Hysteresis in pressure calibration: what you need to know

Pressure calibration is crucial for ensuring the accuracy and reliability of process instruments used across various industries.

One often-overlooked but critical factor in the pressure calibration process is hysteresis. Understanding hysteresis and its implications can help improve the accuracy and consistency of your pressure measurements.

While hysteresis can be found in various types of measurements, such as temperature and electrical signals, this article focuses on its impact on pressure calibration, where hysteresis is most significant.

What is hysteresis?

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 100 kPa pressure instrument at a 50 kPa point, it may show 49.95 kPa with increasing pressure. With decreasing pressure, at the same 50 kPa point, it may show 50.05 kPa. This difference between 49.95 kPa and 50.05 kPa is caused by hysteresis.

Figure 1: A simplified illustration of 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 behaviour of pressure instruments.

Identifying hysteresis

To manage hysteresis effectively, it’s essential first to identify and measure it accurately. Below 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 don’t 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 versus output readings can help visualise 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, it is much easier to identify hysteresis.

Sending calibration results to calibration software also helps, as the software often offers graphical presentation results.

Mitigating hysteresis

While hysteresis cannot be completely eliminated, it can be managed and minimised. Here are some best practices to help you do this:

• Regular calibration: Calibrate regularly, with up and down cycles, to identify hysteresis.
• Instrument selection: Choose high-quality pressure instruments with low hysteresis characteristics for critical applications.
• Consistent procedures: Follow consistent calibration procedures to ensure the repeatability and reliability of results.
• Instrument cleanliness: Ensure that instruments are clean and free from contaminants that could affect their performance.
• Environmental control: Whenever possible, maintain stable environmental conditions during calibration to reduce external influences, although this is not always possible when calibrating instruments in field conditions.

Hysteresis in pressure switches

With any switches, including pressure switches, there is a hysteresis-like feature called ‘deadband’. This means that the switch has been designed so that there is some difference between the opening and closing points with increasing and decreasing pressure. This may seem a lot like hysteresis, or even be called hysteresis, but it is not actual hysteresis.

This deadband is needed and important in pressure switches; otherwise, the switch could start oscillating between open and closed when the pressure is at a certain value. Because switches are used to control specific operations, this is undesirable.

Conclusion

Hysteresis is a critical factor to consider in pressure calibration — especially in the world of process instruments, where precision is paramount. By understanding what hysteresis is, identifying its causes and implementing best practices to manage it, you can ensure more accurate and reliable pressure measurements.

This article was originally published on the Beamex blog at https://blog.beamex.com/hysteresis-in-pressure-calibration. Beamex products are distributed in Australia by AMS Instrumentation & Calibration Pty Ltd.

Top image credit: iStock.com/Goodvibes Photo

Originally published here.