To continue our discuss on calibration of temperature sensors in the process industry, we are going to move on to dry-block calibrators.
Dry-block calibrators incorporate a heating/cooling element, but do not require the use of any liquids such as oil to perform their calibration tasks. The units typically consist of a heating “block”, an internal sensor, and some type of control to reach and maintain desired temperature ranges. Some advanced dry-blocks also have internal reference sensors and associated accuracies thus eliminating the need for an external reference sensor. Because there is no oil or liquids involved, dry-block calibrators are much less messy, and far more portable. Most dry-blocks can easily be transported throughout the plant in a carrying case or on a portable cart. Perhaps one of the best advantages of a dry-block calibrator is the heat/cool tiem to desired temperatures. Dry-blocks reach temperature much faster than traditional liquid baths, thus saving time (and money)whithin the calibration process. While dry-block calibrators have the edge in portability and speed, traditionally the dry-blocks have been less accurate and less stable than the liquid bast counterparts. Recent technology introductions have dramatically improved the performance of dry-blocks in both accuracy and stability. There are some dry-blocks that have the accuracy via external reference sensors where the dry-block can be used without any external reference sensors or readouts. As sensors are added to the dry-block calibrator, there is an opportunity for heat loss (due to loading effect). Again, technology has advanced where the use of Dynamic Load Compensation; the heat loss is minimized throughout the process.
Electronic Calibrators
In some cases, electronic calibrators can be used to calibrate temperature sensors within the plant. These devices can simulate the sensors under test (SUT) and provide readout results on the sensor’s performance. However, the integrity of the sensor is not tested as there is no temperature source to subject the sensor to. These types of calibrators are typically much more portable and less expensive than traditional liquid baths or dry-block calibrators.
Advanced Features of Temperature Calibrators
As referenced above, technology has brought many advances in the temperature calibration arena. Some of these advances include time savings in terms of time to temperature, auto stepping functions, and automatic tests such as switch tests which show each change of state and the hysteresis. Because temperature measurements can be a lengthy process (it takes time to reach temperature point, provide stability, and typically a test consist of 3-5 temperature points), sometimes automation of the calibration process can be obtained. Using associated software, some temperature calibrators can automate the process completely to include as found/as left results and a calibration test certificate. This is often advantageous where many measurements are being done, and allows for increased productivity on the technicians part.
Specifying the right temperature calibrator
In all applications, specifying the right temperature calibrator can come down to several basic criteria. These criteria include:
- Accuracy required. Typically, accuracy of the calibrator should be at least four times that of the SUT where possible.
- Stability and uniformity of the temperature calibrator. These factors will lead to the overall accuracy of any reading performed.
- Determine what types of sensors will need calibration. What size, shape and temperature range? In some cases, there may need to be multiple calibrators to reach the full range of the SUT.
- What is your budget? Typically the cost of the calibrators increase as functionality and advanced features are added into the equation. Additionally, what are the re-calibration costs of the unit? If being used as a calibrator, re-calibration will need to be considered on a regularly scheduled basis.
- What level of portability is required? Will the calibrator be primarily used in the laboratory or instrument show, or will the unit need to be transported to remote sites within the plant or plants?
- What level of automation is desired? Sometimes the ability to automate can offset the cost of a more advanced calibrator, especially if technician productivity can be increased dramatically.
Conclusion
The calibration of temperature sensors in a plant or R&D laboratory can be an important element in maintaining quality control and consistent temperature measurements. Often times regulatory or quality systems dictate that all temperature sensors can be calibrated on regularly scheduled intervals. The tools available to calibrate sensors range in terms of accuracy, cost, portability and speed. The user should first define what the sensors under test look like and what criteria is required to calibrate prior to specifying and procuring a temperature calibrator.
About the author: Keith Hadley is a Regional Manager with the Calibration Division of Ametek and has been in the calibration and calibration equipment industry for over 30 years. He has completed the course work for lead auditing in an ISO-9000 environment and been involved in calibration programs for both pharmaceutical and process control environments.