Platinum properties for temperature sensors

The function of a Pt temperature sensor or heating element is based on the temperature dependence of the electrical resistance of the platinum metal. The relationship can be described by the following characteristic polynomial:

RT = Ro (1+At+Bt2)

Where RT = Measured resistance and Ro = Nominal resistance at 0°C.
For temperatures below 0 Deg C, the equation is R(t) = R0*(1+A*t+B*t²+C*(t-100°C)*t³)

The constants are defined in the international standards for Pt temperature sensors;

• A = 3.9083*10-3 °C-1
• B = -5.775*10-7 °C-2
• C = -4.183*10-12 °C-4

B is so small that for most applications a linear relationship between RT and the temperature can be assumed.

Pt temperature sensors with high nominal resistances have a higher sensitivity than those with lower nominal resistances, because the slope of the characteristic is directly proportional to Ro.

Depending on the substances and processes used in the manufacture of the Pt temperature sensors, slight specific deviations from the ideal constants and the optimum characteristic can occur. These deviations define the working temperature range and the accuracy tolerance class for each sensor type.

Temperature Coefficient

The temperature coefficient (TC or α) is positive and defined as: TC = (R100-Ro)/(100 * Ro).

This represents the slope of the linear approximation of the characteristic polynomial between 0°C and 100°C. The standard DIN EN 60751 for Pt temperature sensors specifies a TC of 0.003850/°C. This TC value applies to all the products described here.

In addition, customized sensors with temperature coefficients of 0.003750/°C and 0.003500/°C as well as other intermediate TC values are available, e.g. according to the JIS standard.

Yageo Nexensos supplies Platinum RTD Elements with the following temperature coefficients: 3850ppm, 3750ppm (Pt1000 only) and 3370ppm (Pt200 for automative only).

Accuracy tolerance classification

Kamet supplies platinum thin-film sensors in the accuracy tolerance classifications B, A, 1/3 DIN and 1/10 DIN. Tolerances of basic values for platinum temperature sensors are specified in DIN EN 60751. The following applies:

• Class B: Δt=±(0.3°C + 0.005 |T|)
• Class A: Δt=±(0.15°C + 0.002 |T|)
• Class 1/3 DIN: Δt=±1/3 (0.3°C + 0.005 |T|)
• Class 1/10DIN: Δt=±1/10 (0.3°C + 0.005 |T|)

T = Temperature in °C of your specific operating temperature

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