Setting the switch points of an electronic 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 setting. It involves a pressure switch with a hysteresis function. Instruments with this configuration are most frequently 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 greater than the reset point (RP) and the start 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 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 a vacuum pump, for example, which must maintain a vacuum between -0.7 bar and -0.5 bar. For the task described, we select a pressure switch with a measuring range 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, in order to switch on 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 the upper limit value of -0.5 bar, the pump should once again start and run until the 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 must be selected, otherwise the pump would not switch on. After entering the switch points, the schematic is obtained, which shows how the pressure switch functions in accordance with the example:

Note
Information about WIKA’s electronic pressure switches can be found on the WIKA website. Would you like to buy pressure switches? In our WIKA online-shop you will find some of our standard designs. If you have any questions, your contact will gladly help you.