Accurate temperature measurement doesn’t start and end with the sensor.
In many manufacturing environments, a PT100 temperature sensor may appear to be working perfectly.
The PLC displays the correct value, the HMI shows no alarms, production continues as normal, but that doesn’t necessarily mean the process temperature is accurate.
Small errors anywhere within the measurement loop can influence process control, product quality and energy efficiency long before anyone notices.That’s why full-loop PT100 calibration is considered best practice for critical temperature measurement applications.
Rather than testing the sensor alone, full-loop calibration verifies the entire measurement chain—from the sensing element through to the value displayed on the operator screen.
What Is a Full-Loop PT100 Calibration?
A full-loop calibration verifies every component involved in measuring and displaying temperature.
Instead of checking only the RTD sensor, the calibration confirms the accuracy of:
- The PT100 temperature sensor
- Extension leads and wiring
- Temperature transmitter
- 4–20mA signal
- PLC scaling
- HMI display
The objective is simple, to ensure the temperature shown to the operator is the actual temperature occurring within the process.
Why Bench Testing Isn’t Always Enough
A laboratory calibration confirms that a sensor is accurate under controlled conditions.
However, once installed on site, several additional factors can affect the final measurement.
Examples include:
- Wiring resistance
- Transmitter drift
- PLC scaling errors
- Signal conversion issues
- Incorrect HMI configuration
If these elements aren’t verified together, the displayed temperature may differ from the actual process temperature.
For many industrial processes, even a small temperature deviation can influence quality, consistency and production performance.
Common Problems Found During Full-Loop Calibration
Many temperature measurement issues don’t generate alarms.
Instead, they quietly affect production over time.
Common examples include:
- Temperature transmitter drift
- Incorrect PLC scaling
- Sensor ageing
- Poor probe installation
- Delayed response caused by excessive transmitter damping
- Wiring faults
These issues can remain hidden until a full-loop calibration is performed.
How a Full-Loop PT100 Calibration Is Performed
1. Preparing the Sensor
The first step is ensuring the sensor is installed correctly within the dry-block calibrator.
Correct immersion depth is critical.
If the probe isn’t inserted deeply enough, ambient air can influence the measurement and create inaccurate results.
A correctly sized insert also ensures good heat transfer between the dry block and the sensor.
2. Verifying the Complete Measurement Loop
Unlike a simple sensor calibration, full-loop testing focuses on the value displayed by the control system.
This means comparing the reference temperature from the dry-block with the temperature displayed on the HMI after the entire signal has passed through the transmitter and PLC.
The objective is to verify that both values match within the required tolerance.
3. Performing a Three-Point Calibration
A three-point calibration is widely used to verify measurement accuracy across the complete operating range.
Testing is typically performed at:
- Lower Range Value (LRV)
- Mid-point
- Upper Range Value (URV)
This confirms both zero accuracy and linearity across the full temperature range.
Finding the Source of Measurement Errors
If the displayed value differs from the reference temperature, the sensor isn’t always responsible.
The error could originate from several locations within the loop, including:
- The PT100 sensor
- The temperature transmitter
- Signal wiring
- PLC scaling
- HMI configuration
A systematic approach allows each component to be verified before adjustments are made.
Best Practices for Reliable Temperature Calibration
Consistent results depend on more than the calibration equipment. Good practice includes:
- Using the correct dry-block insert
- Ensuring sufficient probe immersion
- Allowing the system to stabilise fully before recording readings
- Recording “as-found” measurements before making adjustments
- Investigating the physical cause of errors rather than masking them in software
- Maintaining complete calibration records for traceability and compliance
These simple steps help improve measurement confidence while supporting long-term equipment reliability.
Why Full-Loop Calibration Supports Better Process Performance
Reliable temperature measurement supports:
- Improved product quality
- More consistent process control
- Reduced waste
- Greater confidence in production data
- Better compliance and audit readiness
- Early identification of instrumentation issues
For critical manufacturing processes, calibrating the complete loop provides greater assurance than testing individual components alone.
PT100 Calibration Services from BONNER
For more than 45 years, BONNER has supported manufacturers with temperature calibration, instrumentation and automation services across a wide range of industries.
Our engineers perform full-loop PT100 calibration using certified equipment to verify the complete measurement chain—from the sensor through to the PLC and HMI, helping ensure your process data is accurate, reliable and traceable.
If you’re reviewing your temperature calibration programme or would like to discuss improving measurement confidence across your facility, we’d be happy to help.