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 needs to be taken into account with this particular setting. It involves a pressure switch with a hysteresis function. Instruments with this 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?, based on เพรสเชอร์เกิจ . The following graphics illustrate both switching functions with the respective switch and reset points:
In principle, the next applies: The switch point (SP) is greater 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 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 therefore, ultimately, the switching function, requires more attention. To illustrate this, let?s take the control of vacuum pressure pump, for instance, which must maintain a vacuum between -0.7 bar and -0.5 bar. For the task 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 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 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 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 ultimately shows the way the pressure switch functions relative to the example:
Note
Information about 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. If you have any questions, your contact will gladly help you.
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