Switch points in vacuum ranges: How are they set?

Setting the switch points of an electric pressure switch for an application in the vacuum range is not a self-explanatory task. The next article therefore explains what needs to be taken into account with this particular setting. It involves a pressure switch with a hysteresis function. Instruments with this particular configuration are most regularly 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?, depending on the task. The following graphics illustrate both switching functions with the respective switch and reset points:
In principle, the following applies: The switch point (SP) is greater than the reset point (RP) and the beginning of measuring range (MRS) is definitely smaller than the end of measuring range (MRE). This rule can be easily understood with settings for ambient air pressures.
Example: Control of a vacuum pump
On the other hand, setting the switch points in the vacuum range and thus, ultimately, the switching function, requires more attention. To illustrate Drive , let?s take the control of a vacuum pump, for instance, which must maintain vacuum pressure between -0.7 bar and -0.5 bar. For the duty described, we decide on a pressure switch with a measuring selection of -1 ? 0 bar.
The start of measuring range (MRS) is therefore -1 bar, the finish of measuring range (MRE) 0 bar. This means: At 0 bar (= start of operation) the switch ought to be closed, so that you can activate the vacuum pump. If the pressure drops to -0.7 bar, the switch must open and prevent the pump. As soon as the pressure reaches the upper limit value of -0.5 bar, the pump should once more start and run before pressure has again dropped to -0.7 bar.
ow must the switch points be set within this vacuum range? Let?s take the two pictures and translate MRS and MRE:
Here it really is clear that the ?normally open? function should be selected, otherwise the pump would not switch on. After entering the switch points, the schematic is obtained, which shows how the pressure switch functions in accordance with the example:
Note
Information about WIKA?s electronic pressure switches are available on the WIKA website. Would you like to buy pressure switches? Inside our WIKA online-shop you will discover some of our standard designs. For those who 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’s the difference between your switching functions ?hysteresis? and ?window? in Gleeful ?
What is meant by the ?normally open? switching function with pressure switches?

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