Accurate measurements and consistency in production that means users will want to regularly calibrate sensors and ensure they work properly.
Learning Objectives
- Understand how digital sensor equipment is critical when it comes to plant efficiency.
- Learn how the “set it and forget it” method is not recommended because users can lose critical information.
- Observe that following manufacturer policies and procedures specific to an industry can help guide users.
Digital sensor equipment is critical when it comes to efficiency in a plant but that same equipment, when subjected to external environmental factors can lose precision when exposed to various plant conditions. To ensure accurate measurements and consistency in production that means users will want to regularly calibrate (mark a sensor, gauge or instrument with a standard scale of readings) the instruments being used, but when it comes to finding a regular cadence for sensor calibration, users should start with the basics.
The first thing to do when looking at any control systems from a calibration standpoint would be to locate and document all the instruments (sensors) in the system and determine their impact. Does the sensor impact safety, quality of the end product, or regulatory requirements or all of the above? If the answer is yes, the instrument needs to be calibrated.
Caution against “Set it and forget it” method
As is the case with almost any production schedule, planning is key. If a user is working with a critical process that requires very little to no downtime, replacing instruments in scheduled downtimes will be crucial. There aren’t too many sensors that fall into a category of “set it and forget it.” Some simpler sensors such as proximity sensors, photo eyes, and limit switches would fall under a category of not needing to be calibrated so the “set it,” direction applies. This does not mean they can be forgotten in an environment focused on quality and safe production “forget it” should just not apply. Periodic testing of irregular operations and safety functions should be included in maintenance/downtime schedules.
Most instruments will have a mean time to failure (MTTF) or some equivalent rating on them from the factory (usually hidden somewhere deep inside the manuals). This will give users an approximation of when users can expect the instrument to fail based on hours of operation or number of times cycled. Users should use these numbers to do preventive maintenance and replace critical instruments before they fail.
Any sensors exposed to elements including temperature shifts, vibrations, changes in humidity or moisture levels (reducing the instruments MTTF) deserve additional attention and that awareness should happen periodically to prevent a shutdown from complete instrument failure. Sensors also can experience drift (change in its output due to stresses placed on it) because of these elements and require calibration. To keep the plant running at peak efficiency while keeping operations and maintenance staff sane, setting up a regular cadence to calibrate makes sense. There are really two effective ways to execute this important task.
Timed interval calibration
There are multiple ways to go about calibration intervals. The first is simple, and a lot of companies use it for most of their instrument calibrations. Timed intervals of 1 month, 6, months, yearly, after every batch, etc. can be set and logged at regular intervals for calibration. This method is highly effective for cadenced calibration and for most instruments this will adequately meet their calibration requirements.
Trend analysis calibration
The more effective and efficient way of determining an instruments calibration needs is through a history/trend analysis. This process takes more time and attention to develop. Start by calibrating an instrument at timed intervals and increasing the time until users find the instrument has drifted out of its calibrated range. Based on that information, identify the interval that keeps the instrument constantly in its calibrated range.
Users also can use this method to find instruments that have higher failure rates and fix issues with those instruments. Because the nature of this method requires keen attention to scheduling and logistics that can easily be lost in a flurry of daily production issues, it might be wise to rely on a software package to help ensure consistency. Calibration software packages can help manage these intervals as well as keep records of calibrations for quality and regulatory purposes.
Reliability and verification
Obviously, the reliability of various measurements are based on the calibration meters used to calibrate the instruments. Verifying the accuracy of the instrument measurements at required intervals is an important step that unfortunately many operators and production facilities overlook. Having meters verified by a laboratory with ISO/ISA17025, ISO9000/9001, and NIST national/international measurement standards helps ensure users will take more accurate and reliable measurements when doing calibrations.
No process is 100% without inconsistency risk so even while calibrating an instrument with a verified meter on a consistent basis, users can still expect to run into some uncertainties. That’s when the fun math begins. Measurements and calculations that determine the test uncertainty ratio (TUR) in a calibration measurement will allow users to know if the measurement will be accurate enough to be within required calibration limits of the unit under test (UUT). Generally speaking, a TUR of 4:1 (calibration meter is four times more accurate than the UUT) will ensure thatcalibrations are traceable to NIST standards and fulfill all regulatory compliance.
Calibration best practices, regulations
Users will want to follow companies’ policies and procedures considering any regulatory or requirements specific to an industry. Working with a certified controls systems technician who can verify their calibration meters and records are compliant with organizational and regulatory standards allows companies to rest easy knowing that sensors are sharing accurate data. Ensuring teams are safe, equipment is running effectively and all quality standards are being upheld will give peace of mind while upholding the highest standards of quality and control.
Justin Palensky is a licensed electrician, certified controls systems technician (CCST), Huffman Engineering Inc. Huffman Engineering is a 2022 recipient of the System Integrator of the Year Award and CSIA member; CSIA is a CFE Media content partner. Edited by Chris Vavra, web content manager, Control Engineering, CFE Media and Technology, [email protected].
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Keywords: sensor calibration, process sensors
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