Time for a safety briefing and, yes, I am the cause of the briefing. This is not a blame game post – what happened to me will likely happen to me again and may happen to you as well. The important thing is to learn from it because I got real, real lucky.
I was topping off my new Hatsan Jet II with high pressure air from my Omega tank for maybe the dozenth time or more and thought I was being careful. I am respectful of high pressure air because I am a NAUI certified SCUBA diver and sometimes weld right with Gas, MIG and TIG torches. I have heard plenty of stories over the past 40 years (yeah, I am that old) about tanks, fittings and what not. Guess I got sloppy this time.
I connected the DonnyFL female foster fitting to the male foster fitting on the Jet II and gave a gentle tug to make sure it was seated. You always want to do this by the way. I really didn’t think a lot about it because I have refilled from a tank so many times. Bad routines can lead to bad outcomes and it did here.
I was holding the Jet II off at an angle where I could see the manometer (what they call the pressure gauge in the high pressure / pre-charged-pneumatic (PCP) airgun world. I’d guess the angle was about 30-45 degrees and may 18 inches away – these are all best guesses in hind sight.
I cracked open the valve and was doing a slow controlled fill. Somewhere just over 225 bar (3,263) PSI, the hose’s female fitting explosively slipped off the male fitting on the airgun and rocketed off. It happened so fast that I realized my face was numb, I had taken a direct hit in the polycarbonate right lens of my glasses and had heard something like a gun shot. I turned off the air and went to the bathroom to look in the mirror expecting to see blood. I was geniunely surprised there wasn’t any so then I started looking.
My right lens had a gouge where the fitting hit it and saved my eye. I had a small light bruise on my eyebrow but it really wasn’t bad. Luckily, I had spare glasses and grabbed them to be able to see.
My next thought was “what the hell just happened?” The O-ring from inside the DonnyFL female quick connect foster fitting was still on the Jet II’s male foster fitting. Nothing had broken – no tears, missing metal or rounded edges.
I was the victim of stacked tolerances. This means that any given specification has a +/- allowance that the machinist can go over or under by and still be ok. The stacked tolerance problem is when you have those tolerances of parts combine to then be outside of specification. Please notice that I am not blaming any of the vendors.
The DonnyFL female quick connect foster adapter, which works fine on my Air Venturi fittings and FX Impact Compact Mk II had slightly different dimensions than the Hatsan male fitting. I had noticed since I first bought the Jet II that I needed to firmly seat the DonnyFL female fitting as it didn’t go on easy. With 20/20 hindsight, I should have realied that was a bad sign and switched to the supplied Hatsan female quick connect fitting. I didn’t make the change befoe because I thought the DonnyFL and was good to go since it worked on everything else. Turns out that was a wrong assumption.
I really don’t know why more harm wasn’t done. Maybe the fitting flew straight out, the end of the hose was reached and it whipped back with less energy. Maybe because my glasses flattened around my eye socket it partially absorbed and then distributed the force. I can tell you I feel real lucky that worse didn’t happen.
What did I do as a result?
All of my air fittings are made by Air Venturi – I buy different configurations of them and keep them in stock so that when I have a fitment problem I can change to an Air Venturi fitting.
In this case, I wasn’t about to take the Hatsan apart so I took the Hastan supplied female fitting and put an Air Venturi male quick connect on the end to get a solid connection both to the airgun and to the DonnyFL quick connect. The fittings go together very smoothly and I’ve not had problem after almost a dozen top offs.
Summary of Lessons Learned
The problem of stacked tolerances will always be around and thus one brand of fitting may not work well with another. So here are the lessons learned.
- Be mindful that when you have to push a foster fitting on harder than normal then that likely indicates the fitment is wrong and you must look at your options.
- Use the airgun supplied adapter (if they give you one) and then put a brand of fittings on it that allow you to safely mate the parts together if there is a fitment issue.
- I will use the pressure gauge on the tank and not the airgun going forward even though I thought I was at a safe angle.
- Do not get anywhere near the fitting during refills – especially with your head or face.
I hope this helps you out. It was a wakeup call for me.
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