How Can You Increase the Rate Time in Temperature Loops?

March 6, 2012
Temperature loops benefit more than most loops from the use of derivative action. The rate time can be 1/2 of the total loop deadtime or larger by the use of a wireless smart transmitter, the enhanced PID for wireless, and a proper installation location and thermowell design. For many temperature loops, the rate time is compensating for thermal lags that add process and measurement deadtime to the temperature response.

Temperature loops benefit more than most loops from the use of derivative action. The rate time can be 1/2 of the total loop deadtime or larger by the use of a wireless smart transmitter, the enhanced PID for wireless, and a proper installation location and thermowell design. For many temperature loops, the rate time is compensating for thermal lags that add process and measurement deadtime to the temperature response.

The full beneficial amount of derivative action in temperature loops can be achieved by minimizing the noise (fluctuations and spikes) and quantization error (steps) in the measurement. Sensor locations with good mixing to provide a uniform representative temperature and sufficient insertion length to minimize wall effects can reduce process noise. Wireless smart transmitters can eliminate electromagnetic interference (EMI) and decrease the quantization error by improving the resolution of the measurement. The enhanced PID developed for wireless can spread out whatever quantization error exists by using the elapsed time since the last step instead of the PID execution time as the time interval for the derivative calculation.

Finally, a small signal filter to keep the fluctuations in the PID output from noise within or just slightly larger than the deadband of the final control element (e.g. valve or variable frequency drive) can increase the allowable rate time setting. As a last resort you can also a signal filter to spread out the step from a quantization error until you can install a smart transmitter or implement an enhanced PID. This measurement lag adds deadtime to the loop so it is at best a temporary fix.

About the Author

Greg McMillan | Columnist

Greg K. McMillan captures the wisdom of talented leaders in process control and adds his perspective based on more than 50 years of experience, cartoons by Ted Williams and Top 10 lists.

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