Switch points in vacuum ranges: How are they set?

Setting the switch points of an electronic pressure switch for an application in the vacuum range is not a self-explanatory task. The next article therefore explains what must be taken into account with this particular setting. It involves a pressure switch with a hysteresis function. Instruments with this particular configuration are most frequently installed in industrial applications.
Electronic pressure switches, like the WIKA models PSD-4 and PSD-4-ECO or the A-1200 pressure sensor with IO-Link, operate as ?normally closed? or ?normally open?, based on the task. The following graphics illustrate both switching functions with the respective switch and reset points:
In principle, the next applies: The switch point (SP) is higher than the reset point (RP) and the start of measuring range (MRS) is always smaller than the end of measuring range (MRE). This rule could be easily understood with settings for ambient air pressures.
Example: Control of vacuum pressure pump
However, setting the switch points in the vacuum range and thus, ultimately, the switching function, requires more attention. To illustrate this, let?s take the control of vacuum pressure pump, for instance, which must maintain a vacuum between -0.7 bar and -0.5 bar. For Peril described, we decide on a pressure switch with a measuring selection of -1 ? 0 bar.
The beginning of measuring range (MRS) is therefore -1 bar, the finish of measuring range (MRE) 0 bar. This implies: At 0 bar (= start of operation) the switch ought to be closed, so as to activate the vacuum pump. If the pressure drops to -0.7 bar, the switch must open and prevent the pump. When the pressure reaches top of the limit value of -0.5 bar, the pump should once more start and run until the pressure has again dropped to -0.7 bar.
ow must the switch points be set within this vacuum range? Let?s take both pictures and translate MRS and MRE:
Here it really is clear that the ?normally open? function should be selected, otherwise the pump wouldn’t normally activate. After entering the switch points, the schematic is obtained, which ultimately shows the way the pressure switch functions relative to the example:
Note
Information about WIKA?s electronic pressure switches are available on the WIKA website. Would you like to buy pressure switches? In our WIKA online-shop you will find some of our standard designs. Should you have any questions, your contact will gladly assist you to.
Also read our posts
What is meant by the switching function ?hysteresis? with electronic pressure switches?
What is the difference between your switching functions ?hysteresis? and ?window? in pressure switches?
What’s meant by the ?normally open? switching function with pressure switches?

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