Fail-safe system for use with a sensor

摘要

A circuit for controlling the fail-safe operation of a measurement and control apparatus (not shown) such as is used in a steam raising plant to monitor the water level in a steam generation vessel. It includes a sensor (10) which comprises an electrode which in use is inserted into a water/steam column (not shown) to detect the presence or absence of water. The apparatus detects the presence or absence of water by making a measurement of the impedance experienced in a gap between an insulated tip of the electrode and a surface held at a reference voltage or else connected to ground. The apparatus may be configured to provide an alarm. The circuit comprises comparators (IC1 and IC2), a phase detector (IC3) and a triple redundant drive circuit (12) which drives a relay (14) to indicate a normal or an abnormal/fault condition.