Switch Hysteresis & Ratings

Switch Hysteresis

Hysteresis in a differential pressure switch is a valuable feature because it prevents the switch from constantly toggling on and off around the set point, which could occur due to minor fluctuations in pressure that are normal in most systems. Here’s a breakdown of what this means and why it’s beneficial.

Without hysteresis, a differential pressure switch might rapidly switch back and forth (oscillate) between on and off states if the pressure around the set point fluctuates slightly. This can happen in systems where pressure naturally varies within a small range due to operational conditions, like pumps starting or stopping, changes in fluid velocity, or minor system disturbances.

Hysteresis as a Solution

Hysteresis introduces a “deadband” or a range of pressure values around the set point where the device will not change its state. This deadband acts like a mechanical filter that ignores minor fluctuations in pressure that don’t indicate a significant or actionable change in system conditions.

In summary, hysteresis is a design feature that introduces a deliberate delay in the switch’s response to pressure changes, effectively filtering out minor fluctuations and preventing the device from reacting to insignificant or transient events. This ensures more reliable and stable system performance, reducing the likelihood of equipment damage and enhancing overall operational efficiency.

Hysteresis Range Defined

Hysteresis is range from when a switch turns on to the point that it turns off.   It is stated in % of Full Scale Range(FSR).   For example:    (Switch On increasing pressure – Switch off decreasing pressure.) / Full Scale range

The tradeoff in selecting a higher power reed is a greater hysteresis and reduced switch adjustability.

For example the average Hysteresis of the 3W SPDT switch for Mid-West Instrument Model 120 is 5%, whereas the 60W SPDT switch is 13 %.  In most applications (ie; pressure drop across a filter), hysteresis is not a parameter to be concerned with.  However, large hysteresis limits the ability to set a switch near the bottom of the range.  In some cases a switch can be set on increasing pressure below the defined capability; however it does not clear when the differential pressure drops to 0.

For reed switches the hysteresis cannot be adjusted and will vary from unit to unit.

Note that our specification specifies a nominal and maximum value.

switch hysteresis

Switch Specification Hysteresis

ModelSPST (60W)SPDT (60W)SPDT (3W)SPST (25W)
Model 12015% / 8%(F.S.)20% / 13%(F.S.)10% / 5%(F.S.)N/A
Model 12118% / 10%(F.S.)N/A12% / 8% (F.S.)N/A
Model 130N/AN/A10% / 5% (F.S.)N/A
Model 140/142N/AN/A10% / 5%(F.S.)15% / 8%(F.S.)
Model 22020% / 10% (F.S.)20% / 18% (F.S.)10% / 6% (F.S.)N/A
Model 24020% / 9% (F.S.)25% / 18% (F.S.)15% / 6% (F.S.)N/A
Model 52228%/18% (F.S.)N/A23%/16%N/A