Under Pressure

Written by Tom Gaylord

Reproduced with permission from The Airgun Letter
The FX electric compressor solves the problem of filling precharged airguns

Among the many camps in airgun-dom, the precharged or compressed-air guns are making the greatest advances today. Despite being among the very oldest powerplants, precharged guns suffered abandonment the better part of the century as newer guns rose to the forefront. When they were re-introduced to the world by British makers in the 1980's, few airgunners knew of their long history. They were viewed as revolutionary new airguns that quickly established dominance in most of the precision airgunning disciplines, due mostly to their almost complete freedom from sensitivity of hold -- the bane of spring-air guns. But all was still not well with precharged guns.

Because they were and still are filled from scuba tanks, there has been a general reluctance to switch over to PCP guns, despite all they have to offer. The most common reason given for not switching (or even trying them out) is all the support stuff needed to keep them running, which primarily centers around how they are filled. Spring gunners say they don't want to be encumbered by all the paraphernalia that necessarily accompanies a precharged gun. They can grab just their spring gun and pellets and be free to make a day of it, while their precharged brethren are seen wheeling heavy air bottles, bulky charging devices, and bags full of other items wherever they trek. There is a lot of truth to this argument.

Another unspoken reason for not trying precharged guns is the danger element. Everyone has seen the movie Jaws and remembers how the monster shark exploded when the scuba tank he was chewing was punctured by a rifle bullet. No one wants that to happen to them while innocently filling their airgun.

But the biggest reason airgunners don't embrace precharged guns more than they do is the trouble associated with getting the high-pressure air into their tanks. You have to lug your expensive scuba tanks to a dive shop and pay to have them topped off, even when they are still 3/4 full by diver's standards. How wasteful it feels to watch someone put 800 pounds into a 3,000 pound tank, because you stop using it when it dips down to 2,200 psi. A diver would have had most of his time left, but an airgunner doesn't run his tank down to zero. He has to fill it when it gets down to around 2,000 psibecause that's where the airgun bottoms out. No sense trying to put 100 psi into a rifle to get five good shots, is there?

To help with this situation, the Swedish hand pump came to market a few years ago. It made topping off an airgun much simpler than before, except for one small problem -- it required manual effort!

For many years I attempted to tout the benefits of the hand pump only to become convinced of one thing -- there are some folks who will never use it. The hand pump does require a certain amount of effort, and while it is more technique than brute strength, I have to acknowledge that some people are simply not suited to using one. So the hand pump is not the perfect solution. But an electric pump, or compressor, is!

Not just any compressor can generate the high pressures demanded by precharged airguns. Today, most guns call for a pressure of 3,000 psi (pounds per square inch) or approximately 206 times the normal atmospheric pressure at sea level. Shop compressors can pressurize to 120 psi, and some powerful ones cn go a little higher, but 3,000 psi is a realm they have no hope of reaching. Not all guns need that much pressure, but even the ones requiring less are still in a region where ultra-special equipment is needed.

A new compressor for this kind of work normally starts at $2,500 and up. Several years ago, a Korean compressor that can handle the load was offered for $1,200, and it was hailed as revolutionary. Even so, at less than half the going market price, the Korean machine was still unaffordable for many airgunners. But now FX of Sweden, the same company who makes the hand pump, offers an elecric compressor fr around $800! That brings it down to a level many shooters -- not to mention clubs -- can afford.

Airguns of Arizona provided the FX compressor for this test. When I finally pulled it from its box I was impressed by the weight of the unit and the apparant ruggedness of its electric motor. It has the appearance of a machine that will last a long time. More on that in a bit.

The compressor wasn't ready to be used right out of the box. First, I had to pour the contents of a small bottle of oil into the cooling reservoir, then fill the remaining volume almost to the top with tapwater. The liquid cools the compression chamber of the pump, which give you a much longer run time.


The problem I faced was how to operate the compressor enough to simulate the years of use an airgunner will give. I have neither the guns nor the time to test it that way. But I did find a neat way to do it. I filled scuba tanks!

