Happy With Your Temperature Sensors?
Control Engineering readers seem to have deeply held opinions on temperature sensors, apparently more than other types of instrumentation, or at least they’re less shy about expressing those opinions. It might be because there are fewer choices of technologies when compared with something like flowmeters. To find out what readers think, we posted an online survey to ask users what kinds of temperature sensors they use, and how happy they are with day-to-day performance.
Those who responded agree that sensors fall into three categories: thermocouple, RTD (resistance temperature device), and thermistor. We’ve compared the pros and cons of the three before, so the questions we’re looking at today relate to what people are actually using and if they’re satisfied with the situation. Some answers reinforce widely held beliefs (some may regard them as merely opinions) about the characteristics of sensing technologies, but others run contrary. This survey does not dig into specific manufacturers but is aimed more at general behavioral observations.
The battle between thermocouple and RTD as the most widely deployed is, to borrow some of the terminology from our recent election, a statistical dead heat. The two numbers are extremely close. Thermistors take a very distant third place with only 8.5% naming that as most used. Rounding out the field are unspecified “semiconductor” devices and infrared as write-ins.
Users seem satisfied with what they have, as there are hardly any that say their preferred technology is different than the one they’re using the most. Thermistors lead the more negative category of least liked, at 65%. Thermocouples are next at 24%. RTDs are cited the least as least liked, along with some miscellaneous write-ins.
There are operational parameters that coincide with this distribution and follow some conventional wisdom on performance characteristics. For example, two-thirds of the respondents that say they have to handle relatively high temperatures routinely (>1,000 °F) say they deploy thermocouples the most. This is logical given thermocouples’ ability to handle higher sensing ranges. Below that level, RTDs take a lead, which may suggest that when all things are equal and specific performance requirements are not a factor, they are preferred over thermocouples.
Similarly, RTD users claim a higher need for precise measurements. 80% say they need precision of ±1.9% or better. This coincides with the general belief that RTDs are more precise. (Whether they are or not is debatable and beyond the scope of this article. I apologize in advance if you think I’m perpetuating a negative stereotype.) Thermocouple users are more evenly distributed on the topic, but there is still a strong cluster among the most precise.
When looking at installation practice, two-thirds of RTD users say they install a transmitter at the device in all cases. The balance of RTD users choose between transmitters and matched cable based on distance. Only one claimed to use just matching cable in all applications.
Thermocouple users were more evenly divided on the question of always using a transmitter, always using just matched cable, or a mix of transmitters and cable based on distance.
Fewer than 20% reported issues with EMI, and those that do tend to use transmitters, presumably to mitigate the problem. Fortunately, there were no responses that suggest any old wire will do.
Performance and reliability
The question related to reliability had to be general, but in the real world, some individual device installations will be better than others. As respondents report, some combinations of device and practice yield better results and higher reliability.
For example, among RTD users that install transmitters on every device, all reported that the devices “work well and rarely ever give me problems.” Of those that mix transmitters and cabling, half report that they “have to keep an eye on them and recalibrate as necessary.”
Thermocouples are not as easy to correlate. As mentioned before, there is greater variety of wiring practice and reliability results. Thermocouple users are split between “work well and rarely ever give me problems,” and “have to keep an eye on them and recalibrate as necessary.” Only one said “they can be a headache.” However there is no apparent correlation between these responses and wiring practice. Use of cable or transmitters doesn’t have an effect.
Again, this seems to support the perception that RTDs are more stable, but as mentioned earlier, such points are debatable. The small but apparently loyal group of thermistor users all reported that problems are rare.
Users are roughly divided on maintainability. Half say they can check and replace sensors whenever necessary. The balance says that “most are easy to reach but a few can’t be changed without shutting down part of the process.” Only one responded that “changing a device is a major project so we’re running as best as we can with some points inoperative until the next scheduled shutdown.” That same user also reported that devices “work well and rarely ever give me problems.”
Ultimately the choice of sensor technology has to be a combination of application necessity (e.g., temperature range or precision); internal culture, tradition, and training (e.g., “We always use thermocouples.”); and preferred vendor relationships. On a practical level, the number of devices available for any given application should allow a selection and never force a user into one undesirable path. Such situations may still happen, but this will likely be the result of internal issues and not the products themselves.
Peter Welander is content manager for Control Engineering. Reach him at firstname.lastname@example.org.
Challenges of Temperature Sensing, Dec. 2008