An Introduction To The PCP Regulator. Don’t Ignore The Plenum!

In his previous article, Bob reviewed the challenges in producing (nearly) consistent muzzle velocity from unregulated PCP air rifles. Of course the PCP regulator seems to offer a solution. But there’s more to it than that, as Bob explains in this article.

Trying to produce a constant velocity in a PCP while faced with varying air pressure seems counter-intuitive. Why not use a PCP regulator to produce a constant pressure at the valve, regardless of the reservoir pressure?

For many shooters, the concept is ideal, but not for everyone.

First of all, since the power of a PCP is proportional to the pressure, reducing that pressure will obviously reduce the FPE potential. However, you don’t use all the available horsepower of your car most of the time, so if you don’t need maximum power (FPE), then using a PCP regulator makes a lot of sense.

Also, since bottles rated for higher pressures (eg. 4500 psi) are now readily available, we can drop that down to 3000 psi and still have good power available. Here is an example of some PCPs I built that use 3000 psi bottles and regulators:

An Introduction To The PCP Regulator. Don’t Ignore The Plenum!

The obvious example of a gun better left unregulated is a Big Bore PCP, intended to take larger game. All you need is a couple of powerful shots, which is easily achieved unregulated. However, if you wants lots of shots with a low Extreme Spread, a regulated PCP could be just the ticket.

The Importance of the Plenum

Whenever you want the maximum FPE from a PCP, unregulated is probably the way to go.

The problem is that no PCP regulator can operate fast enough to top up the pressure inside the valve during the 1-2 milliseconds it is open. Only the volume of air downstream of the regulator, between it and the valve seat, which is usually called the plenum, is available for the shot.

If that plenum volume is too small for the FPE you want to create, the pressure drop inside the plenum (during the shot) is too great. This means that the power (and efficiency) is reduced.

Plenums are achieved with a tubular spacer between the regulator and valve, or designed into the gun.

Let’s look at a popular PCP, the Benjamin Marauder. A stock .25 cal MRod can easily be tuned for 45 FPE. One of the most common mods to them is to install an “in tube” regulator, that sits in the reservoir, just ahead of the valve (sometimes replacing the gauge block).

But if we install the regulator right against the valve, there is almost no plenum volume available, maybe just the volume in the valve itself! This type of installation – if you expect to stay close to the original 45 FPE – is bound to disappoint.

Consider the chart below:

An Introduction To The PCP Regulator. Don’t Ignore The Plenum!

With just the valve volume only (maybe 6 cc), you might end up with only 30 FPE, unless you drastically increase the regulator output pressure (called the setpoint).

You will note from the chart that a plenum volume of about 1 cc per FPE (ie 45 cc, at the red diamond) will deliver nearly the same FPE as you had originally. Below ½ cc per FPE, the power drops off quickly.

For in-tube regulators, that range of plenum volume, between ½ – 1 cc per FPE, is the recommended range. Even at ½ cc per FPE you will likely have to increase the setpoint pressure about 10% to compensate for the pressure drop during firing.

What PCP Regulator Pressure to Use?

When you are installing or adjusting a regulator, you need to decide what setpoint pressure to specify or adjust it to. You can estimate that by looking at the shot string you had in the unregulated version. Consider this chart from the previous article:

An Introduction To The PCP Regulator. Don’t Ignore The Plenum!

If your plenum is 1 cc per FPE, you want to select a pressure at about 3-5% higher than the pressure that gave you the velocity you want from the unregulated tune you were using, with the same pellets.

Let’s say you wanted a velocity of 850 fps. You look at the downslope (low pressure) side of the appropriate bell curve. From the chart above, you will notice that 850 fps occurs (on the teal curve) when the pressure is just below 1400 psi.

Assuming you have a plenum of about 1 cc per FPE, you would set your regulator to about 1450 psi. If you wanted 900 fps, using the black curve and pressure line, you would select about 1650 psi.

If you only needed 800 fps, using the purple curve and pressure line, you would choose about 1150 psi for the PCP regulator setpoint pressure.

What would you do if you could only manage a plenum volume of ½ cc per FPE?

In this case, I would suggest that you increase the setpoint by about 10% compared to your unregulated tune. You can get away with even smaller plenum volume, but only if you are after a low to medium power tune, or are prepared to significantly increase the regulator setpoint to compensate.

Obtaining Optimum Shot Count

When you increase the setpoint of the regulator, you reduce the usable pressure range in the main reservoir, called the “headroom”.

For example, if your reservoir fill pressure was 3000 psi, and your regulator setpoint was 1500 psi, the difference of 1500 psi would be the headroom available to use before the reservoir pressure fell below the setpoint pressure and the regulator stopped regulating.

Below that, the pressure at the valve (and the velocity) would start to decrease.

If your setpoint was 2000 psi, the headroom would only be 1000 psi (instead of 1500). This means you only have two-thirds of the air available, so your shot count will suffer.

With an in-tube PCP regulator, more plenum volume means less HP reservoir volume.

The best shot count with an in-tube regulator depends on making a compromise between plenum volume and reservoir volume. For many PCPs, that will occur at about ½ cc per FPE, though more power (and efficiency) is available at 1 cc per FPE.

High Powered Regulated PCPs Are Possible

If we abandon the concept of an in-tube regulator, and use a separate HPA bottle for the reservoir, we can design the gun with a separate plenum of adequate volume. It is then possible to build a powerful, regulated PCP.

Here is a photo of a 6 mm PCP which I built recently which produces 130 FPE.

An Introduction To The PCP Regulator. Don’t Ignore The Plenum!

The key is the large plenum above the bottle. The Carbon Fibre 4500 psi tank is regulated down to supply 2800 psi into the plenum, which is 145 cc. As long as you use an adequate size plenum for the FPE intended, regulated PCPs can be built with lots of power.

Next month I will talk about tuning your regulated PCP.