So What’s The Real Pressure In My PCP?

Almost all of us have observed differences in pressure readings between the gauge on our PCP and that on a fill tank, pump or compressor. Sometimes the difference can be large, sometimes small. But it always raises a question. What’s the real pressure in my PCP?

HAM reader Bernard Fouché has been working on this problem. His answer is to install a high precision gauge inline between the tank and gun.

It’s a simple and effective solution. However it allows our HPA gauges to be calibrated against a high quality, standard gauge. It’s a very valuable tool if you really want to understand your airgun and answer that question: what’s the real pressure in my PCP?

Thanks to Bernard for sharing the detail with us!

I came up with an idea, I call it my “independent external pressure gauge”. While I’m pretty sure that I’m re-inventing the wheel, here it is:

This is made from parts I bought from Huma Air (since I live in Europe) but all of these parts are quite common:

– 1 x digital gauge (the device is shown with a Huma Air gauge, one can use a Sekhmet one, or any other one instead) with a 1/4 BSP male thread.
– 2 x female Quick-Disconnect to BSP 1/8-Inch male threads.
– 1 x ‘T’, with all three 1/8-Inch ports having BSP female threads.
– 3 x 1/8-Inch bonded seals.
– 0, 1 or 2 x QC bling plugs. Zero if you insert the device only in complete circuits, one if you want to read the pressure of a tank (or of a buddy bottle), two if you want to protect the device from dust.

(One can use a threading different from BSP, what matters is that all parts have the same type of threading, pay attention when ordering. The device can be assembled by anyone as long as all of the threads are the same 😉 Do not forget to seal the connections!)

A ’T piece’ similar to one used is shown at the bottom of the picture (3 x 1/8-Inch, all females). This seems to be the part sometimes difficult to find. Since this part from Huma Air is available only with BSP threadings, this made all the parts to be BSP threaded. You may have a different situation, this all depends on what is available to you.

The need for such a device arose from my setup: I have an Altaros booster, a 15 liters tank, different pressure gauge from AliExpress, together with a RTI Prophet with its own gauges. Until now, all of my gauges were analog, from different suppliers and none of them was described as being ‘calibrated’.

When used in the same pressurized circuit, all these analog gauges give different readings. Like with horoscopes, I could choose whatever reading pleased me the most, or the less, depending on my mood. However high pressure circuits require more down to earth information.

I’ve also experienced returning from a dive shop supposed to have filled my tank at 300 bars / 4500 PSI, to discover that the filling I got was closer to 250 bars. However with such untrustworthy gauges, I was unable to grasp the situation for what it was, I had feelings, not facts.

Here is of those gauges often offered on AliExpress:

(The picture may not show it clearly, but this gauge glows in the dark so uncalibrated information is available in all light conditions. I wonder who needs to use a filling station in complete darkness, zombies ?)

Hence I came up with what is shown in the first pictures: a device that can be connected to any pressurized circuit using Quick Disconnects. I chose to use female QD on both sides because the male-to-male part is usually already available in every PCP toolbox, a female-to-female QD is rarer. This way the device can be inserted (when adding a male-to-male connector) or even used as a female-to-female connector. Beware of QC male connectors to 1/8-Inch BSP (or any other threading), there are often non-return valves and you don’t want that.

DIN connection is clumsy and is not used past the main air tank tap, so Quick Connect / Disconnect was a logical choice.

Here is my filling setup, the digital gauge is close to the ground:

With this device, using a QC blind plug, I know what remains in my main tank, or in the buddy bottle that I have. Since the device is small, reading the pressure of a tank wastes a limited amount of air. The device doesn’t not have a bleed screw since there are usually bleed screws already installed at different locations of the circuit to test. However if you want to read the pressure of an air tank having no bleed screw on its tap, you will have to insert one, to be able to degas the circuit before disconnecting the device. The device is small enough to carry everywhere.

I can insert the device in the filling station circuit and accurately fill the Prophet at 300 bars / 4500 PSI (the gauge from RTI is optimistic by 10 bars, probably a measure of safety), as I was able to understand that all of the analog gauges that I have must not be trusted.

The AliExpress originated gauges may be correct, may have a fixed error, or may have an error that varies with the pressure. Mine are either correct or optimistic, which limits the risks I’m  enduring if I’m relying on them. For instance, here is my tank gauge compared to the digital reading:

(My main tank gauge, in red, shows a value closer to 275 bar than the 250.2 bar pressure the digital gauge reports)

Specs for the Huma Air gauge is give a 1% error, the Sekhmet a 0.25% error, however their features and cost differ.

Beside the threadings, one must pay attention to the pressure ratings of all of the involved parts before ordering them and assembling the device, especially digital gauges since some of them are not rated for 300 bars / 4500 PSI. Even if you are using a lower pressure today, consider having something that could be used also for a future air gun to make the device a long term investment or to ease its reselling.

Such a digital gauge connected inline between a buddy bottle and an air gun allows one to know the amount of air used by each shot.

Sekmeth describes its gauge as being able to augment this information with the number of remaining shots, I will try mine soon.

Having the gauge mounted as an external gauge allows tuning of different air guns and to avoid paying such a high end gauge for each gun you own. Since once an air gun is tuned, you don’t fiddle with the settings while you are on the go, you are only interested in knowing if the gun is filled and how many shots you still have, an uncalibrated analog gauge is usually enough.

This project helped me to answer the question: what’s the real pressure in my PCP? So I thought that I could share this idea, it may help someone else.

Bernard, thanks for sharing! In fact, HAM uses a similar inline gauge system for some of our testing. For example, you can see it in our Extreme Booster Pump review. (Photo below). But I have to say that Bernard’s is superior for accuracy!