Pt100 sensors (platinum 100 Ω (PRTD)) are more linear than thermocouples:

Figure 6 Change in the coefficient of linearity of a type S TC and a Pt100

Figure 6 Change in the coefficient of linearity of a type S TC and a Pt100 The relationship between the temperature and the ohm value of the RTDs was calculated by Callendar, then later refined by Van Dusen, **which is why it is known as the Callendar-Van Dusen (CVD) equation.**

$$ {R_T} = {R_0} + {R_0}\alpha \left[ {T – \delta \left( {\frac{T}{{100}} – 1} \right)\left( {\frac{T}{{100}}} \right) – \beta \left( {\frac{T}{{100}} – 1} \right)\left( {\frac{{{T^3}}}{{100}}} \right)} \right] $$

Where RT = resistance at T°C, R0 = resistance at 0°C, α = temperature coefficient at 0°C in Ω/Ω/°C, δ = coefficient of linearisation, β = second coefficient of linearisation for negative temperature values (β = 0 for T > 0°C).

This equation has been converted to enable it to be used more easily with the coefficients A, B and C given by the standard DIN 43760 (IEC 751) and the data sheets of the components:

$$ {R_T} = {R_0}\left[ {1 + AT + B{T^2} – C\left( {T – 100} \right){T^3}} \right] $$

C=0 for T>0°C.

Coefficients for different α values | |||

Coefficient | Value | Value | Value |

α | 0,003850 | 0,003926 | 0,003911 |

δ | 1,4999 | ||

β | 0,10863 | ||

A | 3,9083e^{-3} | 3,9848e^{-3} | 3,9692e^{-3} |

B | -5,775e^{-7} | -5,870e^{-7} | -5,8495e^{-7} |

C | -4,18301e^{-12} | -4,000e^{-12} | -4,2325e^{-12} |

These three α values represent the three main specifications for RTDs.

- 0.03850 Ω/Ω/°C: Standards DIN 43760, IEC 751 and other international specifications, called the European Standard.
- 0.003926 Ω/Ω/°C: Requires platinum of 99.999% purity or better, called the American Standard.
- 0.3911 Ω/Ω/°C: Often called the US Industrial Standard.