The circuit requires so little power from the indicating instrument thnt several alarm points can be used. The control can be located on the primary indicating instrument or at any remote location. The alarm level control can be set accurately and is stable over long periods of time. It can be connected to operate with either a positive or negative signal. The disadvantages of the meter relay can be overcome by using a differential comparator in the form of an integrated circuit which is small, reliable and inexpensive. The non-contact type meter relay is expensive and in both types the alarm level can only be set at the indicating instrument. Contact type relays prevent the indicating meter from exceeding the alarm set point and they are limited to only one up-scale point and one down-scale point. Although meter relays are the most common alarm level controls now in use they have several limitations. It is therefore necessary to provide reliable alarm circuitry if the primary instrument is to be effective. The purpose of most fixed radiation monitoring instruments is to provide in alarm when the radiation levels being monitored exeed a predetermined value.
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