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. In this blog, I’ll dive into what hysteresis is, why it matters in pressure calibration, and how you can manage it effectively.
While hysteresis can be found in various types of measurements, such as temperature and electrical signals, this blog focuses on its impact on pressure calibration, where hysteresis is most significant.
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.
The image below shows a simplified illustration of hysteresis. Increasing and decreasing pressure do not follow the same line - there is a clear difference, which is hysteresis.
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.
Several factors can contribute to hysteresis in pressure instruments, such as:
To manage hysteresis effectively, it’s essential first to identify and measure it accurately. Here are some techniques:
While hysteresis cannot be completely eliminated, it can be managed and minimized. Here are some best practices to help you do this:
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 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. Anyhow, you can learn more on pressure switches in this blog post: Pressure Switch Calibration.
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.
At Beamex, we have worked with pressure calibration for 50 years, so I am confident when I say that we know something about it.
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