Ask Greg McMillan

Greg's Response:

Temperature is often the most important of the common measurements because it is an indicator of process stream composition and product quality. Temperature measurements are also essential for equipment protection and performance monitoring.

In the process industry 99% or more of the temperature loops use thermocouples (TCs) or resistance temperature detectors (RTD). The RTD provides sensitivity (minimum detectable change in temperature), repeatability, and drift that are an order of magnitude better than the thermocouple (Table 1). Sensitivity and repeatability are 2 of the 3 most important components of accuracy. The other most important component, resolution, is set by the transmitter. Drift is important for controlling at the proper setpoint and extending the time between calibrations. Operators often adjust setpoints to account for the unknown drift. When the TC is calibrated or replaced, the modified set point no longer works. The RTD is also more linear and much less susceptible to electro-magnetic interference.  The supposed issue of a slightly slower sensor response will be addressed in the next post on how to get the best installation.

Thermistors have seen only limited use in the process industry despite their extreme sensitivity and fast (millisecond) response, primarily because of their lack of chemical and electrical stability. Thermistors are also highly nonlinear but this may be addressed by smart instrumentation.

Optical pyrometers are used when contact with the process is not possible or extreme process conditions cause chemical attack, physical damage, or an excessive decalibration, dynamic, velocity, or radiation error of a TC or RTD.

Simulations that include sensitivity, repeatability, drift, and EMF noise can show the advantage offered by the RTD.

For much more knowledge, see the ISA book Advanced Temperature Measurement and Control, Second Edition (use promo code ISAGM10 for a 10% discount on Greg’s ISA books).