PT100 Accuracy and Probe Classification

It is important to understand that just like K-Type thermometers there are 3 main areas of uncertainty when using PT100 thermometers.

The uncertainty of the unit
The uncertainty of the probe
Uncertainty of any calibration procedure

So lets break it down.

1) The uncertainty of the unit

This one is nice and simple, but often misunderstood. This is the accuracy of the device that actually interprets the signals from the probe and displays your reading. This is the number you will most commonly see listed in product descriptions. In the case of our Pt100 Thermometer, this is ±0.05°C from -80°C to 200°C or 0.1°C at other temperature ranges. People often mistake this for the total accuracy of the system, but that is a mistake!

2) The uncertainty of the probe

This is where things start to get a little more complex. Each probe has a different level of tolerance. IEC 60751:2008 is considered the international standard and specifies the tolerances for platinum RTD elements. The standard specifies 4 classes of accuracy. Class B, Class A, 1/3 DIN and 1/10 DIN. As we learned in the article “What is a Resistance Temperature Detector (RTD)?” Platinum RTD’s follow a temperature-resistance curve which dictates what level of resistance in the sensor represents which temperature. Each probe has the capacity to drift away from this curve reducing accuracy. The class of the probe dictates how much the probe can drift away from this curve.

For example... A Class B probe should represent a temperature of 0°C at a resistance of 100ohms but with a tolerance of 0.3°C the actual temperature could be between -0.3°C to +0.3°C. A 1/10 DIN probe should represent a temperature of 0°C at a resistance of 100ohms but with a tolerance of 0.03°C the actual temperature could be between -0.03°C to +0.03°C. Don’t forget to add this to the Accuracy of the unit for your final accuracy. In the examples below we used our own Pt100 model. Class B: (±0.05°C + ±0.3°C) = ±0.35°C at 0°C 1/10 DIN: (±0.05°C + ±0.03°C) = ±0.08°C at 0°C The table below shows the tolerances of each Class of probe at various temperatures. If you cross reference the temperature you are likely to be measuring and add them to the tolerance of the Pt100 unit you can calculate which Class of probe will be suitable for your application.

RTD Probe Accuracy based on Class
ACTUAL	Class B	Class A	1/3 DIN	1/10 DIN
-200 °C	1.30 °C	0.55 °C	0.39 °C	0.38 °C
-150 °C	1.05 °C	0.45 °C	0.23 °C	0.21 °C
-100 °C	0.80 °C	0.35 °C	0.15 °C	0.12 °C
-90 °C	0.75 °C	0.33 °C	0.14 °C	0.10 °C
-80 °C	0.70 °C	0.31 °C	0.13 °C	0.09 °C
-70 °C	0.65 °C	0.29 °C	0.12 °C	0.08 °C
-60 °C	0.60 °C	0.27 °C	0.11 °C	0.07 °C
-50 °C	0.55 °C	0.25 °C	0.10 °C	0.06 °C
-40 °C	0.50 °C	0.23 °C	0.10 °C	0.06 °C
-30 °C	0.45 °C	0.21 °C	0.09 °C	0.05 °C
-20 °C	0.40 °C	0.19 °C	0.09 °C	0.04 °C
-10 °C	0.37 °C	0.17 °C	0.08 °C	0.03 °C
0 °C	0.30 °C	0.15 °C	0.08 °C	0.03 °C
10 °C	0.35 °C	0.17 °C	0.09 °C	0.04 °C
20 °C	0.40 °C	0.19 °C	0.10 °C	0.04 °C
30 °C	0.45 °C	0.21 °C	0.11 °C	0.05 °C
40 °C	0.50 °C	0.23 °C	0.12 °C	0.06 °C
50 °C	0.55 °C	0.25 °C	0.13 °C	0.07 °C
60 °C	0.60 °C	0.27 °C	0.14 °C	0.08 °C
70 °C	0.65 °C	0.29 °C	0.16 °C	0.09 °C
80 °C	0.70 °C	0.31 °C	0.17 °C	0.10 °C
90 °C	0.75 °C	0.33 °C	0.18 °C	0.11 °C
100 °C	0.80 °C	0.35 °C	0.19 °C	0.12 °C
110 °C	0.85 °C	0.37 °C	0.20 °C	0.13 °C
120 °C	0.90 °C	0.39 °C	0.21 °C	0.14 °C
130 °C	0.95 °C	0.41 °C	0.22 °C	0.15 °C
140 °C	1.00 °C	0.43 °C	0.24 °C	0.15 °C
150 °C	1.05 °C	0.45 °C	0.25 °C	0.16 °C
160 °C	1.10 °C	0.47 °C	0.26 °C	0.17 °C
170 °C	1.15 °C	0.49 °C	0.27 °C	0.18 °C
180 °C	1.20 °C	0.51 °C	0.29 °C	0.19 °C
190 °C	1.25 °C	0.53 °C	0.30 °C	0.21 °C
200 °C	1.30 °C	0.55 °C	0.31 °C	0.22 °C

3) Uncertainty of the Calibration Procedure

Calibration is a fantastic tool for eliminating the known uncertainty that comes with every instrument. Your PT100 Thermometer is tested against a known reference standard and a certificate of calibration is produced. For a high accuracy instrument such as our Pt100 model, we offer a UKAS Calibration Service.

What does my certificate tell me

From our previous calculations we know that at the standard calibration temperature points we offer, our own PT100 with a Class B probe would normally have the following accuracy uncertainties.

-20.00°C ±0.45°C
0.00°C ±0.35°C
100.00°C ±0.85°C
200.00°C ±1.35°C

This certificate of calibration shows several pieces of key information.

The Calibration Temperature - This is the temperature the thermometer is being tested at
The PT100 Thermometer reading - This is the reading the PT100 thermometer gave at this known temperature
The uncertainty remaining at the calibrated temperature points

An Example extract from a certificate

This would be displayed as follows (this is an example only)

Calibration Temperature	Thermometer Reading
-20.00°C	-20.01°C
0.00°C	0.00°C
100.00°C	99.99°C
200.00°C	199.98°C

The uncertainty of measurement for the thermometer is. Digital: Below 255°C = 0.06°C - Above 265°C = 0.11°C Not only does this informs the user of any adjustments to be made to the reading at the desired temperature. It also reduces the uncertainty at each of the tested points from between ±0.45 to ±1.35°C to an amazing 0.06°C. This uncertainty is calculated by UKAS guidelines and is provided with a level of confidence of approximately 95%.