How to Select the Best Temperature Sensor for Process Control Applications – 8500918

Temperature Measurement: Select the Best Temperature Sensor for Process Control Applications Industry Technology Paper

Thermocouples vs Resistance Temperature Detectors

Many users simply look to fill the basic needs of their application and do not worry much about their choice of temperature sensing technology. That is, they will make a selection based simply on temperature range and their own bias, perhaps based on their familiarity with a particular sensor type.

At a minimum, an informed sensor choice should first consider the following:

• Measurement range, including the range extensions of shutdown, startup, and process upset.
• The response time.
• The sensor stability, accuracy, and sensitivity in the application environment.

When we start to cross the boundaries between choosing one type of sensor over another, the optimum choice between thermocouple and RTD can be difficult. There is a lot of overlap between these sensors at the more popular lower end of the operating temperature range. So for sensors that cover the same operating range, and applications where response time is not a driving issue, plus stability, accuracy, and sensitivity are acceptable, we really have to drill deeper and compare characteristics between sensors to find the best fit for a given application.

In general, if your application requires the highest accuracy, cost is not a concern, and your operating ambient is less than 800°C, then the choice of an RTD over a thermocouple sensor is probably the right one. The RTD is more accurate, more stable, more repeatable, and offers a more robust output signal with better sensitivity and linearity than a thermocouple. However, the RTD does have a narrower operating range with a lower maximum operating temperature, it is generally more expensive, and it does require excitation which might drive the need for an external power source (a Wheatstone bridge for example). Please review Part 2 of this series for other differentiating features of RTD sensors.

If you instead decide that a thermocouple is best for your application, perhaps because of its lower cost, wider temperature range, faster response time, and simpler construction, plus its many physical sizes and wider range of configurations available, then you might start by picking a Type K thermocouple until you can find a specific reason to choose another type. That is, type K is the most common and least expensive of available T/C types, and it also has a wide operating temperature range with high sensitivity. It is constructed from nickel-based metals which have good resistance to corrosion and are cheaper than the comparable platinum-based metals. So with this in mind, why would you choose anything else? Well, it does have one lead that is magnetic (the Red or negative lead), and this might not work well around electric motors. It is also vulnerable to sulfur attack and should not be used in sulfurous atmospheres.

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