How to use the pt100 rtd temperature calculator
- Choose Temperature from ohms or Ohms from temperature, then the sensor: Pt100, Pt500, Pt1000 or another resistance at 0 degrees C.
- Enter the measured resistance or the temperature and the temperature unit.
- For a two wire sensor, enter the total resistance of the leads in Advanced options so that it is subtracted.
- Read the temperature in Celsius, Fahrenheit and kelvin, the sensor resistance and how many ohms it changes per degree.
Formula
Above 0 C: R = R0 x (1 + A t + B t^2)
Below 0 C: R = R0 x (1 + A t + B t^2 + C (t - 100) t^3)
A = 3.9083e-3, B = -5.775e-7, C = -4.183e-12
Inverse above 0 C: t = (-A + sqrt(A^2 - 4 B (1 - R / R0))) / (2 B)
These are the Callendar-Van Dusen coefficients from the IEC 60751 standard for platinum sensors. Below 0 C the page finds the temperature by repeated tangent steps (Newton's method).
The reference points of a Pt100
A Pt100 reads 100 ohms at 0 C and 138.5055 ohms at 100 C, while 175.856 ohms corresponds to 200 C and 60.2558 ohms to -100 C. A reading of 138.51 ohms is 100.01 degrees C, and the sensor changes by 0.37928 ohm per degree there.
A Pt1000 reads 1097.35 ohms at 25 C. If a two wire Pt100 shows 138.51 ohms and the leads add up to 0.3 ohm, the sensor is at 138.21 ohms, which is 99.221 degrees C.
Pt100, Pt500 and Pt1000
The number is the resistance at 0 degrees C. All three follow the same curve scaled by that number, so a Pt1000 has ten times the resistance and ten times the change per degree. The larger value makes lead wire resistance matter less, which is why two wire circuits often use Pt1000.
Resistance is read with a divider or bridge, which you can explore with the voltage divider calculator and the Ohm's law calculator. Humidity reasoning with temperature is covered by the dew point calculator.
Wires and accuracy
In a two wire connection the resistance of the leads adds to the sensor, which reads as warmer. At about 0.39 ohm per degree for a Pt100, each 0.39 ohm of lead resistance is a degree of error. Three and four wire connections cancel the leads. A real sensor also has a tolerance class and can warm itself with the measuring current, which this page does not include.
Assumptions and limits
- The sensor follows the standard platinum curve with the IEC 60751 coefficients, for temperatures from -200 to 850 degrees C.
- The lead resistance you enter is the total of both leads and is subtracted from the measurement as is.
- Tolerance class, self heating and long term drift are not modeled.
- A resistance outside the range of the sensor is refused rather than guessed.
Frequently asked questions
What is the resistance of a Pt100 at 100 degrees C?
It is 138.5055 ohms, using the standard coefficients. The IEC table rounds it to 138.51 ohms.
How do I convert Pt100 resistance to temperature?
Divide the reading by 100, then solve the quadratic R/R0 = 1 + A t + B t squared for t above 0 degrees C. The page does this and uses an iterative method below zero.
What is the difference between Pt100 and Pt1000?
Only the scale: a Pt1000 has 1,000 ohms at 0 C instead of 100 and changes ten times as much per degree. The curve has the same shape.
Why does a two wire sensor read high?
The leads are in series with the sensor, so their resistance adds to the reading. Enter the total lead resistance and the page subtracts it, or use a three or four wire connection.