Calibration
calibration, where hysteresis is most significant. WHAT IS HYSTERESIS?
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Hysteresis is a phenomenon where the output of a system depends not only on its current input but also on its history of past inputs. In simpler terms, it means that a pressure sensor might not return to its original state after being subjected to varying pressures. This lag or difference can affect the accuracy of the measurements. For example, if you increase the pressure to a certain value and then decrease it back to the same value, the instrument might show a different reading compared to the initial one. This difference is hysteresis. For a practical example, if you calibrate a
100kPa pressure instrument at a 50kPa point, it may show 49.95kPa with increasing pressure. With decreasing pressure, at the same 50kPa point, it may show 50.0 kPa. This difference between 49.95kPa and 50.05kPa is caused by hysteresis.
HYSTERESIS IN PRESSURE CALIBRATION In the world of process instruments, hysteresis can have a significant impact on calibration. Pressure instruments – such as transmitters, sensors, and gauges – are expected to provide precise and repeatable readings. However, due to hysteresis, the readings can vary based on the instrument’s past pressure exposures. This can lead to errors and inconsistencies in your pressure measurements, which can be critical in processes where precision is key.
CAUSES OF HYSTERESIS IN PRESSURE INSTRUMENTS
Several factors can contribute to hysteresis in pressure instruments, such as:
Material properties: The materials used in the construction of pressure-sensing elements can cause hysteresis due to their inherent properties.
Design factors: The design and construction of pressure-sensing elements, including their mechanical components, can influence the level of hysteresis. Often in pressure sensors, the pressure stretches mechanical parts that can have a mechanical hysteresis, causing pressure measurement hysteresis.
Contamination: Dirt or other contaminants inside the instrument can cause hysteresis by obstructing the movement of mechanical parts, leading to inaccurate readings.
Environmental influences: Temperature changes, humidity, and other environmental conditions can affect the hysteresis behavior of pressure instruments.
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hile hysteresis can be found in various types of measurements, such as temperature and electrical signals, this article focuses on its impact on pressure
HYSTERESIS IN PRESSURE CALIBRATION
IDENTIFYING HYSTERESIS To manage hysteresis effectively, it is essential first to identify and measure it accurately. Here are some techniques:
Up and down calibration: Conduct calibration by increasing and decreasing the pressure to identify any differences in the readings at the same pressure points. Please note that if you do not wait long enough for the readings to stabilise, any delay or lag in the measurement instrument can look like hysteresis.
If you generate pressure with a hand pump, you need to be careful not to overshoot (or undershoot) when generating calibration points, or you may lose some of the hysteresis effect. For example, you need to approach the increasing points from below, and not overshoot and come back down.
Calibration cycles: Perform multiple calibration cycles to observe any discrepancies or repeatability issues in the readings. If there are any repeatability issues with the instrument, it may look like hysteresis. Therefore, it is good practice to perform several calibration repeats to reveal repeatability issues. Fully automated pressure calibration obviously makes it easier and saves time when performing multiple repeats.
Graphical analysis: Plotting the pressure input vs. output readings can help visualize hysteresis. It may be very difficult to see the hysteresis in numerical results. If you have a pressure calibrator that displays the calibration results in graphical format (such as a Beamex MC6 family calibrator), it is much easier to identify hysteresis.
Sending calibration results to calibration software also helps, as the software often offers graphical presentation results (at least Beamex Calibration Management Software does).
The image shows a simplified illustration of hysteresis. Increasing and decreasing pressure do not follow the same line - there is a clear difference, which is hysteresis.
September 2024 Instrumentation Monthly
Pressure calibration is crucial for ensuring the accuracy and reliability of process instruments used across various industries. One often overlooked but critical factor in this calibration process is hysteresis. Understanding hysteresis and its implications can help improve the accuracy and consistency of your pressure measurements. Beamex’s Heikki Laurila dives into what hysteresis is, why it matters in pressure calibration, and how you can manage it effectively.
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