2 SPDT Relays to Control Operating Pressure Range

The setpoint relays in a meter like that are typically used just to signal an annunciator, not as a control system.

It will sort of work, but it will endlessly hunt back and forth. You need what is called a PID controller in order to maintain a pressure. Think of your cruise control on your car (assuming you have used one). Without it, what you are planning is like having a lead foot on the accelerator pedal that is either all the way on or all the way off the pedal and difficult to move. So your car is either going full speed on or coasting with no control. That's sort of OK until you reach a hill, at which time you are either accelerating rapidly with the pedal on full, or decelerating rapidly with no power because of the hill. With the cruise control, you set a speed, it takes control of the throttle and evens out the swings. Ideally you would have some sort of analog device, such as the throttle, that can vary the output in proportion to the need. But even with just a single on-off condition (assuming a pump or valve), a PID controller can dampen the swings by anticipating the on-off states and the response time of the system to give you as stead maintenance of the pressure as possible. Without it, your system will go full on at the low set point, the pressure will surge which will trigger the high set point and turn off, then immediately drop again to the low setpoint , repeat ad nauseum.

Thanks for the reply.

I understand your lead foot analogy. I'm not worried about that. Full on and full off is completely fine. The rate of pressure reduction and pressure increase is slow enough that overshooting isn't really an issue and a little overshoot is actually fine. I'm just trying to roughly keep it in the range. In a 12 hour period it only cycles ~10 times so rapid, precise control isn't needed.

This is a one off experiment to satisfy management so buying a PID controller isn't really an option. If I set to 20 and 30 and it's going to 15 and 35, I'll just bump the set points in a little.

I am sure you can do this with your meter with dual relays. The first thing you have to identify for yourself is the electrical power required to operate this pump. This will determine the contactor you will need to acquire to directly operate the pump. The normally open main contacts of this contactor will provide the current path to operate your pump. The relay contacts of your meter will certainly not be capable of handling the current and specifically the interruption arc of stopping this pump. Now I would get a contactor that also includes a set of NO signal contacts. (These contacts will be open when the main contacts are open.)

Now for how the system should operate, your description does not make sense to me.

Start at 100 and run the pump until it reaches 20. At that point the pump/valve turn off and it sits until the pressure rises to 30 due to inherent system leakage.

The only control systems I've ever worked with that did not start at zero were vacuum pump systems, and they started at 1 atmosphere or 760 torr and then went down with pumping. Depending on the quality of the vacuum you wish to achieve, there will be multiple pumps, valves and even heating tape that should be controlled to achieve 10E-8 torr or better. I doubt that you have a vacuum pump condition. Regardless of what you actually have, allow me to propose a pump increasing system for an explanation of how a simple contactor with one signal NO contact and your dual relay meter can automatically raise your meter reading.

Let X be your actual meter reading. When your pump is ON X increases in value. If X<A then the pump turns ON. If X>B the pump turns OFF. Now either through usage or leakage when the pump is OFF X will stay the same or decrease. To prevent your pump from cycling ON/OFF many times A<B. (The difference between A and B will give your system a hysteresis.) Now how you implement this is by wiring the NC A contact, the NC B contact, your power source, and your contactor coil in series. Wire in parallel with just the NC A contact your NO added signal contact of the contactor. You now have an automatic pumping scenario. You may wish to add some indicator lights to show operation, or additional manual switches that start and stop your pump.

A little background: It is a vacuum system, so it starts at 760 and then needs to maintain the vacuum range/depth. I previously had this function set up with a different meter using a single setpoint that had adjustable hysteresis. The previous meter didn't hold calibration well enough so I had to switch. I have and appropriate NO motor contactor, pump, and valve for it to reach and maintain the desired conditions.

I'm close to following your explanation, but didn't quite follow the wiring order. I'll stick with your increasing pressure description with A<B:

power to NC A to NC B to motor contactor signal voltage. the parallel arm would branch off of NC A. So starting with NC A it would be: NC A to NO SSR signal voltage, SSR power to motor contactor power?

If this is the case, as soon as I hit A=20, the power will be cut in both arms of the system and there would be no power and it would never get to 30? I'm missing something here even when I try to draw a schematic. I must be misunderstanding something with the order or where the parallel arm branches off.

The NC A contact are paralleled by the contactor NO signal contact (NO SC). So at start up the current loop to turn ON the contactor coil is power supply (PS), NC A, NC B, coil, PS RTN. Once the contactor is ON and before reaching the A threshold there will be two current loop paths that keep the coil ON; 1) PS, NC A, NC B, coil, PS RTN 2) PS, NO SC [this will be closed while the contactor is ON], NC B, coil, PS RTN. When you cross the lower threshold, A, the coil will remain ON by the second loop. When you reach threshold B the NC B contacts open and thus your contactor looses power and the NO SC also open. Thus as you drift from above B, to below B and above A the contactor and your pump remain OFF. Only once the sensor signal drop below A does the cycle start all over again. Now since this method uses the NC contacts of your sensor logic you will probably want some added switch to turn things completely OFF.

Oh and JRAEF is spot on that this is a full throttle only control loop, but many low quality vacuum application scenarios will work quite well with this full ON throttle arrangement.

You need to turn the pump on at a high level, keep the pump running until the low level is reached, then turn the pump off at a low level and keep the pump off until the high level is reached again.

That action is called "pump down".

The classic wiring schematic for pump down control with 2 level switches, which is what your meter relays appear to be, is below:

You need an additional double pole relay (K in the schematic) with contacts capable of handling the pump current.

At high level, both LS1 and LS2 are 'made' or closed, so the relay coil K energizes and the pump turns on. As the level falls, LS 2 opens, but the pump continues to run because of the latching contact K in the control circuit.

The latching contact K in the control circuit opens when the level falls low enough to open LS1.

The pump remains off until the level rises enough to close both LS1 and LS2.

The diagram is for liquid level, but the concept is the same for pressure control as you have described it.

"Pumping down control" really couldn't be more obvious, I should have thought of that. Hopefully my last questions: 1) Would a siemens 45EG20AF work for 110VAC control and 110VAC motor? It appears to be a double pole single throw 2) If so, the markings on it are L1, L2, T1, and T2. Would L1/T1 be one pole and then L2/T2 be the other pole with the connections on the side being the control voltage?

If not the correct DPST relay. Any chance you could point one out from McMaster, Omega, or Grainger that would work for 110VAC control, 110VAC motor that pulls 34.9A at startup and 7.8A at full load?