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[W. Mark Hellinger]

I've been looking to source an air compressor pressure switch with a 3 position lever, i.e.:  includes a low pressure shut-down. I'm coming to the conclusion that such a thing might not exist.  Maybe it doesn't exist for good reason, but I can't imagine why not?  However, I've looked and looked and can't find such a thing... and figured if any bank of knowledge would know, it's likely here.

Anyhoo:  My well pump controller has the three positions:  Off, Start, and Auto.  When starting at zero pressure in the system, you gotta hold it in the start position till they system builds 10 lbs. or so, then you can flip the switch to auto... basically a Murphy Switch concept.  Thus the system shuts down in the event of a system rupture or such.  I'm thinking same-same would be excellent for my shop air compressor, but I can't find such a thing.  And of-course one can't use a water pump controller/pressure switch for an air compressor because the water pump switch doesn't have the unloader needed for the air compressor (so the motor isn't starting under a hard load).  If I've gotta do-it-myself, I'll series wire-in a "low pressure resettable pressure switch"... cause I really like the idea of a catastrophic shut-down with the air compressor... but it would be more civilized to have it all in one... if such a thing exists.

[Tim McCulloch]

I've been looking to source an air compressor pressure switch with a 3 position lever, i.e.:  includes a low pressure shut-down. I'm coming to the conclusion that such a thing might not exist.  Maybe it doesn't exist for good reason, but I can't imagine why not?  However, I've looked and looked and can't find such a thing... and figured if any bank of knowledge would know, it's likely here.

Anyhoo:  My well pump controller has the three positions:  Off, Start, and Auto.  When starting at zero pressure in the system, you gotta hold it in the start position till they system builds 10 lbs. or so, then you can flip the switch to auto... basically a Murphy Switch concept.  Thus the system shuts down in the event of a system rupture or such.  I'm thinking same-same would be excellent for my shop air compressor, but I can't find such a thing.  And of-course one can't use a water pump controller/pressure switch for an air compressor because the water pump switch doesn't have the unloader needed for the air compressor (so the motor isn't starting under a hard load).  If I've gotta do-it-myself, I'll series wire-in a "low pressure resettable pressure switch"... cause I really like the idea of a catastrophic shut-down with the air compressor... but it would be more civilized to have it all in one... if such a thing exists.

You're wanting a low pressure cut off so your compressor won't run continuously in the event of an air distribution failure?

Consider looking at HVAC refrigerant pressure switches.  You'd need a supporting relay or contactor, possibly.

[W. Mark Hellinger]

You're wanting a low pressure cut off so your compressor won't run continuously in the event of an air distribution failure?

Yes, per exactly Tim.  I.e. (for example):  So I don't walk into the shop first thing in the morning, sniff and go:  "Humm... smells like smoldering oil... why's that???"  Quick look around... go into the room that houses the air compressor:  "Oh, there's your problem... the air compressor is SMOKING... profusely!  Why's that?  Cause the stuff that goes around and around and makes noise is glowing orange... cause it's really hot... cause it's seemingly been running for a long time... oh, and I see the system pressure is ZERO... and its' usual noise when running is notably different... sounds like a rod about ready to exit the crankcase... and discover that's because it is (compressor guts are about ready to seek daylight)... and that's because the compressor is out of oil having discharged that lubrication stuff from the inside to the outside when the seals melted out of it."  Shut the compressor off.  "Oh here's the problem: The pneumatic box stapler that got left plugged-in... it's trigger assembly fell apart and dumped the system air... seemingly sometime last night."  Oh yea, and: The end of month electrical bill was considerably higher... too... adding insult to injury.

Consider looking at HVAC refrigerant pressure switches.  You'd need a supporting relay or contactor, possibly.

Will do... thanks for the tip!  Hadn't thought about HVAC stuff... probably cause I have basically zero knowledge/experience with that subject.

[Tim McCulloch]

Will do... thanks for the tip!  Hadn't thought about HVAC stuff... probably cause I have basically zero knowledge/experience with that subject.

The HVAC models may not be in the range you want, but I'd bet the same manufacturers will have a wide selection.

[Bill Meeks]

There are also relatively cheap low pressure cut out switches for well water pumps that might be suitable from manufacturers such as Square D. I checked some of these and they are rated for use up to 220 PSI (not to exceed pressure). Search for "Square D low pressure cutoff switch". These require a manual reset when they trip out on low pressure, but that is probably what you want anyway.

My working life (before I did cybersecurity/IT stuff) was maintaining and then teaching industrial process control systems for power plants. If you want a step up into the pro level, consider something like Barksdale pressure switches: https://www.barksdale.com/products/pressure/mechanical-pressure-switches or Static-O-Ring (SOR): https://www.sorinc.com/products-category/pressure-vacuum-switches/. To put these in speaker terms, Square D is Alto and Barksdale or SOR are Meyer  .

