Why Measure Moisture using Microwaves?
There are several methods of measuring moisture in online process control apart from the microwave method, these include resistive, capacitive, infra-red and nuclear techniques. Each of these have specific problems and therefore do not provide an ideal solution for the plant controller.
- This technique is prone to error because water does not act as a consistent conductor. For example distilled water is a non-conductor of electricity, whereas salt water will conduct electricity. This technique therefore is severely affected by any contaminants in the water.
- This is a more common method of moisture measurement but it is still affected by contaminants in the water. It also has a limited effective working moisture range.
- This is a very common method of measuring moisture but as this is a reflective technique, it will only provide a surface measurement which will not be representative of the total moisture in the product. The technique is expensive and requires specially calibrated equipment and a dust or moisture free environment.
- The Nuclear moisture measurement technique offers an accurate method of moisture control. However it is not suitable for many industries, can be extremely expensive and it also requires a large volume of material to operate successfully.
Compared with these other methods of moisture measurement, the Hydronix microwave method is the least affected by impurities, colour, particle size or temperature and is totally safe.
The Microwave Method of Moisture Measurement
To accurately measure moisture, it is necessary to determine the number of water molecules present in the material. To enable this, the material to be measured is passed across the ceramic faceplate or head of the microwave moisture sensor which radiates an extremely low powered electromagnetic field. Due to the dipolar effect of a water molecule, the resonant frequency of a microwave resonator changes with variations in moisture content. It is these variations that are detected by the sensor electronics. They are then measured in terms of 'unscaled units' which are scaled by a process of calibration to provide a precise readout of the moisture present. The resulting signal is sent via an analogue (0-20mA [0-10v] or 4-20mA) or RS485 digital communications link back to the plant control system, allowing adjustments to the water addition process to be made automatically.