Author Topic: REALLY! High Pressures  (Read 541 times)

Alan

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REALLY! High Pressures
« on: January 06, 2019, 04:37:27 PM »
As our resident expert, Bob Sterne, recently pointed out, Boyle’s law ( the pressure [P] of a gas is inversely proportional to the volume [V] ) only works with an “ideal” gas. In the real world, at some point ( depending on temperature, and the gas in question [helium, nitrogen, etc.] ), the law falls apart. In other words, doubling the pressure doesn’t cut the volume in half! This fact is important to remember.

A few months back, a prototype airgun was proffered which used a 7,500 PSI (≈520 bar) air supply. Air (gas), in this case, was the air we breathe. Had the airgun used helium or nitrogen, it might have been more interesting.

In case you missed the point here, you need to ask yourself a question. All else being equal, how many more shots could you take, using a tank pressure of 7,500 PSI (≈520 bar), versus one using 4,500 PSI (310 bar)? Now don’t be surprised at the answer! Depending on the tank volume, using the air we breathe, perhaps only 4 to 6!

What bothers me the most, is that some PCP airgun enthusiasts have a death wish! I’ve actually seen past-due (recertification) SCBA tanks filled to 5,000 psi (345 bar), and no over-pressure relief valve in sight! If one of these tanks let loose, the result could cause fatal injuries to anyone close by.

Imagine what would have happened, if this was the case in this video?
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I shudder to think!


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Alan

I have an Omega compressor. If you're a fellow Guild member, and you pass through Roswell, NM, I'll fill your portable tank as a courtesy.

Nicesurprice

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Re: REALLY! High Pressures
« Reply #1 on: January 07, 2019, 05:51:38 AM »
We can fill up to 420 Bar.
Need to find a bottle witch can handle it ????
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Alan

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Re: REALLY! High Pressures
« Reply #2 on: March 29, 2019, 04:44:01 PM »
Sorry for the late response... When you get much past 3,00 psi, the amount of additional "shots" per pound, drops off rather rapidly. The common vernacular with respect to the issue is called, Compressibility Factor.

We need to remember a simple fact—the "air" we breathe is made up of a myriad of gasses, with nitrogen being the prime one. When mixed with oxygen, and trace gasses (plus a whole lot of solids in the form of pollution) at high pressure levels, Boyle's law falls apart.

It is possible to use helium (somewhat expensive) or nitrogen (a bit cheaper), and actually get more "shots" as it were, for any given volume and pressure. But other than the compressing costs, air is sort of free.
  • Roswell, New Mexico
Alan

I have an Omega compressor. If you're a fellow Guild member, and you pass through Roswell, NM, I'll fill your portable tank as a courtesy.

Frank in Fairfield

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Re: REALLY! High Pressures
« Reply #3 on: March 29, 2019, 05:28:27 PM »
Alan,
Not at our FT matches but, just shooting on the range, I have seen many expired tanks and those were filed well beyond 4500psi.
I packed up and left immediately.
Sadly, most RSO do not have knowledge of air tank pressures.
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steveoh

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Re: REALLY! High Pressures
« Reply #4 on: March 30, 2019, 05:19:43 PM »
I admit that this high pressure stuff makes me nervous as all get out.

I just bought a used 4500 psi 45 min fireman tank and it’s off getting hydro’d. I get four years out of it for $80. The DAQs eat up air so fast, I could see having four or five big tanks for an outing.

Lot of guys are buying cheap expired tanks so they can afford pcp airguns. Worries me a bit.

Someone from Europe said their tanks are rated for 30 years. Forget where I saw that. I know you can now buy 15+ year old tanks that have been recertified and I wonder how they determine if such a tank is safe. Very curious.

Then there are a plethora of Chinese tanks which may or may not be DOT certified.
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Alan

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Re: REALLY! High Pressures
« Reply #5 on: March 31, 2019, 05:17:45 AM »
It is this "issue" which I believe is the reason so many manufacturers are bringing out 12 volt-powered, PCP compressors.

There have been gas engine powered "breathable" compressors for a number of years, due to the scuba industry. However, as Steve can tell you, these are not for the lite-pocketbook types!!
  • Roswell, New Mexico
Alan

I have an Omega compressor. If you're a fellow Guild member, and you pass through Roswell, NM, I'll fill your portable tank as a courtesy.