If you are holding a PT100 probe and a multimeter, the baseline fact you need is this: a standard PT100 RTD (Resistance Temperature Detector) outputs exactly 100.00 Ω at 0°C, scaling at roughly 0.385 Ω per degree Celsius. Unlike thermocouples that generate millivolt potentials, RTDs are passive resistive devices. To find the temperature, you measure the resistance and cross-reference it against a standardized lookup table. The global authority for this curve is the IEC 60751 standard (also adopted as DIN EN 60751), which dictates the Callendar-Van Dusen equation for platinum resistance.
The Master PT100 Ohm Temperature Chart (IEC 60751)
Before using the table below, understand how to read the columns. The Nominal Ohms column assumes a pure platinum sensor with an alpha coefficient (α) of 0.00385 Ω/Ω/°C. The tolerance columns are expressed in ohms, not degrees. To find the maximum temperature error in Celsius, divide the ohm tolerance by the approximate sensitivity (0.385 Ω/°C). Bookmark this section for quick bench lookups across the standard -50°C to +150°C operating range.
| Temperature (°C) | Nominal Resistance (Ω) | Class B (F0.3) Tolerance (±Ω) | Class A (F0.15) Tolerance (±Ω) |
|---|---|---|---|
| -50 | 80.31 | ±0.45 | ±0.25 |
| -25 | 90.20 | ±0.35 | ±0.20 |
| 0 | 100.00 | ±0.30 | ±0.15 |
| 25 | 109.73 | ±0.35 | ±0.20 |
| 50 | 119.40 | ±0.45 | ±0.25 |
| 75 | 128.99 | ±0.55 | ±0.30 |
| 100 | 138.51 | ±0.65 | ±0.35 |
| 125 | 147.96 | ±0.75 | ±0.40 |
| 150 | 157.33 | ±0.85 | ±0.45 |
Which Tolerance Column Applies to Your Installation?
Choosing between Class B and Class A (or tighter variants) dictates both your hardware cost and your wiring topology. According to Omega Engineering's RTD reference guidelines, the tolerance class defines the baseline accuracy at 0°C and the slope of the error band as temperature changes.
Class A (F0.15): Required for food processing, pharmaceutical, and laboratory applications. It guarantees ±0.15 Ω (±0.4°C) at 0°C. Class A sensors demand 3-wire or 4-wire connections and high-quality termination blocks to prevent thermal EMF errors.
1/3 DIN (F0.1): A non-IEC but widely adopted industry standard that restricts the tolerance to one-third of the Class B curve (±0.10 Ω at 0°C). Use this only with 4-wire Kelvin measurement setups or precision RTD transmitters like the PT100-to-I2C MAX31865 breakout boards.
Derating Accuracy and Compensating for Lead Resistance
A common point of confusion on the bench is how derating rows modify the base value. In RTD terminology, "derating" applies in two distinct ways: tolerance expansion and lead-wire offset.
1. Tolerance Band Derating
The base tolerance (e.g., ±0.15 Ω for Class A) is only valid at exactly 0°C. As you look at the derating rows moving toward the extremes of the chart, the physical tolerance band widens. At 150°C, a Class A sensor’s tolerance derates to ±0.45 Ω. This happens because minor impurities in the platinum wire cause non-linear deviations from the ideal Callendar-Van Dusen curve at elevated thermal energies. If your process runs constantly at 140°C, your Class A sensor is effectively performing with a ±1.1°C error margin, not the ±0.4°C printed on the box.
2. Measured Ohm Derating (The 2-Wire Trap)
If you are using a 2-wire RTD setup, the resistance of your copper lead wires adds directly to the sensor's resistance. You must derate your measured value by subtracting the lead resistance before consulting the nominal column.
Worked Example: You measure 112.50 Ω on a 2-wire PT100 Class B sensor using 15 meters of 24 AWG copper lead wire.
- 24 AWG copper has a resistance of roughly 84.2 mΩ/m.
- The 15-meter run requires a 30-meter round trip (out and back).
- 30m × 0.0842 Ω/m = 2.52 Ω of lead resistance.
- Derated Measurement: 112.50 Ω - 2.52 Ω = 109.98 Ω.
Looking up 109.98 Ω on the chart gives a true temperature of roughly 25.6°C. If you had failed to derate the lead resistance and looked up 112.50 Ω directly, the chart would indicate 32.4°C—a massive 6.8°C error caused entirely by ignoring the copper wires.
This is exactly why 3-wire and 4-wire configurations exist. A 3-wire setup allows the transmitter to measure the resistance of one lead and mathematically subtract it from the total loop, while a 4-wire setup uses a Kelvin connection to eliminate lead resistance from the measurement circuit entirely.
What the Table Cannot Tell You
The IEC 60751 chart assumes an ideal, perfectly submerged sensor in a thermally stable environment. In the real world, three physical phenomena will cause your multimeter reading to diverge from the chart, regardless of your tolerance class.
- Self-Heating Errors: RTDs require an excitation current to measure resistance. If your transmitter or ohmmeter pushes more than 1 mA through a PT100, the I²R power dissipation will physically heat the platinum element. In a stagnant gas environment (like an air duct), a 2 mA excitation current can cause 0.5°C to 1.0°C of self-heating error. Always verify your transmitter's excitation current; modern precision units keep it below 0.5 mA.
- Thermal Shunting: The metal sheath of the RTD probe conducts heat. If you insert a 6mm stainless steel probe into a small-diameter plastic pipe carrying cold water, the probe acts as a heatsink, drawing ambient room heat down the sheath and into the sensing element. The chart will report a temperature higher than the actual fluid. Mitigate this by ensuring the immersion depth is at least 10 to 15 times the probe diameter.
- Strain and Vibration Degradation: Platinum wire is incredibly fragile. If your RTD is mounted on a vibrating compressor manifold, the mechanical shock can stretch the wire or break the internal ceramic potting. This introduces a permanent, creeping offset to the base 100.00 Ω value. If your 0°C ice-bath calibration suddenly reads 100.40 Ω and the error is consistent across the chart, the sensor has suffered mechanical strain and must be replaced.
