Automating and Documenting Temperature Transmitter & Sensor Calibrations

By Jim Shields, Project Manager - Process Calibration, Fluke Temperature transmitter calibrations are often performed without taking into account temperature sensor performance.   For many processes, this has been an acceptable practice, although the sensor typically contributes more errors than the transmitter.  This practice can be problematic for critical process measurements or those that need a higher degree of confidence or accuracy.  Testing the transmitter and its respective process sensor together provides a more complete test. If performed with the proper test equipment, error contributions from the sensor can be minimized by matching the device to the transmitter electronics.  

Temperature Testing Pro Tips

  • Resistance Temperature Detectors (RTDs) are almost always more accurate than thermocouples (TCs). As long as the temperature measured is within the range of the RTDs, they are a better choice when accuracy matters.
  • Thermocouples have a wider temperature range and are more durable than RTDs.
  • Thermocouples are a good choice for applications with rough service that experience destructive vibrations and repeated temperature cycling.
  • Above ambient, dry-well calibrators stabilize at temperature faster with increasing temperatures than decreasing temperatures.
  • If the stabilization time for the dry-well is difficult to estimate, consider selecting “Manual Test” on the 754 and wait for the temperature to stabilize before recording the measurement.
  • HART smart transmitters with RTD probe inputs may allow for entry of the probe’s certification constants. By inputting these constants, the sensor is matched and measurement system errors are minimized.

A dry-well calibrator is used with a Fluke 754 to automatically test a transmitter with sensor. 

How to perform a test:

To use a dry-well calibrator, such as a Fluke 754, to automatically test a transmitter with sensor: Step 1: Remove the process measurement sensor and install into the dry-well temperature calibrator. Step 2: Connect the mA measurement jacks of the 754 to the transmitter and connect the dry-well communication cable between the 754 and the dry-well. Step 3: Press HART to query the transmitter for its configuration. Step 4: Enable Loop Power on the 754 as required. Step 5: Press HART again and configure the calibrator for the test; select “Measure mA, Source Dry-Well.” Step 6: Select “As-Found” to configure for documenting the test. Be sure to set the delay time long enough to allow the dry-well to change temperatures and stabilize. Step 7: Record the As-Found test either using Auto-Test, with the settling time for the dry-well in the test delay time, or manually while observing for stable test temperatures. Step 8: After the As-Found test select “Adjust” and select “Yes” when prompted to use a dry-well for adjusting the transmitter input. The applied temperature and adjustment will adjust the transmitter input block to output the correct measurement by adjusting the transmitter and sensor’s output together. Step 9: After adjusting the input with sensor, adjust the transmitter mA output using “Output Trim.” Step 10: After adjustment is completed, record the post adjustment of the transmitter As-Left and errors of the input. Sensor and output errors of the transmitter should be nominalized, resulting in improved temperature measurement performance.

Fluke 754 and Fluke Calibration dry-well calibrating a 4-20 mA transmitter as well as a temperature sensor and probe.

About the Author Jim Shields is a product manager for the Process Calibration group for the Fluke Corporation. He has worked in the field of electrical, temperature and pressure measurement for more than 25 years. source:automation.com