Learn more about the accuracy of temperature measurement using RTDs (Resistance Temperature Detectors). This video explains the international standard IEC 60751, which defines three accuracy classes for PT100 sensors: Class B, Class A, and Class AA. Class B has a tolerance of ±0.75°C at 100°C (±1.35°F at 212°F) , while Class A and Class AA offer higher accuracies of ±0.4°C (±0.72°F) and ±0.2°C (±0.36°F), respectively. Another important factor influencing RTD accuracy is the wiring configuration. A four-wire connection is recommended for highest accuracy, as it compensates for line resistance variations.
In general: proper planning is crucial for accurate RTD measurements!
Video Transcript
Today it's about the accuracy of temperature measurement using RTDs. Class A, class B, wiring technologies... there's some things to discuss. The accuracy of the PT100 sensor itself is written down in an international standard. The international norm 60751. In this you will find three classes of accuracy. The first one is the so-called class B. All of these accuracy classes look the same way. The best possible accuracy is around 0 degrees Celsius, 32 degrees Fahrenheit, the temperature of melting ice. From this point there are funnels opening with an accepted tolerance. For example here at class B at 100 degrees Celsius, it's around +/- 0.75 of a degree. The next one obviously should be class A, which is more or less double as accurate. Again we have this funnel opening and you see at 100 degrees we are around 0.4 degrees +/- as an accepted tolerance. The customers ask for something better for many years but you see, there was no letter left.
So what should the people who have written down the standard do yeah? They introduced a class double A. This is exactly three times better than the class B. Again with this funnel construction and you see here we are around +/- 0.4 degrees in these kind of applications. Additional to this it's very important how to connect the sensor. If you just connect the two wires of the resistance, you have an influence of the line resistance itself. And when the lines are made of a metal these lines also react on temperature changes. So it's really not recommended at all to use a Pt100 measurement with this two-wire technology because the resistance of the wire will destroy the accuracy of your measurement.
A better solution is the so-called three-wire connection and here it works like this. The transmitter is measuring sometimes or constantly what is the line resistance in the moment. Because here in this loop we are measuring two times of the line resistance only. So it can compensate this from the result which is measured in the second loop which is two times of the line resistance and the sensor itself. This works good for many applications if all of these line resistances are really absolutely the same at any time. This cannot be guaranteed for very long cables, for mineral insulated cables extending a certain length, and for changes of the terminal resistance, if the screws are really set with the same force.
So the best method is the four wire connection. In this the transmitter is working in a different way, so it totally compensates the influences of any line resistance. They must not be the same any longer, it doesn't matter. It's totally compensated. The highest accuracy can be reached using the four wire connection. If you think that is all you need to know about the accuracy of RTD measurement, that's not true! The biggest mistakes can be made during the planning phase, see the next video!
