Differential Pressure Switch Wiring

First, I doubt that the contacts of these switches will be capable of handling directly the current draw of your compressor motor. This will mean that you will need an additional relay or contactor to do what you desire. There is an additional reason to have a relay that will become obvious soon. Make sure that this relay has a Normally Open (NO) set of contacts that are not used to power the motor itself. To standardize the pressure switch contact names, the NO contacts are open when at atmosphere and the NC contacts are closed at atmosphere. Wire the low pressure NC and relay NO contacts in parallel with each other. Wire this combination in series with the high pressure NC contacts, an ON/OFF toggle switch, the relay coil and a suitable power supply to turn on the relay coil. Wire the remaining contacts of the relay so that the compressor is running when the relay is ON.

You will notice now that the current flowing through your pressure switch contacts will be just the current to engage the added relay coil. You should now walk though this logic to see that your compressor will be ON when the pressure is below both switch pressures, OFF when above both switch pressures and ON only when charging up to the high pressure.

Now I haven't included any current protection devices (fuse or circuit breaker) but this should be added where needed. The high pressure level switch should be at a pressure level less than the safety release valve pressure which should also be less than the vessel maximum safe pressure.

I'm not sure that you have the right pressure switch. Compressor control is conventionally done with a gauge pressure (single pressure port), "wide adjustable deadband" pressure switch. A gauge pressure switch has only one pressure port. The wide adjustable deadband allows a single electrical switch to operate for pump-up control.

You describe your switch as a "differential pressure switch". A differential pressure switch has two pressure ports, a high side and a low side:

Which one do you have? The two port differential H122K is not appropriate for this kind of service.

The H122 switches do not have sufficient deadband to perform pump-up action with a single switch.

UE's adjustable deadband option is the 1519 option. Does your switch have the 1519 option?

Assuming you have a single port, gauge pressure switch, without the adjustable deadband you'll need a two pole control relay to accomplish what is called pump-up control action.

You need to turn the compressor on at a low pressure, keep the compressor running until the high pressure cut-off point is reached, then turn the compressor off and keep the compressor off until the low pressure cut-in point is reached, which will re-start the cycle. That action is called "pump up".

The classic wiring schematic for pump up control with 2 separate pressure switches, where one pressure switch is set for a cut-in and the other set for cut-out, is shown below (the diagram is for liquid level, but the concept is identical to pressure):

Note that the contacts are normally closed below the trip pressure setpoint. You need an additional double pole relay (K in the schematic) with contacts capable of handling the compressor motor current or the compressor starter's current.

At zero pressure, the compressor is turned on because LS2 is below its cut-out setpoint (contacts closed) and LS1's contacts are closed because it is below its cut-in setpoint. The latching contact K closes as soon as the control relay energizes. As the pressure rises above LS1's setpoint, LS1's contacts open, but the compressor keeps running because LS2 is still closed and the latching contact K completes the relay's control circuit.

When pressure rises above the LS2's setpoint, LS2 opens and the pump control relay turns off. The latching contact K opens. The compressor turns off. All pressure switches have some deadband, and as soon as the pressure drops below the deadband reset point, LS2 closes again (990?, 980 psi?). But the compressor won't turn on until the pressure drops below LS1's setpoint, at which time LS1's contacts close, the compressor starts and the cycle begins again.

Be sure that the LS1 switch in the diagram is the front low switch in the H122:

You wire to the common (C) and Normally Closed (NC) terminals on each switch.

In response to Rafael L Lama's proposal to wire dual switches with no control relay.

1) The proposal, as stated, produces a short circuit across the line:

2) Perhaps the intention was to wire the common terminal(s) to the LOAD, the compressor motor, shown as M in the diagram.

If so, then yes, the compressor motor will turn on at any pressure below the cut-in point (as shown above), but will turn off at any pressure above the cut-in point, as shown below.

If there was a clever way to do pump-up or pump-down with only dual switches, the economy would not support the host of "pumping relays" or wiring diagrams using a control relay with a latching contact, like I posted. But no one's come up with a scheme in the past century to do so.

Any other takers?