Now you need to know that this compressor was not designed to fill a scuba tank. It was intended to fill guns directly, and it does so very handily. Even the gargantuan bottle on the Air Force Talon is a light job using the FX. But an 80 cubic foot scuba tank is beyond the design range of the compressor --make no mistake. However, there are smaller tanks, and we happen to have one. Our field target tank is a six cubic foot buddy bottle that fills my Harrier almost three times before the pressure starts falling. It's about 1-1/2 times the internal volume of the Talon reservoir, so it behaves like a very large gun reservoir, rather than a scuba tank. The compressor doesn't care what it fills -- just how long it has to operate till the job is finished. The buddy bottle was the perfect "load" for this test.

The FX arrived with a micro-bore hose, for less air loss, ending in a female quick-disconnect coupling. It will conect to any gun having a male QD fitting, like all modern Daystates, but I screwed an adaptor directly into a scuba tank K-valve yoke. Then the compressor went to work. It will fill the tank from dead empty in 14-17 minutes, but that is a rare situation. Usually there will be about 2,000 psi in the tank and a topoff comes in 5-7 minutes.

The compressor putts along like a small diesel engine under little to no load, as long as you remember to bleed the air lines before starting and the vessel you are filling is relatively empty. There is a problem topping off a tank that has 2,700 psi in it already, as the compressor has to generate enough pressure to overcome what's in the tank before it can start filling it. If you start with a too-full vessel, it is like trying to run the hundred-yard dash up a steep hill. The pressure builds up too fast and slows everything down.


Once I learned to only fill relatively empty vessels (either guns or tanks that are in need of a fill) there was no problem. It worked so well, in fact, I got cocky. I filled the 80 cubic-foot tank seen on the cover! I did it in easy stages, operating the compressor for five minutes at a time, then allowing it to cool for 20 minutes, or so. The tank had 2,500 psi to begin with, and it took three sessions to top it off -- the last one being very tricky because the tank was really too full to work with. But I did it and it worked.

Actually, the compressor has no idea what it is connected to, so the large tank puts no more strain on it than a small gun reservoir. What does matter is the time it takes to finish the fill.

The FX compressor has a built-in cutoff switch that shuts the motor off when the compressor reaches the maximum pressure. This is a handy device for those who want to do other things while their gun fills. Because of the way it works, however, it can also shut the compressor off when whatever you are starting to fill is already close to the limit. Many PCP guns have to be pressurized several hundred psi above the pressure in the reservoir before the inlet valve is forced open and actual filling can begin. If you try to fill a gun that has almost 3,000 psi in it, the overpressure required to open the inlet valve can trip the cutoff switch, shutting down the compressor before the inlet valve ever pops open. It can be like trying to fill a drinking glass from a 55-gallon drum! You may have to exhaust the gun (or small scuba tank) to a lower pressure level before trying to refill it. It's the reason that filling the 80-cubic-foot tank was a one-time-only experiment.



When the hand pump first hit the market, it wasn't long before someone posted plans on the Internet to electrify it. Those plans turned out to look very similar to the layout of the current FX compressor, giving rise to conjecture that this is just a hand pump with a motor attached. Not so!

Not only is the electric compressor water-cooled for longer operation, it also has a different compression tube than the pump. There are many similarities, but the two are not interchangable. This is a purpose-built compressor, and it may be operated safely up to four times as long as the hand pump -- within certain constraints. While the pump tube unit can tolerate 20 minutes of continuous operation, the water separator will have to be purged sooner in most situations.

Just like the hand pump, the compressor seperates water from the air it compresses. Periodically, this water must be removed from the unit by bleeding the compressor through a knurled screw in the back of the machine. How often this must be done depends on the humidity where it is operating, but inventor Fredrik Axelsson recommends not exceeding seven minutes between purges. To purge, shut the compressor off (there is a switch) and open the bleed screw. If you are filling a buddy bottle, close its valve before bleeding or all the air will run out of the tank, too.

A lot of moisture was collected by the separator in my testing. Living in Maryland, where the humidity is always high, I can count on wringing lots of water from the air during a fill. Because the machine is electric, I didn't mind the long blast of air that rushed out when I opened the bleed valve, since I had invested no effort putting it in.

