Air Pressure question.

[Just for fun]

I'm not sure where this needs to go, has nothing to do with machining.

I have hand full of air pressure gauges and they all seem to read a little different. Checking the air pressure on a water well pressure tank yesterday I decided that I would compare a couple of gauges. Now here is the weird thing, my tire pressure gauge that's attached to an air hose I use as a reference. I checked a couple other gauges on my little air tank, and they are close but when checking the air pressure on the pressure tank which should be 38 psi with no water. I got 38 psi with the tire gauge, but 1st try with a little digital gauge I got 32 psi, rechecked with my little tank the two were very close, back to the pressure tank tire gauge 38 psi, digital 35. WTH

I thought of what the reason might be as I was typing this. On the pressure tank I noticed that it seems to be a little slow coming up to pressure. Maybe the orifice on Schrader valve is blocked a little and the digital gauge takes a reading at say 1 second or less and doesn't read the full pressure.

Thanks for the help! I think I got. LOL

I have another question. Do you guys have any recommendations on a good air pressure gauge?

 

[benmychree]

It is in the nature of cheap pressure gages to be inaccurate, likely only test gages are likely to be consistently accurate. A dead weight gage tester can quite accurately be used to calibrate pressure gages.

 

[aliva]

A test gauge would be one way to get an accurate reading, but the cost may be prohibitive. Buy from a quality manufacture, Ashcroft comes to mind. Some have calibration screws, but that just ups the price. Cheap gauges are not very accurate. Depends on how much accuracy you want to pay for.

 

[ddickey]

With no pressure on them do they all read zero?

 

[markba633csi]

The digital gauge might be using a slow sampling that could give goofy results- your tire gauge is probably pretty close
When in doubt, trust the analog

 

[tq60]

A person with one clok knows the time, get a second and you are not sure...

We had a tire gage reading low, tried to add when Dash said low, seemed to take too much, checked dash, and now higher than the tire gage...now in the trash.

Buy a decent quality air pressure gage with top pressure a bit above your working pressure.

If the compressor maxes at 125, then get the 150 or 160, no need for the 300.

Next, get a decent chuck (the part that interfaces with the valve) and attach it to the gage.

That is your reference.

Tire "stick" gage is a piston against a spring in a tube, not very accurate unless better Milton, but these still can be adjusted and be wrong.

Your reference can be used to confirm the others.

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[Flyinfool]

Where I work we build ASME certified pressure vessels. One of the requirements for testing is that the test gauge should be near double the planned reading. Gauges are most accurate around 50% of travel. And this is even with expensive test gauges costing over $1,000 each. We have different gauges for testing we need to use a 750 PSI gauge to test a system of 300 to 400 psi, we would have to switch to a 1,000 PSI gauge to test a 450 PSI system. Or a 500 PSI gauge to test a 250 PSI system. All of out gauges are the type that can be calibrated. We have one set of master gauges that are sent out to be certified on a regular basis and all of the others are calibrated against the master set very often.
Another rule is that you always need 2 gauges on the test setup. if they have a different reading they are both considered bad and have to be re calibrated or replaced. We build vessels from 150 to 1000 PSI, most of what we make is 400 PSI. This is a lot more than what you are looking at but the principles are the same.
You need to make of a cheap and dirty manifold so that you can test 2 gauges at the same time so that you can be sure they are both seing the same pressure. Every time you test the pressure you bleed off a bit of that pressure to fill the gauge, so the next test will have slightly different conditions. On my water tank I have an analog gauge permanently attached so I can check it at a glance every time I walk by. By using a tire gauge you are letting out a bit of air with every test. If you are that worried about the pressure being just right, then set up the tank with 2 permanent gauges and as long as they read the same then you can assume they are still accurate, as soon as one shows different you need to replace them both because you do not know which one is off.

If the system works good at 38 PSI on the tire gauge, then make that the standard and see what a permanent gauge reads when the tire gauge says 38 and make that the new standard and just use the tire gauge once a year to check the permanent gauge for drift. With the permanent gauge you can also see just how far the system pressure fluctuate during normal use.

 

[Just for fun]

The real question was why I could check my air tank and just between the tire gauge and the little digital they would read the same, step inside the building and check the pressure tank they would be so far off. Like I said in my first post, I believe it is because there is some sort of blockage in the Schrader valve, and it takes a little bit to come up to full pressure. During that time frame the digital gauge takes a sample and registers the reading before it is at full pressure. I have several air gauges granted none of them are the high end/high dollar units. This is the first time in 30 years I have tried the digital gauge.

@Flyinfool The tank is a pressure tank for a water well system, the pressure switch for the well is a 40 to 60 lbs switch. The air pressure is always set 2 lbs below cut-in. I could install a permanent gauge I suppose, it's not something that I could use on a regular basis though. The only way to read the air pressure is to shut the well down, drain the tank of water and read/set the air pressure. Unless I start having some sort of problem, I read it and adjust it once a year.

 

[tq60]

Visit a pump supply or commercial irrigation supply and buy a water rated gauge and install it.

8 to 13 bucks or so.

Glycerin filled so it is stable reading.

These are for our irrigation system.

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