Why hysteresis occur in pressure sensors? Causes of hysteresis error in pressure sensor

Hysteresis is, generally speaking, the difference between the switching point and the reset point expressed as a positive or negative percentage of the specified pressure range. Hysteresis is caused by the netural reluctance of a pressure sensing material.

In other words, when a switch changes state, for example from Open to Closed, the point at which it restores the Open state (the original state) will not be exactly at the original point. The difference between these two points is the hysteresis.

Its proper setting in the pressure switches prevents the system from oscillating and thus becoming unstable. This would work if the switching point and the reset point were set to the same value.

Hysteresis in pressure sensors – example

As an example, let us take a pressure switch for a drinking water pump with the switching point and reset point set at 6 bar. When this value is exceeded, the pressure switch will activate the switching output and stop the pump. This will lead to a pressure drop below 6 bar and the pump will be restarted. With such settings, where switching point = reset point, this process will repeat continuously at short intervals with the slightest fluctuation in pressure. This will lead to pump instability and rapid wear.

Therefore, in order to avoid early pump failure, it is necessary to set a process-adapted switching point and reset point. In this way, the hysteresis loop is controlled stably.

Pressure sensor hysteresis

The phenomenon of hysteresis is encountered in both mechanical and electronic pressure transducers, as well as in temperature, level or flow sensors.

Mechanical versus electronic pressure transmitters in the context of hysteresis

Mechanical pressure sensors usually have a hysteresis predefined by the manufacturer, while electronic pressure transmitters are often configurable.

Mechanical pressure transmitter

The hysteresis of mechanical pressure switches can be found in the manufacturer's specifications. It is usually set at 10-20 % of the value the switch point has.

Electronic pressure transmitter

Electronic pressure measurement devices, such as the DELOS SI display pressure transmitter, offer the possibility of fully configurable hysteresis in the nominal measuring range. The hysteresis can be set using an external program or the integrated display.

Advantages of adjustable hysteresis in electronic pressure transmitters

The advantage of adjustable hysteresis is that it allows the measurement range to be shifted depending on the application. This provides the ability to accurately determine the switching and reset point, giving the operator greater control over the overall performance of their engineering system.

DELOS SI pressure transmitter – switching outputs

For the high accuracy pressure transducers JUMO DELOS SI, the switching output of the device can be selected:

• Type: normally open or normally closed

• Function: window or hysteresis

Hysteresis switching function – applications

Many pressure sensors also have a safety function and are used, among other things, to monitor a critical threshold value. The most commonly used pressure switching function, due to its ease of application, is hysteresis.

For mechanical pressure transmitters, hysteresis is the only switching option. Electronic pressure transmitters still have a 'window' switching function.

Hysteresis switching function vs. window function – principle of operation, differences

The hysteresis switching function monitors whether a limit value (switching point) is exceeded, while the "window" function monitors whether the measured process is between the lower and upper limits in the set range. The pressure switch changes status (e.g. open -> closed, active -> inactive) when the limit value or the upper or lower limit in the 'window' is exceeded.

Therefore, the use of hysteresis will be suitable for applications that only need a single setpoint setting, while the window function is suitable for applications that require lower and upper limits.

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