How does an air pressure regulator work?
by B.B. Pelletier
This question popped up earlier this week, so I thought I would address it today.
What IS a pressure regulator?
An air pressure regulator is a mechanical device that controls the air pressure and volume available to the firing valve of a precharged airgun. Because the pressure is always the same, a gun with a regulator shoots with very consistent velocity, plus it may get a few more shots on a fill. As long as there’s enough pressure in the reservoir, it operates as described. When the reservoir pressure drops below the regulated pressure, the regulator remains open all the time and the gun becomes unregulated. It continues to fire, although most shooters notice a change in the discharge sound when their gun is “off the reg.”
Okay, let’s examine this closer. The reg allows high-pressure air to flow into a small chamber called the firing chamber. When the pressure in the firing chamber reaches a certain level, the regulator closes the air flow. The firing valve of a regulated PCP opens to release the pressurized air from the firing chamber. This air is at optimum pressure for the way the firing valve has been tuned…the size of the air passageway through the valve, the strength of the valve return spring (which affects the speed at which the valve closes) and the volume of the firing chamber. The gun is perfectly tuned when the air from the firing chamber expends all of its energy (to the extent that is practical) just as the pellet reaches the end of the barrel.
The output of a regulated airgun is affected by both the pressure inside the firing chamber and the volume of the firing chamber. Change either one and you change the tune of the gun. Although the illustration shows the firing chamber to be relatively large, in fact, it is much smaller than shown. In the AirForce Micro-Meter valve, the entire firing chamber fits in a small compartment inside the valve body, and even then they had to take steps to reduce the chamber volume so the gun wouldn’t be too powerful. The Micro-Meter valve and tank are not regulated, but they operate by a similar design that severely restricts airflow to the firing chamber.
How does an air pressure regulator actually work?
The pressure regulator contains a powerful spring that acts on a valve. However, this spring is not powerful enough to keep the check valve closed against the high pressure of the air reservoir. Air forces its way through the check valve and into the firing chamber. When the pressure builds inside the firing chamber, it pushes on the check valve, helping the spring close the valve against the air reservoir. To control how much pressure is in the firing chamber, the spring is adjusted until the desired pressure closes the check valve.
Regulators take up space!
Air volume is lost to make room for the regulator. The tradeoff for that lost space is more shots, because the reg lets the firing valve operate on a lower air pressure. Sometimes, it’s an even trade and no additional shots are gained. Other times, the regulated gun delivers a few additional shots. However, the firing valve must be tuned to work well at the reg output pressure. It’s never a given that the combination of the reg setup and the firing valve tune work well.
What prevents shooters from filling their guns to an even higher pressure to get more shots? Nothing, except the regulator itself. The spring has to be small to fit inside the reg, yet it also has to be powerful. Most regs use a special type of spring called a Belleville washer, or more correctly, a Belleville spring washer. These are cone-shaped washers stacked in different ways to achieve desired spring rates through a range of values. Once set, the spring operates within a narrow band. Belleville washer springs are strong, relative to their diameter, which makes them perfect for the inside of a regulator. However, they do have limits. They’ll do their job within the limits of their design, but they won’t hold back pressure that’s much higher than they were designed for. If you overfill the reservoir, the firing chamber will be filled with higher-pressure air and there goes the sensitive balance of the gun’s tune.
It isn’t a question of “if” a regulator will fail, it’s a question of “when.” All regs fail, and when they do they have to be serviced. I have owned regulated airguns that worked fine for about five years, but then the O-rings went bad and the regs had to be rebuilt. Leaking seals are a real problem with a reg, as are failed Belleville washers, though in my experience they’re not as much a problem as seals. The point is that all guns with regulators eventually require maintenance, though they may give many good years of service.
Because reglators are complex and less reliable, the USFT rifle doesn’t have one. Instead, it operates on lower-pressure air for which a firing valve can be tuned to give an incredible number of uniform shots. That’s an interesting direction that no other airgun manufacturer has yet taken. Walther, on the other hand, has gone the opposite route of greater dependence on regulators to moderate their 300-bar (4,350 psi) operating pressure level. Both approaches work and both have good reasons for their use. It isn’t a question of whether it is better to have a regulated gun or not; it’s really up to the shooter to understand the technology he’s shooting, so he can get the maximum benefit from it.
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