A plastics recycling plant used ducted hot air to dry a stream of shredded and washed post-consumer polyethylene bags. It was important to keep the temperature high enough for efficient drying, yet not so high that the plastic melted. The operator commented that the temperature controller was not doing its job, and no combination of PID or other control parameters would stop the severe air temperature cycling. When the thermocouple was replaced by one with an exposed, low-mass hot junction, response time went from minutes to seconds. In this case, another contribution to good process design was the short time lag between a power change and the sensing of temperature change downstream.
Thermal lag has two components.
These principles apply equally to fluids and solids in motion. In fact, PID temperature controllers routinely handle exponential lags, but it takes a more advanced model to compensate for dead time and avoid cycling. For fast temperature sensing, thermocouples generally beat RTDs because of their low mass, simple construction, point sensing and ease of exposure of the sensing element. With RTDs, their construction demands protection tubes for most industrial locations. This protection and the distributed construction of the platinum wire contribute to slow response.