Ask Greg McMillan

Greg's Response:

Temperature RTD sensors should use 4 wires and TC sensors should use extension wires of same material as sensor with length as short as possible to minimize the effect of electromagnetic interference (EMI) and other interference on the low-level sensor signal. The temperature transmitter should be mounted as close to the process connection as possible, ideally on the thermowell.

To minimize conduction error (error from heat loss along the sensor sheath or thermowell wall from tip to flange or coupling), the immersion length should be at least 10 times the diameter of the thermowell or sensor sheath for a bare element.  For high velocity streams and bare element installations, it is important to do a fatigue analysis because the potential for failure from vibration increases with immersion length.

The process temperature will vary with process fluid location in a vessel or pipe due to imperfect mixing and wall effects. For highly viscous fluids such as polymers and melts flowing in pipes and extruders, the fluid temperature near the wall can be significantly different than at the centerline. Often the pipelines for specialty polymers are less than 4 inches in diameter, presenting a problem for getting sufficient immersion length and a centerline temperature measurement. The best way to get a representative centerline measurement is by inserting the thermowell in an elbow facing into the flow If the thermowell is facing away from the flow, swirling and separation from the elbow as can create a noisier and less representative measurement. An angled insertion in can increase the immersion length over a perpendicular insertion. A swaged or stepped thermowell can reduce the immersion length requirement by reducing the diameter near the tip. Thermowells with stepped stems also provide the maximum separation between the wake frequency (vortex shedding) and the natural frequency (oscillation rate determined by the properties of the thermowell itself).

Tight fitting spring loaded sheathed sensors should be used to eliminate any air gap between the sensor sheath and the inside wall and bottom of the thermowell that dramatically increases the response time.

Simulations that include response time and velocity and composition at thermowell tip can confirm the best installation. 

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).

1. Ensure distance from the equipment outlet (e.g. heat exchanger exit) and sensor at least 25 pipe diameters for a single phase to promote mixing (recombination of streams).
2. Verify the transportation delay (distance/velocity or volume/flow) from the equipment outlet (e.g. heat exchanger exit) to the sensor is less than 4 seconds.
3. Ensure the distance from the desuperheater outlet to the sensor provides a residence time (distance/velocity) that is greater than 0.2 sec
4. Use a RTD for temperatures below 400 *C to improve threshold sensitivity, drift, and repeatability by more than a factor of ten compared to TC if vibration is not excessive.
5. For RTDs at temperatures above 400 *C, minimize length without increasing conduction error and maximize sheath diameter to reduce error from insulation deterioration.
6. Be extremely careful using RTDs at temperatures above 600 *C. A hermitically sealed and dehydrated sensor can help prevent increase in platinum resistance from oxygen and hydrogen dissociation but reliability and accuracy may still not be sufficient.
7. For TCs at temperatures above 600 *C, minimize decalibration error from changes in composition of TC by best choice of sheath and TC type.
8. For TCs at temperatures above 600 *C, ensure sheath material compatible with TC type.
9. For TCs above temperature limit of sheaths, use the ceramic material with best thermal conductivity and design to minimize measurement lag time.
10. For TCs above the temperature limit of sheaths with gaseous contaminants or reducing conditions, use possibly purged primary (outer) and secondary (inner) protection tubes to prevent contamination of TC element and still provide a reasonably fast response.
11. In furnaces and kilns ensure location and design minimizes radiation and velocity errors.
12. Use immersion length long enough to minimize heat conduction error (e.g., L/D > 5).
13. Use immersion length short enough to prevent vibration failure (e.g., L/D < 20).
14. Ensure velocity is fast enough to provide a fast response (e.g., > 0.5 fps) and is fast enough to prevent fouling for sticky fluids and solids (e.g., > 5 fps).
15. For pipes, locate the tip near the centerline.
16. For vessels, extend the tip sufficiently past the baffles (e.g. L/D > 5).
17. For columns, extend the tip sufficiently into tray or packing (e.g. L/D > 5).
18. For TC, use ungrounded junction to minimize noise.
19. To increase RTD reliability, use dual RTD elements except when vibration failure is more likely due to smaller gauge in which case use redundant separate thermowells. 
20. To increase TC reliability, use sensors with dual isolated ungrounded junctions.
21. For maximum reliability, greater intelligence as to sensor integrity and to minimize the effect of drift, use 3 separate thermowells with middle signal selection.
22. Document any temperature setpoint changes made by an operator for loops with TCs so that they can be diagnosed as to possibly originating from TC drift.
23. Realize the color codes of TC sensor lead and extension wires change with country ensuring drawings show correct codes and electricians are alerted to unusual codes.
24. Use spring loaded sheathed TC and RTD sensor that fits tightly in thermowell to minimize lag from air acting as insulator (e.g., annular clearance < 0.02 inch).
25. If an oil fill is used minimize thermowell lag, ensure tars or sludge at high temperature do not form in thermowell and tip is pointed down to keep oil fill in tip.
26. Use Class 1 element and extension wire to minimize TC measurement uncertainty.
27. Use Class A element and 4 lead wires to minimize RTD measurement uncertainty.
28. Use integral mounted temperature transmitters for accessible locations to eliminate extension wire and lead wire errors and reduce noise.
29. Use wireless integral mounted transmitters assembled and calibrated by measurement supplier to eliminate wiring errors and errors and provide portability for process control improvement and to reduce calibration, wiring installation and maintenance costs.
30. Use “sensor matching” and proper linearization tables in the transmitter for primary control loops and in Safety Instrumented Systems achieving accuracy better than 1 *C.