[Matthew Knischewsky]

Yes, per exactly Tim.  I.e. (for example):  So I don't walk into the shop first thing in the morning, sniff and go:  "Humm... smells like smoldering oil... why's that???"  Quick look around... go into the room that houses the air compressor:  "Oh, there's your problem... the air compressor is SMOKING... profusely!  Why's that?  Cause the stuff that goes around and around and makes noise is glowing orange... cause it's really hot... cause it's seemingly been running for a long time... oh, and I see the system pressure is ZERO... and its' usual noise when running is notably different... sounds like a rod about ready to exit the crankcase... and discover that's because it is (compressor guts are about ready to seek daylight)... and that's because the compressor is out of oil having discharged that lubrication stuff from the inside to the outside when the seals melted out of it."  Shut the compressor off.  "Oh here's the problem: The pneumatic box stapler that got left plugged-in... it's trigger assembly fell apart and dumped the system air... seemingly sometime last night."  Oh yea, and: The end of month electrical bill was considerably higher... too... adding insult to injury.
Will do... thanks for the tip!  Hadn't thought about HVAC stuff... probably cause I have basically zero knowledge/experience with that subject.

Now I'm thinking about a low pressure cut off for my compressor. It's now on my list of things to turn off before going away because this scenario played out a number of years ago. We went away for a few days and an air line ruptured. You've described fairly accurately the carnage.

[Steve-White]

I bleed the air tanks & pull the breaker when I exit the shop. 

[W. Mark Hellinger]

There are also relatively cheap low pressure cut out switches for well water pumps that might be suitable from manufacturers such as Square D. I checked some of these and they are rated for use up to 220 PSI (not to exceed pressure). Search for "Square D low pressure cutoff switch". These require a manual reset when they trip out on low pressure, but that is probably what you want anyway.

Understood.  And that was my original plan was "just" replace the existing Square D pressure switch on my air compressor for a Square D pressure switch like on my well pump which has that low pressure cut-off feature... easy-peazy... 'cept it doesn't seem Square D, nor any others, makes a basic pressure switch for air compressors with that integrated low-pressure cut-out/shut-down... i.e.:  Has both the air compressor application pressure release valve and low pressure cut-out.  It's either one or the other:  Well pump type of which there are models available with the low-pressure cut-out, or: Air compressor type that have the pressure release valve but none available with a low-pressure cut-out... which seems strange to me, but then I'm guessing there's a good reason... maybe?  Or maybe it's a case of: "That's just the way it's always been"... as-in the the seeming (to me) case of the Velcro-like grill retention buttons on many or possibly most SVT tube bass heads and SVT speaker cabinets... which in my experience either work per one or the other: Really well... like too well, or are largely or completely dysfunctional... which I think I've figured that one out and come-up with a good fix.

[Brian Jojade]

This could be handled in a couple of different ways.  Measuring pressure certainly is one way.  But what if you develop a slow leak?  Tank maintains enough pressure, but the pump starts running and runs just fast enough to maintain the leak.  Never enough to shut it off.  Same net result in the end.

Maybe an electronic solution would work.  ie, the pump is only allowed to run for X number of minutes per hour or it trips off, requiring a manual reset.  If you happen to be using the compressor excessively in that hour, it's a minor inconvenience, but at the same token, maybe the compressor will need a rest anyway.

Building something with logic that determines when and how long the compressor is running wouldn't be that hard.  It could then also log when it's running and you can determine if there is a slow leak somewhere as you'll then see a change in when the compressor decides to run while the shop is closed.

[Bill Meeks]

Understood.  And that was my original plan was "just" replace the existing Square D pressure switch on my air compressor for a Square D pressure switch like on my well pump which has that low pressure cut-off feature... easy-peazy... 'cept it doesn't seem Square D, nor any others, makes a basic pressure switch for air compressors with that integrated low-pressure cut-out/shut-down... i.e.:  Has both the air compressor application pressure release valve and low pressure cut-out.  It's either one or the other:  Well pump type of which there are models available with the low-pressure cut-out, or: Air compressor type that have the pressure release valve but none available with a low-pressure cut-out... which seems strange to me, but then I'm guessing there's a good reason... maybe?  Or maybe it's a case of: "That's just the way it's always been"... as-in the the seeming (to me) case of the Velcro-like grill retention buttons on many or possibly most SVT tube bass heads and SVT speaker cabinets... which in my experience either work per one or the other: Really well... like too well, or are largely or completely dysfunctional... which I think I've figured that one out and come-up with a good fix.

The Square D switch should work with either medium (water or air). In fact, in most water well pump scenarios the pressure switch is actually measuring/sensing the pressure of the air bubble in the tank bladder and is not in contact with liquid water.

I was thinking adding an additional low pressure cut-out switch in series with the existing switch. The way I understand the cut-out is that it is "closed" with higher pressure and then "opens" on low pressure and locks out in the open position.  In series with your existing switch, I thought this would provide the protection you need. The only potential side effect would be when you shutdown the system manually, pressure would bleed down and the low pressure cut-out would activate and lock-out, thus preventing a normal restart until manually reset. The only other potential gotcha would be what the high pressure cut-off limit is on the water well switch. It might open up too soon for a typical air compressor as I think the water well switches tend to top out on the high pressure setpoint adjustment around 80 PSI or so.

Perhaps I am misunderstanding Square D's description of how the low pressure cut-out switch works.

[ProSoundWeb Community]