There are a number of sensors (2, 22) such as thermocouples which can provide a putative measurand signal which is within a predicted range for such signals whilst the sensor is incorrectly operating, such as as due to an open circuit. Techniques and processes are available to determine by interrogation sensor operational validity, but these can distort the measurand signal if correct. By time division multiplex techniques the present arrangement takes a putative measurand signal from a sensor (2, 22) in order that either within the same time division or more normally a separate time division, an interrogation of the sensor is performed in order to determine accuracy and therefore validity of the sensor 22. In such circumstances, the putative measurand signal from the sensor 22 is dynamically verified by the interrogator 23 to allow greater confidence with respect to interpretation, if an invalid sensor operation is determined, then a pre-defined value for that sensor failure can be output for the data signal.