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 regularly installed in industrial applications.
Electronic pressure switches, including 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?, according to the task. The next graphics illustrate both switching functions with the respective switch and reset points:
In principle, the following applies: The switch point (SP) is higher than the reset point (RP) and the start of measuring range (MRS) is definitely smaller compared to the end of measuring range (MRE). This rule could be easily understood with settings for ambient air pressures.
Example: Control of vacuum pressure pump
Alternatively, 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 vacuum pressure between -0.7 bar and -0.5 bar. For Thrilled described, we decide on a pressure switch with a measuring range of -1 ? 0 bar.
The start of measuring range (MRS) is therefore -1 bar, the end of measuring range (MRE) 0 bar. This implies: At 0 bar (= start of operation) the switch ought to be closed, so that you can switch on the vacuum pump. If the pressure drops to -0.7 bar, the switch must open and prevent the pump. When the pressure reaches the upper limit value of -0.5 bar, the pump should once again start and run until the pressure has again dropped to -0.7 bar.
ow must the switch points be set in this vacuum range? Let?s take both pictures and translate MRS and MRE:
Here it really is clear that the ?normally open? function must be selected, otherwise the pump wouldn’t normally activate. After entering the switch points, the schematic is obtained, which shows the way the pressure switch functions relative to the example:
Note
Information regarding WIKA?s electronic pressure switches can be found on the WIKA website. Do you want to buy pressure switches? Inside our WIKA online-shop you will find a few of our standard designs. Should you have any questions, your contact will gladly help you.
Also read our posts
What’s meant by the switching function ?hysteresis? with Distrustful ?
What’s the difference between the switching functions ?hysteresis? and ?window? in pressure switches?
What is meant by the ?normally open? switching function with pressure switches?

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