How to determine if a PT100 sensor is outside its tolerance range

The key to determining if a PT100 sensor is outside its tolerance range is to compare its measurement error at a specific temperature with the allowable error for its accuracy class specified in the international standard (IEC 60751). If the measured error is greater than the allowable value at that temperature, it is considered outside the tolerance range.

The tolerance of a PT100 sensor is not a fixed value, but a function of temperature. Therefore, it cannot be judged solely based on empirical values ​​at a single temperature point; it must be calculated and compared in conjunction with its nominal accuracy class (e.g., Class A or Class B) and the actual operating temperature.

I. Standard Clarification: IEC 60751 Tolerance Formula

The tolerance determination for all industrial PT100 sensors is based on the IEC 60751 standard. Common grades and their error formulas are as follows:

Where t is the absolute value of the actual temperature (unit: °C), for example, at 100 °C, |t| = 100.

II. Detailed Judgment Steps

1. Determine the sensor's nominal accuracy grade. Check the product nameplate, specification sheet, or purchase record to confirm it is Grade A, Grade B, or another grade.

Unless otherwise specified, it will be treated as Grade B by default (most commonly used in industry).

2. Obtain Calibration or Measured Data

Measure the actual resistance value of the PT100 at a known standard temperature (e.g., via a thermostat, freezing point flask, etc.),and convert it to the corresponding temperature value

For example: Under a standard 100℃ environment, the measured resistance is 138.85Ω, and the corresponding temperature found in the table is 101.0℃.

3. Calculate the measured temperature error

ΔT = T_measured - T_standard

ΔT = T_measured - T_standard

In the example above:

ΔT = 101.0 - 100 = + 1.0℃

ΔT = 101.0 - 100 = +1.0℃

4. Calculate the allowable error at this temperature point

Substitute into the corresponding tolerance formula:

For a Class B sensor, at 100℃:

Allowable error = ± (0.30 + 0.005 × 100) = ± 0.80℃

Allowable error = ±(0.30 + 0.005 × 100) = ±0.80℃

5. Comparison and judgment

Measured error: +1.0℃

Allowable range: [-0.80℃, [+0.80℃]

Because +1.0℃ > +0.80℃ → exceeds the tolerance range, it is judged as unqualified.

III. Quick Reference Table for Common Temperature Point Tolerances (Taking Grade B as an Example)

Table: Temperature (℃) Tolerance (Grade B) Maximum Tolerance Resistance Deviation (Ω)

0 ±0.30℃ ±0.12Ω

25 ±0.425℃ ±0.16Ω

50 ±0.55℃ ±0.21Ω

100 ±0.80℃ ±0.31Ω

200 ±1.30℃ ±0.50Ω

Tip: A multimeter can be used to measure the resistance on-site, and this table can be used to quickly determine whether the resistance is out of tolerance.

IV. Key Factors Affecting Judgment Accuracy

Even if the measured value is close to the critical point, the following interfering factors must be eliminated to avoid misjudgment:

Table Factor Impact Countermeasures

Lead Wire Resistance Two-wire connection introduces additional error. Use three-wire or four-wire measurement.

Insufficient Thermal Equilibrium Sensor not sufficiently heated. Wait for sufficient time (usually 5–10 minutes).

Insufficient Insertion Depth Temperature sensing element not fully entered the constant temperature zone. Insertion depth ≥ 150mm or ≥ 10 times the probe diameter.

Inaccurate Standard Source Large temperature fluctuations in the constant temperature bath. Use a calibrated standard platinum resistance thermometer as a reference.

V. Possible Causes and Handling Suggestions for Exceeding Tolerance

Table Cause Manifestations Suggestions

Prolonged High-Temperature Use Leading to Aging Significantly increased error in the high-temperature range. Shorten the calibration cycle and consider replacement.

Mechanical Damage or Vibration Fatigue Resistance jump, slow response. Check the protective tube and internal structure.

Contamination or Coking Slowed response, zero-point drift. Clean the probe and improve the installation environment.

Improper Calibration or Inaccurate Standard Systematic deviation. Send the sensor to a third-party metrology institution for verification. Once it is confirmed that the tolerance is exceeded, the sensor should be taken out of service and repaired, replaced or recalibrated, especially in critical temperature control systems (such as pharmaceuticals and battery management) to avoid production accidents.