7. Microwave systems

Microwaves are electromagnetic in nature, with frequencies between 300 MHz and 300 GHz. Like light, they are little affected by diffuse dust, unsaturated steam, pressure, temperature and the chemical composition of the atmosphere through which they pass. Although the level detectors using them are on the whole rather expensive compared with other solutions, they have undergone considerable development, prices have fallen and the high frequency makes it easy to claim very good measurement uncertainty, in the millimetre range. Robust, they can withstand high temperatures.

There are four ways to use them:

• in level detection, like a photoelectric cell through the walls ;

• in measurement, by emitting pulses whose return time is determined, as in most ultrasonic systems;

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