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Digital Microwave Moisture Measurement for Concrete, Asphalt and Process Industries from Hydronix

Although microwave moisture measurement has been proven to be superior to a resistive or capacitive method, the analogue measurement technique has a number of limitations when compared to a Hydronix sensor which uses digital measurement technology.

Moisture causes a change in the dielectric property of a material

An analogue microwave sensor measures moisture through a combination of frequency shift 'f'' (the left to right movement of the response) and amplitude attenuation 'A' (the change in the height of the response) of the resonator. This combination is measured as a single analogue response and therefore frequency shift and attenuation cannot be separated. In the 1980’s Hydronix introduced an innovative digital microwave sensor that enabled the frequency shift component to be accurately measured using precise digital techniques. This development resulted in an improvement in accuracy and also a very significant extension of the moisture range for which the sensor would give a true linear response as moisture levels increased.

Less advanced sensors simply measure a change in amplitude at a fixed frequency. Measuring across a custom frequency range gives more accurate results than those sensors that work within the confines of a single frequency using an open band such as those used by Burglar Alarm Systems (433MHz) or Wi-Fi ( 2.4GHz). The components required to manufacture a sensor that measures a frequency shift as opposed to a change in amplitude at a fixed frequency are more complex but provide a superior result.

The difference between a digital and an analogue measurement

The difference between a digital multi-frequency measurement and an analogue measurement at a fixed frequency (f1) measuring just the change in amplitude can be seen in the diagram on the right. As the moisture increases the frequency shifts from f1 to f2, f3 and then f4. The frequency shift between each is similar in magnitude. A sensor with a digital measurement technique will continually scan the frequency response and will track the equal changes in frequency as the material becomes wetter.

For the same moisture changes, a single frequency sensor simply measuring the change in the amplitude at frequency f1 will measure the changes from A1 to A2, A3 and then A4. It can be seen that the sensor progressively loses the ability to register a change in reading as the material becomes wetter. Typically, a good analogue sensor will lose the ability to register additional changes in moisture from about 12% onwards. This not only affects the sensors ability to register changes in moisture above this level but it also means that the entire moisture curve lacks linearity as shown below.

A non-linear measurement

Sensors that use a non-linear measurement technique require the reading to undergo significant mathematical manipulation in order to output what appears to be a linear response to changes in moisture. This highlights a key difference between an analogue sensor that claims to be digital because it can process the signal and output a seemingly linear measurement and a sensor (such as those manufactured by Hydronix) that uses a digital microwave measurement technique that is inherently linear.