Compressor tank pressure margins

by Mike
(Kansas City)

I have a Craftsman (Devilbiss)single stage oil-free compressor, about 12 years old. It has an ASME rated tank and has a Furnas pressure switch that is labeled 100/125.


I believe the unit is advertised as a "125psi compressor."

The problem is that the unit has never cut in until the tank pressure drops to 90psi. That difference between 90 and 100 is just enough to cause performance problems with some of the tools I'd like to use.

I've tried adjusting the (single) screw on the pressure switch up, but even a very slight adjustment results in the cutoff pressure raising up well above 125psi. I'd install a different switch if I could find one that truly had a 100psi cut-in pressure, but I can't.

So that leaves the question, would this tank be safe to run at pressures up to 150psi so that I can be confident about increasing the adjustment on the existing switch? Is there really a difference between a 125psi tank and a 150psi tank (which seems to have become more commonplace in the last few years)?

Thanks for any advice!

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Mike...

Typical pressure switch adjustments are in the 1/4 turn range. Sounds like you may be adjusting the nut too much?

On the tank there should be an ASME plate indicating the safe pressure of that tank. If no such plate, go with the compressor manufacturers limits.

What I suggest you do is put the term "how to adjust pressure switch" into your search box for guidance. There are many pressure switches on the market that allow you to adjust both cut in and cut out.

Mind you, reducing the range between cut in and cut out means your compressor will cycle more often, with frequency of maintenance and life expectancy of the air compressor being affected.

Cheers,

Bill

Comments for Compressor tank pressure margins

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Nov 17, 2011
Update
by: Mike

Thanks, Bill.

After I posted the question I pulled the compressor out from the wall where I could get a better look at it.

There was indeed a metal plate tack welded to the tank, indicating it was certified for 150psi at both high and low temperatures.

Even better news is that I apparently bought an exact replacement pressure switch around the same time I bought the compressor, so I went ahead and installed it. Whereas the original switch seemed very difficult to adjust properly, I was able to adjust the new one easily with about 3/8 of a turn, so that it now cuts in at exactly 100 and cuts out at about 134.

Now I just need to figure out why it seems to lose about 1psi of pressure per minute (and did with the old switch too), but at least now I know it's not the switch. Hopefully it's just the tank drain or a loose fitting, and not the check valve. Thanks again.

______________________

Good stuff, Mike. A 1 PSI a minute leak does sound like a slow leak.

B.

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Increasing the tank pressure?

Tank Pressure - Is there anyway of increasing the tank pressure to 150 or 200 lbs?

I recently renovated an old laundry mat and took the compressor that ran the presses home for use in my garage. I seen that it worked and thought why not. It is rather old but is your basic compressor with the top mounted pistons, and motor with pulley and such.

Anyway its pressure gauge goes to 300 and while I do not want to pressure 300 lbs it cuts out at 125, and cycles as it should. My question is this. Is there anyway of increasing the tank pressure to 150 or 200 lbs?

I plan on using it for impact wrenches air ratchets and such. I have had small 125 lbs compressors such as those small "compressor and tool packs" that you could get at Lowes near christmas, but as soon as I would start using an impact it would deplete the tank and need to completely refill.

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Hi and thanks for writing in.

If the compressor can handle the pressure, and if you know how to adjust the pressure switch, then yes, you can increase the pressure output of your compressor so that it cuts-out at a higher pressure when the tank is full.

However, that you have a higher pressure in your tank means that, if you set your downstream pressure at 90 PSI for example, you will have a few more seconds of pressurized air before your compressor tank drops below cut in pressure and your unit has to run to fill the tank.

It is a common assumption that higher pressure means longer tool use before recharging, and while in theory that is the case, what you really need is a much bigger reservoir (and that means a longer recharge between uses), or a larger capacity compressor that can supply the flow you need, or recharge the tank faster.

Cheers,

Bill

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Cubic Feet of air in an air compressor tank

by John
(Seattle)

How much cubic feet of air per gallon in a tank at 125 PSI, i.e. 21 gallon tank at 125 PSI or 20 gallon tank at 150 PSI? I am trying determine the air compressor tank size that best suits my needs.

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Bill says...

Good question, John, which is why I have written extensively about it on this website.

Why not have a look at the Tanks menu on the new site map, and follow the link to Tank Size to Cubic Feet?

Cheers,

Bill

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Compressor builds pressure ok loses air pressure overnight

Compressor builds pressure ok, shuts off at 140 psi. Over an 8 hour span pressure drops to 40 psi. This is an old 3hp 150 psi craftsman twin.
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Well, I suspect that you either have a leak in one of the fittings on compressor or somewhere in your air line... or, you have a tank check valve that isn't seating completely, and overnight, when the compressor is off and the unloader valve is open, air is bleeding out.

One part liquid dish soap to 100 parts water will give you a mix that you can drip onto fittings to check for leaks.

If no leaks, with an empty tank and compressor off, pull the tank check valve and clean/replace it.

Cheers,

Bill

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