no effect. Precise moisture can be measured in temperatures up to 120°C. TRIME® TDR technology is used in Solitrend product which helps to increase product quality and reduce energy efforts due to precise moisture measurement.

The senor used in Solitrend is in direct contact with the material. Calibration is a simple linear equation. Measurement field adapts itself to the shape of material and is quite large. It is also undisturbed by dripping water, air and dirt. The probes are extremely rugged for exceptionally long operational life. It also replaces time consuming lab analysis and manual sampling with continuous online measurement.

Both In line as well as portable probes measurements are available.

Application

• The building industry. Moisture measurement in Ready Mix Concrete.
• In line moisture content measurement in bulk material or goods like wood chips, pellets, animal food etc.
• The glass and ceramics industry like silica sand, moulding sand, ceramics mass.
• Food Industry. Malting plants, Corn, Sludge drying, Fluid bed dryers (inlet/outlet).

Limitations of other methods

Capacitance method – This method works with single measuring frequencies between 5-80MHz. The dielectric constant is influenced both by moisture and salinity. It is very difficult to distinguish between the two influences and precise moisture measurements are difficult to achieve.

Microwave method – The frequencies of microwave techniques are generally higher than 2GHz. This technique leads to loss in resolution and accuracy of measurement having negative influence on the readings.

Conductance or Resistance method – This method have very low measuring frequencies of about 4kHz. There is a strong influence of different salinity of the bulk material on the measurement. The difference in salinity is caused by different mineral contents temperature. Another problem of resistance method is the low penetration depth of the measuring field.

Near-Infrared method (NIR)-This method shows a very lower penetration depth and, therefore, considerable influences of material surface and thickness.

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