This is the presentation of the Gehmann 4 stage turbo pump, model M100.
First, let’s see what a high pressure pump or HPA pump is. PCP air guns need a very high pressure for the air to push the projectile, usually a pellet, down the barrel and all the way to the intended target. Usually, the minimum pressure for a shot is around 70 or 80 bar or around 1000 PSI. But in order to be able to shoot multiple times, the air storage container needs to store air at a much higher pressure. The most common storage pressure is around 200 bar (2900 PSI) but there are models that use up to 300 bar (4300 PSI).
200 bar is 200 times the normal atmospheric pressure – that is really high . A normal bicycle pump gets to around 3 or 4 bar. So to be able to generate such high pressure, a special pump is needed.
This type of pressure is also used in SCUBA diving tanks and in medical oxygen tanks. So it’s a very specialized domain. These applications usually use bigger volumes so they use special compressors to get to these pressures. A compressor is like an automated pump, so the principle is the same, it’s just an electric motor that is doing the work instead of your muscles.
For airguns, there are 3 ways to fill the air cylinder: a manual pump, an air tank that was filled previously somewhere else or a small compressor. The air tank is relatively cheap but you have to fill it somehow somewhere, so that is a hassle. The small compressor is quite expensive, especially the good ones. There are less expensive compressors made in China that can be good, but most aren’t.
I chose the manual pump because it’s cheap and quite hassle free.
The pump I got is the Gehmann M100 4 stage PCP pump. This is a special pump in the sense that is made especially for competition PCP air guns – both air pistol and air rifles. It does not have a hose, only a standard 200 bar 5-thread DIN connector. All competition guns have an adaptor between this connector and the air cylinder.
This is the Feinwerkbau adapter. All Feinwerkbau PCP cylinders since the 1990s, both for pistols and rifles, use this adapter. One end is screwed into the DIN connector and the cylinder is screwed on the other end. So the cylinder is fixed tight to the pump, like this, before pumping.
Other manufacturers have other types of adapters, like Steyr and Walther, where the adapter inserts into the cylinder. I don;t know which design is the best, but ,unlike the Feinwerkbau, for Steyr and Walther you cannot bleed the system with the cylinder still screwed on, because it will empty the cylinder.
The pump comes already assembled in the box. Only the footplate needs to be attached at the base with the provided screws. Also in the box is a sheet of paper with the operating instructions in english and german.
As i said, this is a 4 stage pump. Usually, most HPA pumps have 3 stages, meaning that there are 3 concentric cylinders and the air passes from one stage to the next in the course of pumping. I don;t pretend to understand how this works. There is a video made by someone else that tries to explain how this works, but i don;t think it’s 100% correct
This being a 4 stage pump complicates things even further. And the interesting thing is that there is this screw on the top of the handle that can enable or disable the 4th stage. How that works, i have no idea.
The instructions say that when using 4 stages, the pump pushes around 300 cubic centimeters of air, while when using 3 stages it pushes only around 200 cubic centimeters. The idea is to start with 4 stages (top screw ON – counter clockwise) until 150 bar and then switch to 3 stages (top screw OFF – clockwise) to pump with more ease.
I usually don;t use this screw. I keep it screwed in – i guess this means it’s OFF and using only 3 stages. I tried it in the other position and could not tell the difference. It seems to me to require the same amount of effort to push.
Speaking of effort, i don;t find it difficult to pump. It is true that I am on the heavy side, so that might help. To pump the P8X cylinder from 100 bar, which is the almost empty position, to 200 bar, which is the max safe pressure, it requires 125 strokes that takes me around 5 and a half minutes. Usually i take a break in the middle at around 160-170 bar to not overheat the pump
After the pressure has reached the desired value, open the screw under the manometer to bleed the system. The high pressure air will vent from this hole on the side, so be careful not to have your fingers in the vicinity. Then you can easily unscrew the adapter and cylinder.
The bleeding screw also functions as a water separator valve, as this pump is fitted with a moisture separator to eliminate oxidation during pumping. So when the system is bled, the moisture is also vented out.
The instruction manual indicates that no maintenance is needed on this model as it has been treated with lifetime grease. However, the o-rings inside the pump do not last forever, so seal kits are available for purchase for around 30 euros. https://buinger.com/Gehmann-Seal-kit-for-hand-pump-M100
The manual states that the pump is not to be dismantled by the owner, so the seals should be swapped by a specialized technician. There is however, on the Gehmann site, a video on how to replace all the o-rings https://alt.gehmann.com/downloads/M220/m220-new-pump-avi.zip
As i mentioned, overheating is a problem that must be avoided. Pressurizing air produces heat in accordance with the thermodynamic laws, and heat is the enemy of the rubber o-rings. It is advised to not pump continuously in excess of 5 minutes.
Overall, i find this pump quite good. It is of good build quality, feels solid and i used it for over an year now with no problems. As long as the pumping is not too frequent and the volume to fill not too large, it is a very good, easy and cheap way to fill an air gun cylinder.
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