Platinum sensors have established themselves as resistance thermometers.
The correlation between temperature and resistance in platinum thermometers is not proportional but is described by a higher order polynomial.
With:
= Resistance of the thermometer
= Resistance of the thermometer at 0 °C
= Individual parameters of the thermometer or norm
= Temperature
The equation is also called the Callendar-Van-Dusen Equation (cvd) and has been used since the 1920s.
The characteristic curve is standardized in DIN EN 60751, which describes industrial platinum resistance thermometers and platinum temperature sensors. It was first published in 1996 and is valid today with its last revision as DIN EN 60751:2009-05.
In the standard, the characteristic curve is divided into two temperature ranges, from -200 °C to 0 °C (shown in blue) and from 0 °C to 850 °C (shown in red).
In order to better classify the measuring resistors, the so-called nominal resistance R0 was introduced in the standard. It describes the nominal resistance of the temperature sensor at 0 °C. For example, a Pt 100 temperature sensor at 0 °C has an electrical resistance of 100 Ohm. The following nominal resistances are mentioned in the norm:
Pt 10 = 10 Ohm at 0 °C
Pt 100 = 100 Ohm at 0 °C
Pt 500 = 500 Ohm at 0 °C
Pt 1.000 = 1.000 Ohm at 0 °C
To achieve the defined nominal resistance, the length or diameter of the platinum wire in the measuring resistor is adjusted. This changes not only the resistance but also the sensitivity of the sensors:
Pt 10 = 0,04 Ohm / K
Pt 100 = 0,4 Ohm / K
Pt 500 = 2 Ohm / K
Pt 1.000 = 4 Ohm / K
In addition to these standards, precision thermometers also use different nominal resistances such as Pt 25, Pt 2.5 or Pt 0.25. Many of these precision thermometers meet the requirements of ITS-90 and are then called SPRT or standard thermometers. For laboratory applications Pt 25 thermometers are often preferred. There are several reasons for this. Pt 25 often represents the best compromise between stability, sensitivity and self-heating.
The difference between the standardized thermometers and the normal thermometers according to ITS-90 is shown by the so-called temperature coefficient . It is defined in the standard by a resistance measurement at 0 °C and 100 °C:
With:
= Increment of the thermometer in 1/K
= Resistance at 100 °C in Ohm
= Resistance at 0 °C in Ohm
The -value of industrial temperature sensors according to the standard is 3.85 
. In contrast, the -value of standard thermometers according to ITS-90 is 3.92875 .. This value corresponds to the sensitivity of spectrally pure platinum in this temperature range.
