Post by Professor Lake Shore on Jan 31, 2017 16:17:55 GMT -5
Why does my temperature instrument display “S. Over,” “T. Over,” “S. Under,” or “T. Under”?
These messages indicate that the instrument cannot display a valid temperature because the sensor value exhibits one of these conditions, in most instances caused by a problem with the sensor wiring or the sensor itself:
“S. Over” – the sensor value is greater than the physical limit of the input.
“S. Under” – the sensor value is at zero or is negative.
“T. Over” – the sensor value is beyond the highest temperature point on the curve assigned to the input (for NTC devices, this means the sensor value is lower than the value shown in the highest curve breakpoint, and for PTC devices, this means the sensor value is higher than the value shown in the highest curve breakpoint).
“T. Under” – the sensor value is beyond the lowest temperature point on the curve assigned to the input (for NTC devices, this means the sensor value is higher than the value shown in the highest curve breakpoint, and for PTC devices, this means the sensor value is lower than the value shown in the highest curve breakpoint).
If these conditions display, then next step is to test to see if there is an open or short in the sensor leads and/or the isolation of the sensor has been compromised. How to do this is explained in the “procedure for testing a diode sensor” and “procedure for testing an RTD sensor” FAQs.