Switch points in vacuum ranges: How are they set?

Setting the switch points of an electric pressure switch for a credit card applicatoin in the vacuum range isn’t a self-explanatory task. The following article therefore explains what should 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?, with regards to 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 higher than the reset point (RP) and the beginning of measuring range (MRS) is definitely smaller compared to the end of measuring range (MRE). This rule can be easily understood with settings for ambient air pressures.
Example: Control of vacuum pressure pump
On the other hand, 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 example, which must maintain vacuum pressure between -0.7 bar and -0.5 bar. For the task described, we select 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 switch on 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 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 A Cut Above be set in 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 activate. After entering the switch points, the schematic is obtained, which shows how 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? In our WIKA online-shop you will discover some of our standard designs. When you have any questions, your contact will gladly help you.
Also read our posts
What is meant by the switching function ?hysteresis? with electronic pressure switches?
What is the difference between the switching functions ?hysteresis? and ?window? in pressure switches?
What’s meant by the ?normally open? switching function with pressure switches?

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