Ask Greg McMillan

Greg's Response:

Process efficiency and capacity are often determined by a temperature control loop performance’s effect on the formation and purification of the product. Good composition control in biological and chemical reactors and in distillation columns is achieved by tight temperature control.

There is a significant misunderstanding as to the best selection, specification, and installation of temperature measurements aggravated by the lack of inclusion of the 5Rs (resolution, repeatability, rangeability, response time, and reliability), drift, and nonlinearity in measurement specifications. The inclusion of the 5Rs in dynamic simulations can help motivate the use of the best temperature measurements. Dynamic simulations should include several time constants that are the thermal lags associated with the thermowell, modeling objects to simulate the sensor resolution and repeatability, a steadily increasing error to simulate sensor drift, and a constant signal to simulate sensor failure. The detailed following list of Top Ten Mistakes alerts us to the significance and extent of the problem besides directing us to the best temperature measurement.  

Top Ten Mistakes in Temperature Measurement Specification

1. Thermocouple (TC) instead of Resistance Temperature Detector (RTD) for < 1600 oF.
2. Direct wiring of sensor to TC or RTD I/O cards.
3. Loose fit of sensor in thermowell.
4. Sensor not touching bottom of thermowell.
5. Focus on TC versus RTD sensor instead of thermowell response time.
6. Thermowell length and installation results in sensor seeing jacket temperature.
7. Thermowell length and installation results in low velocity at tip.
8. Transmitter span too large.
9. Transmitter remote mounted.
10. Thermowell wall at tip too large

The best temperature measurement uses a head mounted narrow span RTD transmitter on a stepped thermowell with a tight fit spring-loaded sheathed platinum resistance temperature detector (PRTD) mounted in a piping elbow to sure sufficient length and velocity (e.g., liquids > 5 fps).

The ISA book Advanced Temperature Measurement and Control Second Edition details how to get the best temperature measurement and control. Here is Table 2.8 from the book: