Pressure gauges: Can you believe them?

by Harley Ayre

You may have seen a message like this on one of the airgun forums:

I found the sweet spot for filling my Condor is not the 3,000 psi recommended by the factory but 2650. I asked AirForce how many shots I will get at full power, and they said it will be the same as if I filled to 3,000. How can that be? Somebody's got to be lying!

The only one that's lying is that shooter's pressure gauge. And so is yours. In fact, they're ALL off to some extent - which is the topic we will examine today - pressure gauges and how to deal with them.

To be accurate, pressure gauges must be calibrated
All measuring devices must be calibrated to a known (established) standard to be considered accurate. The small pressure gauges we use to monitor the filling pressure of our airguns are never calibrated - even at the time they're manufactured. In fact, most of the cheap little gauges we use are incapable of being calibrated or adjusted in any way! Instead, the accuracy tolerances are built into the gauges, based on their design.


Most of the small pressure gauges sold with refill adaptors are not going to be accurate to the exact pressure. You should learn your gauge to determine the final fill pressure for your airgun.

The most common gauge
A standard industry-grade D gauge is commonly found in inexpensive airgun fill devices. It has an accuracy tolerance of plus or minus 5 percent. At 3,000 psi, that gauge can be off by 150 psi in either direction and still be considered okay. Even worse, gauges will most likely have a greater error at their extremes than in the middle of their range. If your 3,000 psi gauge tops out at that pressure, that's where it will most likely have the greatest error. If the gauge goes up to 4,000 psi, 3,000 will fall closer to the middle of the range and should be more accurate.

A better gauge
You can buy a better grade of pressure gauge, but the mid-scale accuracy of pressure indication again comes into play. Industry standard-grade B gauges have 2 percent mid-scale accuracy and 3 percent over the first quarter and the last quarter of the pressure scale. To get an accurate reading out of this gauge, you would want a dial reading of 4000 psi so it moves your useful 3000 psi reading toward the middle of the scale.

What can you do?
In truth, there isn't a lot anyone can do about this situation except to understand it and to act accordingly. To illustrate that, let me give you a related example. I have a dial-type John Deere thermometer hanging on an exterior wall of my house. When I stepped outside this morning, the needle was reading 46 degrees. I could tell it was a lot colder than that, so I tapped the thermometer - the needle dropped to 28 degrees, a temperature that was in close agreement with the National Weather Service report for my town. I've learned over the years that I have to tap this instrument to jog the needle off wherever it sits to get the true reading. That doesn't mean I'm getting the correct temperature even then, but it's a whole lot closer that way.

You have to find the gauge reading at which your gun is filled to give the best performance. This is one legitimate reason to own a chronograph. You find the max fill point by first filling to the recommended maximum, then chronographing every shot. I once owned a .22 caliber Air Arms Shamal that came without instructions. I filled it to 3,000 psi and the first shot with a Crosman Premier was a disappointing 660 f.p.s. The gun was supposed to shoot around 800 f.p.s., so I figured it was overpressurized. The valve could not open all the way against the excess pressure. I then shot a string of 66 shots through a chronograph and the rifle gained velocity continuously up to number 52. When all was finished, I could see a good band of 23 shots ranging between 780 and 809 - a spread of 29 f.p.s. The band began at shot 42, which turned out to be 2,600 psi, so that became my max fill pressure WITH THAT GAUGE! If I had changed fill devices, there would have been a little fiddling to establish the max fill level with the new gauge, though nowhere near as much as when I had no clue where the limit was.

Oh, but there's more!
You can't even count on all gauges performing to the level their specification says they should. It would be nice if they did, but in honesty they don't. Let's face it - these gauges are not checked after manufacture. They're simply assembled, boxed, packed into cases and shipped - that's it! That may bother some people but a heck of a lot of manufactured stuff is handled the same way and we don't notice it. The only time manufacturers get serious about what they make is when lives and safety are at risk - which is why gauges like the ones we use cannot be used for aviation applications and also why there are safety burst disks installed in pressure vessels of a certain size.

Once you find the ideal fill pressure, your gun will perform like all others!
It seems strange that a gun filled to 2,650 could shoot the same as one filled to 3,000, but now you understand that those numbers are probably not correct. The one thing that does remain more or less constant in this ocean of moving numbers is your specific gauge. If it says the fill level is 2,650 today, it will be the same a week from now and a year after that. Learning your gauge is crucial to your success with a PCP airgun.

It's not really a problem.
The owner of a PCP airgun has the duty and responsibility to know that gun better than anyone - including the manufacturer, in certain circumstances. The owner's manual is a guide, but common sense is also needed. PCPs are very much like muzzleloaders that use black powder. Each gun is unique and must be learned by its owner - after which it will always perform at its best. That also goes for the support equipment - like fill devices!