What are the tolerances and the standards?
The European standard, also known as the DIN or IEC standard, is considered the world-wide standard for Pt100. This standard, DIN/IEC 60751 (or simply IEC751), requires the RTD to have an electrical resistance of 100.00 Ω at 0°C and a temperature coefficient of resistance (alpha) of 0.00385 Ω/Ω/°C between 0 and 100°C.
There are actually four resistance tolerances specified in DIN/IEC751:2008. These tolerances apply for thermometers of any value of R0 (you can download tables here below):
Class AA = ± ( 0.10 + 0.0017 * | t | ) °C
Class A = ± ( 0.15 + 0.0020 * | t | ) °C
Class B = ± ( 0.30 + 0.0050 * | t | ) °C
Class C = ± ( 0.60 + 0.0100 * | t | ) °C
The combination of resistance tolerance and temperature coefficient define the resistance vs. temperature characteristics for the RTD sensor. The larger the element tolerance, the more the sensor will deviate from a generalized curve, and the more variation there will be from sensor to sensor (interchangeability). This is important for users who need to change or replace sensors and want to minimize interchangeability errors.