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How to Calibrate and Measure the Hysteresis (Deadband) of a Pressure Switch Sensor?

sebrinadabe594

Hi everyone,

I am currently designing a pressure monitoring solution for an industrial pneumatic control system. I'm using a mechanical pressure switch with a setting range of 0.2 - 2.0 MPa.

My goal is to ensure an alarm is triggered when the pressure reaches 1.2 MPa, but I am currently facing issues with poor repeatability and contact chatter. I want to keep the hysteresis (deadband) within 0.1 MPa to prevent the compressor from frequent "short-cycling."

My current toolset includes: a digital multimeter, a manual pressure pump (with an analog gauge), and some basic wiring tools.

My questions are:

  1. How do I accurately measure the hysteresis? I noticed that the pressure point where the switch "closes" is different from where it "opens." Is this normal? How should I record these two values to calculate the deadband?

  2. How can I reduce false triggers caused by electrical noise? The switch is connected to a PLC digital input. Due to high-power motors nearby, the PLC often receives false trigger signals. Should I install an RC snubber or resolve this via software filtering?

  3. Choosing between Normally Open (NO) and Normally Closed (NC): In a safety system, which wiring configuration is generally recommended? If a cable breaks, which setup allows the system to fail-stop more safely?

  4. Impact of environmental vibration: The pressure switch is installed near a pump body with significant vibration. Will vibration shift the setpoint of the switch? Do I need to perform a remote installation using a capillary tube?

  • 0

    Usually short cycling is caused by not having enough hysteresis.

    The only way to accurately measure hysteresis is with an accurate pressure sensor and a way of adjusting the pressure in small increments. The pressure sensor total error must be less than 0.01MPa to set 0.1Pa hysteresis.

    If you are getting noise triggering you must fix this first. Without a great deal more information I can't comment on how to do that.

    The safety case for NO or NC is dependent on application, in a simple single sensor system where the safe case is to stop the motor you would use a NO sensor so that with abroken wire the motor will not run.

    You can tell if the switch is affected by vibration by isolating it mechanically and observing how its behaviour changes.

    MK

  • 0

      Short cycling in an air compressor system can be caused by too tight of a pressure variance band (hysterysis), but it can also be cause by system storage capacity versus system demand issues, leaks in the system, poor seals and excessive blowby in the compressor, leaking check valves, etc.

    For what it's worth, these are my opinions...
    #1:  Yes.  It is normal for mechanical switches to have a different pickup and dropout value.  As  says, you need to measure this.  Try your manual pump but your accuracy will depend on how consistantly you read your gage.  Hopefully the resolution is fine enough for you to get somewhat accurate - or at least repeatable - measurements.

    #2:  It's a mechanical switch.  There's going to be contact bounce.  My opinion - Resolve it in software.

    #3:  It's pneumatic.  Have both.  It's one more wire on the run.  Use a tee as a manifold.  The system should already have a pressure relief valve, so there's already one layer of safety there should you chose to just use one switch.

    #4:  At the pressure level you are working at, the influence of environmental vibration is negligable.

    Good luck!