The majority of conventional liquid level monitors in industry operate using a gamma-ray absorption technique. Gamma-rays with energies utilised in these machines are strongly absorbed by water/liquids. In a γ -ray measurement a collimated beam is momentarily exposed to the container that is to be measured, with some of the beam below and some above the nominal liquid level. The γ-rays are detected using a scintillation tube after passing through the container and its contents. Thus if more of the γ -ray beam is absorbed by a higher liquid level, the counts drop, and vice-versa. The type of machine is in the main a pass/fail monitor — that is it passes a can if the liquid level is above a predetermined height, and rejects it if it is not. There are several problems associated with the use of γ -ray based systems, most notably environmental considerations, the fact they are potentially hazardous and subject to strict legislation.
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