RTD Temperature Coefficient Calculation

What is the coefficient factor of RTD?

• Resistance Temperature Detectors (RTDs), also known as platinum resistance thermometers (PRTs), are widely used temperature sensors that rely on the principle that the resistance of a metal, in this case, platinum, increases with temperature. 
• RTDs functions according to the theory that the resistance of a metal will increase as temperatures increase. 
• The Temperature Coefficient of Resistance (TCR), denoted by αo, is a crucial parameter for RTDs. 
• It represents the average resistance change per degree Celsius over a specified temperature range, usually 0°C to 100°C, divided by the resistance of the RTD, Ro, at 0°C.

How is RTD coefficient calculated?

Here’s a step-by-step explanation of how to calculate the TCR for a PT100 RTD, along with an example calculation:

α_o = (R₁₀₀−R₀) / (R₀ X 100^oC)   – Equation no. 1

R0 is the resistance of the RTD at zero degrees Celsius (ohm).

R100 is the resistance (ohm) of the RTD at 100 °C.

Note: For the purpose of this conversation, we will only be referring to RTD PT100.

The relationship between resistance (Rt) and temperature (t) for a PT100 RTD is described by the following formula:

R_t= R₀(1+α_o.t)   – Equation no. 2

Where:

R_t  is the RTD‘s output resistance in Ohms at temperature t.

R₀ is the resistance of the RTD at 0°C (ohms).

α_o is the temperature coefficient of resistance (TCR) at 0°C (per °C).

t is the temperature in degrees Celsius.

Example Calculation

Platinum RTD PT100 has a resistance of 100 ohms at 0 degrees Celsius and 139.1 ohms at 100 degrees Celsius.

1. Find out how much resistance the RTD has at a temperature of 60 degrees Celsius.
2. Perform the TCR (Temperature Coefficient of Resistance) calculation for platinum.
3. Determine the temperature at which the resistance is 120 degrees Celsius.

What is temperature coefficient of Pt100?

Perform the Temperature Coefficient Calculation on the RTD PT100.

From Equation no.1:

α_o =  (R₁₀₀−R₀) / (R₀ X 100^oC) 

α_o = (139.1 -100) / (100 X 100)

α_o = 0.00391 per degrees Celsius 

Determine the RTD’s resistance at 60°C

From Equation no.2:

R_t= R₀(1+α_o.t) 

R₆₀ = R_o(1 + αt) 

R₆₀ = 100(1 + 0.00391×60) 

R₆₀ = 123.46Ω

Determine the temperature at which the resistance is 120

From Equation no.2 :

R₁₂₀ = R_o(1 + αt) 

120 = 100(1 + 0.00391t)

1 + 0.00391t =120/100

0.00391t = 1.2-1

t = 0.2/0.00392

t = 51.02 degrees Celsius 

Refer the link for RTD Calculator: Converting Process Temperature to measured Output Resistance.

Refer the link for RTD Calculator: Converting Measured Resistance to Process Temperature.

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