Setting the switch points of an electric pressure switch for an application in the vacuum range is not a self-explanatory task. The following article therefore explains what should 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, such as 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 higher than the reset point (RP) and the beginning of measuring range (MRS) is always smaller than the end of measuring range (MRE). This rule can 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 a vacuum pump, for example, 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 should be closed, so as to switch on the vacuum pump. If the pressure drops to -0.7 bar, the switch must open and stop the pump. When 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 in this vacuum range? Let?s take both pictures and translate MRS and MRE:
Here it is clear that the ?normally open? Disastrous should 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 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 a few of our standard designs. When you have any questions, your contact will gladly help you.
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