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The Truth About RTDs

I hope this paper does not upset too many people; because like Jack Nicholson told Tom Cruise in a Few Good Men, some people "Can’t handle the truth." You would think that the way some of us approach Resistance Temperature Detectors or RTDs, that the technology train left these very accurate sensors at the station. Let's start with why it is necessary to understand the “Truth about RTDs”.

The first truth is that RTDs are growing at a much higher rate than their thermocouple counterparts. It is expected that RTDs will grow at a compound annual growth rate (CAGR) of 4.2% reaching a volume in the Americas of 200 million US dollars in 2011. This will surpass the thermocouple which is expected to have a negative CAGR of 2.0% and dollar volume of 185 million during the same time frame.

What can affect this growth? Well, basically two fundamental issues; price and technology. Let’s talk about price first. The truth is that a ” RTD purchased for a replacement in a thermowell is less than 5% more expensive than its thermocouple counterpart. Gone are the days of a 25% price differential. Today’s instrumentation plays an important role as well. Users can replace the existing sensor with the sensor of their choice with little or no hassle.

Technology is a more complicated and detailed issue. It involves a myriad of points and counter points which due to their complexity and length prevent this paper from addressing all areas. It is accurate to state that the RTD inherently can never measure up to the thermocouple when it comes to response time and mechanical strength. It is safe to say that today’s RTD has improved to a point that it can handle most process applications. The real reason the RTD is the sensor leader is its’ accuracy and stability characteristics and this paper will deal with the truth in those areas.

Stability is defined by ASTM as the sensors ability to “remain within the tolerances” for a four week period. The RTD has a big advantage here. Stability and accuracy are very dependent on chemical composition. The tolerance for most RTD element manufacturers is for a purity of platinum that is 99 point 8 places pure. While it is relatively easy to control this degree of purity for an RTD, it is not so for a thermocouple. For instance a type K thermocouple consists of 90% nickel and the rest is made up of chromium, manganese and silicon. It is difficult to repeat the proper mix of these components and consequently the thermocouple is less stable and less accurate than the RTD.

In the process industry it’s a given that the RTD is more linear and more accurate than its thermocouple counterpart. But hidden facts are the RTDs’ accuracy options. Table 1 provides the accuracy options for 100 ohm platinum RTD. Table 2 provides the formula for determining the uncertainty at a specific temperature point. You will note that the most accurate point is at 0 C. There is a reason for this. Most RTDs today are of the thin film construction. They are made by depositing platinum on a substrate and trimming to 100 ohms at 0 C.


RTD Tolerances


IEC 60751 Tolerances


Although Table 1 only goes up to 200C, you can safely use a Class A or B RTD as high as 600C with an accuracy of 1C at 400C for the Class A.

Now for the truth that may shock you. These accuracies are element only and do not include any errors associated with the manufacturing process. Virtually all RTD manufacturers quote the accuracies in the above tables with the assumption that they will remain within the uncertainty tolerances after the manufacturing process is completed. For typical process needs this is a good assumption. But what if you wanted improved accuracy and tolerance certainty post manufacturing?

The chart below provides accuracy options.




The truth is only Smart Sensors can give you guaranteed post manufacturing accuracy guarantees. We urge you to visit our website at www.smartsensors.com and see how we provide Temperature Measurement the right way.

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