RTD’s are based on the principle that the resistance of a metal increases with temperature. The temperature coefficient of resistance (TCR) for resistance temperature detectors (denoted by αo), is normally defined as the average resistance change per °C over the range 0 °C to 100 °C, divided by the resistance of the RTD, Ro, at 0 °C.
where,
R0 = resistance of rtd at 0 °C (ohm), and
R100 = resistance of rtd at 100 °C (ohm),
Note: Here we are discussing about RTD PT100 only.
As a first approximation, the relationship between resistance and temperature, may then be expressed as (see Figure 2):
where: Rt = resistance of rtd at temperature t (ohm),
Ro = resistance of rtd at 0 °C (ohm), and
αo = temperature coefficient of resistance (TCR) at 0 °C (per °C)
Example
A platinum RTD PT100 measures 100 Ω at 0 °C and 139.1 Ω at 100 °C.
calculate the resistance of the RTD at 50 °C.
Calculate the TCR for platinum.
calculate the temperature when the resistance is 110 Ω.
Calculate the Temperature Coefficient of RTD PT100
From Equation – 1 :
Calculate the resistance of the RTD at 50 °C
From Equation – 2 :
R50 = Ro(1 + αt) = 100(1 + 0.00391×50) = 119.55Ω
Calculate the temperature when the resistance is 110 ohms
From Equation – 2:
Rt = Ro(1 + αt) ⇒ 110 = 100(1 + 0.00391t)
Rt =1 + 0.00391t = 1.1 ⇒ 0.00391t = 0.1 ⇒ t = 25.58 °C.
Also Read: Thermocouple Working Principle