If you still have more to fill after the purge, simply reset all the valves and restart the compressor. After 20 minutes of more or less continuous operation, though, give the unit a break to let the heat escape the cooling liquid.

What this means in practical terms is that clubs that charge 10-meter target gun resevoirs can operate the compressor almost nonstop, and, indeed, there are clubs doing that already. One in Germany reports a year of service with zero maintenance. Airguns of Arizona charges PCP rifles all day long with theirs and, again, a single unit has handled the entire workload.

The testing I was able to pack into two months of buddy-bottle filling was probably more like the use of an active airgunner over a half-year, and the machine took everything in stride. I think it might be possible to wear the compressor out if you habitually ran it much longer than the recommended time period, but even then you would be up against a very robust design.



This isn't a tool you can lug around very much. I did move our test unit a bit to demonstrate it before several interest groups, but the 62-pound weight made me think before I picked it up. Because of the cooling tower, the compressor has to be transported in the upright position, so the use of a hand truck is very limited. It's best to just find a place to run it and leave it there.

There is a grab handle on one side of the base of the unit and the electric motor serves as the other point to pick it up. A word of caution, though, with all the water in the cooling tower the compressor is a little top-heavy, so watch that it doesn't suddenly rotate in your hands and bang you in the head.

If you operate it on a framed floor, you'll notice some vibration in the floor as the compressor operates. It's not noisy, but it does set up a definate pulsing every time the pump unit compresses. I found placing it on a concrete slab floor in my basement was the ticket, though I'm sure a heavy wooden workbench would also absorb a lot of the vibration.

The 8.4-amp electric motor turns at 1680 rpm and draws 550 watts of standard 60-cycle 110 AC household current. You don't have to use a grounded (3-prong) plug, but you may notice the power drain in nearby incandescent lights on the same circuit. The motor has cooling fins, although I never noticed that it got more than slightly warm during operation.



AOA supplied us with the connector I requested, but they can also supply the compressor with pretty much every standard type of PCP connection. For those who shoot 10-meter exclusively, a DIN hole to which you can attach all your adaptors will be the obvious choice. The compressor comes this way, plus there are other combinations. You can order yours the way you want it, or just tell AOA how you intend using it and they can help design an adaptor set that's just right for you.



Remove three screws and the protective screen in back comes off, providing access to everything that moves. The belt tensioner is '56-Chevy-simple and obvious. The hoses are all accessible, and the pump unit appears dirt-simple to replace.

Behind the air bleed mechanism, a thick rubber boot protects the electrical connector that houses the pressure cutoff switch. It comes adjusted from the factory, but also has a small range of manual adjustment for fine-tuning, should the need arise. The trick is to have it cut off at 3,000 psi so your rifle is never over-pressurized. For guns with a lower operating level, like the Harrier, the manual shutoff switch allows you to control the max pressure for your needs -- so long as you don't exceed the design maximum of 3,000 psi.



You wouldn't think there would be much to say about an air compressor, but there is. First of all -- this one is affordable, especially compared to the rest of the market. Second, it's very easy to use. Put it in one place and just use it like a microwave. Third, it's rugged and reliable -- just what you want from a support item.

The final remark I'll make is this -- many shooters have avoided compressed-air guns because of the trouble involved in filling them. Others are shooting the guns but experience hardships keeping up with the air requirements. Clearly, some kind of electric compressor is neededfor these shooters. The market has no shortage of compressors that will do the job, but only one at this price. For the individual airgunner or the club that has to watch its expenditures, the FX compressor is a device whose time has come.

Our thanks to Airguns of Arizona for the opportunity to test the FX compressor.



FX electric compressor

Pro -- Solidly built, reliable, easy to operate, low cost, runs on common household current

Con -- A heavy piece of equipment

Availability -- No Longer Available

The air filling hose (or filler port, if that is how you elect to have the compressor set up) comes out

of a brass fitting on the front of the unit at the lower left. A gauge tells you how the fill is proceeding.

The air bleed valve is located in back of the compressor on the same side as the fill port. The power

cord ends in a European wall plug that has an American adaptor, but the unit has a 60-cycle, 110

volt motor, so there is perfect compatibility with household current.

©2000 Airguns of Arizona.

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