Temperature Calibration Guide: Thermocouples, RTDs and Infrared Pyrometers

Temperature sensors are the most widely used instruments in process industries. From furnace control to chemical reaction monitoring, temperature directly affects product quality, energy efficiency and operational safety. However, over time sensors experience drift, which can produce incorrect readings if not periodically calibrated.

1. Preparing the calibration bench

Before starting any calibration you need a calibration bench (dry block or fluid bath), a reference standard traceable to national or international standards, and the appropriate cables and connectors for the sensor type.

The calibration bench must cover the sensor temperature range and offer stability of at least ±0.1 °C at the calibration point. Dry blocks are ideal for field use: portable, fast and fluid-free. Fluid baths offer better uniformity and are preferred in the laboratory.

Verify that all equipment is within its current calibration period. An expired reference instrument invalidates the entire calibration.

2. Point-by-point calibration procedure

Insert the unit under test and the reference standard together into the calibration block. Ensure both sensors are at the same depth and in thermal contact with the block.

Program the bench to the first calibration temperature and wait for thermal stabilization — time varies from 2 minutes (dry block, small sensors) to 15 minutes (fluid bath, large sensors). Once stable, record both the sensor reading and the reference reading.

Repeat for at least 3 to 5 points distributed across the measurement range. A typical pattern would be: 0%, 25%, 50%, 75% and 100% of range.

3. Thermocouples: cold junction compensation

Thermocouples measure the temperature difference between the measuring junction (hot junction) and the reference junction (cold junction) in the instrument. If cold junction compensation (CJC) is incorrect, the error equals the ambient temperature.

Verify that the measuring instrument applies correct CJC. Most modern thermocouple analyzers and calibrators incorporate CJC with ±0.5 °C accuracy. If using an external reference thermocouple with compensation thermistor, ensure it is properly calibrated.

4. RTDs (Pt100/Pt1000): lead wire verification

RTDs (Resistance Temperature Detectors) base their measurement on the change in platinum resistance with temperature. A common error is degradation of the connection lead wires, which increases resistance and produces readings higher than the actual value.

Before calibrating, verify the sensor resistance at 0 °C (should be 100.00 Ω ± tolerance for Pt100). If resistance is significantly higher, the cables may be damaged. Also verify the number of wires (2, 3 or 4): 4-wire configurations eliminate the effect of cable resistance.

5. Infrared pyrometers: emissivity configuration

Pyrometers measure temperature by detecting thermal radiation emitted by an object. The emissivity factor (ε) is the efficiency with which a material emits radiation, with values between 0 (perfect reflector) and 1 (ideal black body).

Incorrect emissivity adjustment is the most common source of error in pyrometers. Some typical values:

• Oxidized steel: ε = 0.85 - 0.95
• Polished aluminum: ε = 0.05 - 0.10
• Copper: ε = 0.07 - 0.15
• Refractory: ε = 0.85 - 0.95

To calibrate a pyrometer, ideally use a calibrated radiation source (blackbody calibrator). In the field, validate the reading using a known-emissivity tape applied on the measured surface.

6. Uncertainty calculation and documentation

Calibration uncertainty includes:

• Calibration bench uncertainty
• Reference standard uncertainty
• Instrument resolution uncertainty
• Thermal stabilization uncertainty
• Reading reproducibility

Document all calibration points, equipment used, ambient conditions and the combined expanded uncertainty (k=2, 95% confidence level). This certificate is the metrological traceability evidence required by quality standards such as ISO 9001, ISO 17025 and sector-specific standards.

7. When to calibrate?

Calibration intervals depend on process criticality, operating conditions and regulatory requirements. General guidelines:

• Heavy service (high temperatures, vibration, corrosion): every 6 months
• Moderate service (controlled processes, protected environments): every 12 months
• Light service (environmental monitoring, HVAC): every 12-24 months

Analyze the drift history of each instrument to adjust intervals: if a sensor consistently shows small drift, you can extend the interval. If drift is significant, shorten it.

Conclusion

Temperature calibration is an essential activity to ensure process quality, energy efficiency and regulatory compliance. Using correct procedures, traceable equipment and properly documenting results maintains measurement reliability and reduces costs associated with out-of-spec products.

MARPATECH SAC offers temperature calibration services with metrological traceability for thermocouples, RTDs and pyrometers, supporting the Peruvian, Argentine and Colombian industries with specialized technical support.