Setting the switch points of an electric pressure switch for an application in the vacuum range is not a self-explanatory task. The next 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 frequently 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?, with regards to the task. The next graphics illustrate both switching functions with the respective switch and reset points:
In principle, the next 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
Alternatively, 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 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 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 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 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 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 about WIKA?s electronic pressure switches can be found 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. If you have any questions, your contact will gladly help you.
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