Setting the switch points of an electric pressure switch for an application in the vacuum range isn’t a self-explanatory task. Threatened following article therefore explains what has to be taken into account with this 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?, according 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 start 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 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 the duty described, we select a pressure switch with a measuring selection of -1 ? 0 bar.
The beginning of measuring range (MRS) is therefore -1 bar, the end of measuring range (MRE) 0 bar. This means: At 0 bar (= start of operation) the switch should be closed, to be able to activate the vacuum pump. If the pressure drops to -0.7 bar, the switch must open and stop 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 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 would not activate. After entering the switch points, the schematic is obtained, which shows the way the pressure switch functions in accordance with the example:
Note
Information about WIKA?s electronic pressure switches are available 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. For those who have any questions, your contact will gladly help you.
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