Controller Problem.

Answers
 

1. If a temperature controller has a set-point of 121°C, and the control sensor is a platinum resistance thermometer, whose resistance is increasing with time due to ageing;  does the process temperature increase, stay at 121°C or decrease as the PRT ages?  The process temperature decreases because as the PRT's resistance increases due to ageing the controller lowers the temperature to reduce the resistance it is used to control the temperature to the set-point.
2. If a temperature controller has a set-point of -20°C, and the control sensor is a platinum resistance thermometer, whose resistance is increasing with time due to ageing;  does the process temperature increase, stay at -20°C or decrease as the PRT ages?  The process temperature decreases as per the explanation above.

Answers next week but also consider how a temperature indicator would respond.  The indicated temperature will increase  because the PRT's resistance has increased.
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