Abstract: A circuit for indicating when recalibration of an electrical sensor is necessary includes an analog driving circuit adapted to be coupled to the sensor, and an analog indicating device coupled to the analog driving circuit for indicating when the sensor should be recalibrated. The analog driving circuit provides a signal proportional to the current received from the electrical circuit. The indicating device is responsive to the output from the analog driving circuit and provides an output which is a function of both the sensor output and the elapsed time that the sensor has been operated.

Abstract: In a calibration warning device for a thermocouple, the thermocouple is connected to input terminals (10) of an operational amplifier (12) provided with negative feedback. The output is applied to the base of a transistor (14) controlling the current supplied to a mercury coulometer (18). When the coulometer indicator reaches full-scale deflection, the thermocouple is due to be recalibrated. The invention utilizes the face that the drift curve of a thermocouple can be matched to a sufficiently high accuracy by a suitable choice of transistor characteristic and amplifier gain, as is explained in the specification.