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Calibration Pressure ramp


To calibrate a pressure switch you need to provide a slowly changing pressure ramp, moving across the operating points of the switch. Depending of the switch type, you need to first supply a suitable pressure to start the calibration. Often you can start from atmospheric pressure, but in some cases, you need to pump a high pressure and start slowly decreasing the pressure towards the operation point. Or you may need to provide a vacuum to start from. This depends on the switch to be calibrated. There are different ways to provide the input pressure. You can use a calibration hand pump with a fine adjustment control, you may use shop air supply with a precise pressure controller, or you can use an automatic pressure controller. It is vital to provide a slow pressure ramp so


that you can see the precise pressure whereby the switch operated. If the pressure changes too quickly, you cannot accurately capture the pressure point when the switch operated. Certainly, some tools (like the Beamex MC6) can automatically capture the exact pressure during the very moment when the switch changed its status. Anyhow, remember to change the pressure


very slowly when you are approaching the operation points of the switch. You may change the pressure faster when you are not yet close to the operation points.


Measuring the switch output


You need some tool to measure the switch terminals. If it is a dry switch, with an open and close output, you may use an Ohm meter. If the output is electrical, you will need to find a tool that can measure the output. In some cases, it may be a voltage meter, or current meter. For electrical outputs, it is sometimes a bit difficult to find how to measure the output. You should anyhow be able to recognise the two states of the output and to see when the state changes. With some tools, you can program a


trigger level that suits the switch in question which enables the status change to be captured automatically. This is how the Beamex MC6 works.


Capturing the operation points


In the switch calibration, you need to capture the input pressure at the very moment when the output state changes. You can try to capture the input pressure


manually, e.g. when the switch state changes, you stop the ramp and look what is the input pressure (on the device/calibrator that is measuring the input pressure). Most likely there is some delay in your reflexes, so the pressure is already different than what it was during the switch operation moment. That is the main reason you should provide a very slow input pressure, so it has not changed that much during the delay of your reflexes.


Instrumentation Monthly June 2020


Move pressure very slowly towards the return point, until the switch status toggles. Record the return pressure.


Make required number of repeats - repeat the two previous steps.


Vent pressure.


Disconnect the test equipment. Return switch back to service. Naturally, you need to document the switch


calibration results.


METROLOGICAL TRACEABILITY IN CALIBRATION – ARE YOU TRACEABLE? The calibration uncertainty is a vital part in every calibration. If the calibration equipment (and calibration method and process used) is not accurate enough for the pressure switch calibration, then the calibration does not make much sense. There is no point in using a two per cent accurate calibrator to calibrate a one per cent accurate instrument. Learn more about calibration on the Beamex


blog: blog.beamex.com. Beamex


www.beamex.com 39


Some devices can capture the input pressure automatically at the very same moment when the switch output changes its state. Needless to say, the Beamex MC6 family of calibrators can do that. The MC6 can interpolate between the pressure measurement readings. A digital pressure measurement device measures the pressure a few times every second. It may happen that the switch operates in between the two consecutive pressure measurement readings. In that case, the MC6 looks at the time stamp of the switch operation and interpolates between the two consecutive pressure measurement results to get the exact pressure value during the switch operation moment.


Delayed output


Some industrial switches may have a delay added to the output so that it does not work too quickly. You should find out if your switch has delay as then the calibration needs to be done even slower than normally. With some added delay, by the time the output toggles, the input pressure is already far away from the point that actually triggered the output to toggle.


STEPS IN PRESSURE SWITCH CALIBRATION


Here is a condensed list of steps in pressure switch calibration:


Depressurise and disconnect for safety.


Connect the pressure source and the pressure calibrator to the switch input.


Connect the device to measure the switch output status.


Exercise the switch a few times - pump full pressure and back to zero. Not with safety switches.


Pump normally pressure close to operation point.


Move pressure very slowly across the operation point, until the switch output toggles. Record the operation pressure.


Also, you need to calculate the errors found in the calibration and compare that to the max allowed tolerance for that switch to see if it Passed or Failed calibration. In the case of the switch failed the calibration, then you need to either adjust the switch or replace it. Even if it passes the calibration, you should still analyse how big the error was. If the error was close to the tolerance limit, or if it had drifted much since last calibration, it is good to adjust it to avoid a fail result in the next calibration. And as with every calibration, based on the


calibration result history, you should consider if the calibration period should be changed. You do not want to waste resources on calibrating it too often, but also you don’t want to calibrate it so seldom that you get a failed calibration result. A failed calibration result should anyhow always start an investigation of the consequences. This can be expensive and work intensive.


DOCUMENTATION, METROLOGICAL TRACEABILITY, CALIBRATION UNCERTAINTY As documentation is included in the formal definition of calibration, it is a vital part of every calibration. This is also valid in pressure switch calibration. Typically, in the form of a calibration certificate. The calibration equipment used should have a valid metrological traceability to the relevant standards, otherwise the calibration does not ensure traceability in the switch calibration.


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