How to calculate the tolerance of RTD temperature sensor

European standards, also known as DIN or IEC standards, are considered the global standard for Pt100 temperature sensors. This standard DIN/IEC 60751 (or IEC751 for short) requires RTD to have a resistance of 100.00 Ω at 0 ° C and a resistance temperature coefficient (alpha) of 0.00385 Ω/Ω/° C between 0 and 100 ° C.

Four resistance tolerances are actually specified in DIN/IEC751:2008

Grade AA = ± ( 0.10 + 0.0017 * | t | ) °C

Grade A= ± ( 0.15 + 0.0020 * | t | ) °C

Grade B = ± ( 0.30 + 0.0050 * | t | ) °C

Grade C = ± ( 0.60 + 0.0100 * | t | ) °C

The combination of resistance tolerance and temperature coefficient defines the resistance and temperature characteristics of RTD temperature sensor. The greater the element tolerance, the greater the deviation between the sensor and the generalized curve, and the greater the change (interchangeability) between sensors. This is important for users who need to change or replace sensors and want to minimize interchangeability errors.

Tolerance test method
Calibration of RTD temperature sensors involving tolerance testing is reserved for low accuracy applications. These values are defined by one of the common models, such as ASTM 1137 or IEC 60751 curves. RTD temperature sensors calibrated in this way are typically used in industrial style applications
Two common accuracy classes are A and B:

These include errors caused by R0 deviation and slope error. Generally, we will see probes rated as Class A.
For example, 0.1 ASTM Grade A. Fractional accuracy can only be achieved in sensors, but it is difficult to achieve in probes. The calculation is simple.

Example: Calculate the accuracy of 0.1 ASTM Class A probe at 100 ° C

For ASTM standards:

Grade A = ± [0.13 + (0.0017 · t)] °C
= (0.13 + (0.0017 · t)) · 0.1
= (0.13 + (0.0017 · 100)) · 0.1
= (0.13 + 0.17) · 0.1
= 0.0.0

Measuring reference probe temperature=100.0 ° C
Measured indicated temperature=100.5 ° C
Calculation error (difference between indication and reference)=0.05 ° C
Tolerance at 100.00 ° C calculated using ASTM standards=0.03 ° C
Determine tolerance status=Failed (0.05 ° C>0.03 ° C)
The above example calculates RTD tolerances at reference and indicated temperatures and determines the tolerance status of RTDs.