View Full Version : air pressure

09-25-2003, 06:16 PM
Anyone know how much pressure is in a gas strut that holds rear hatches open? I'm thinking of trying to modify one to use as a temporary pressure testing tool. I need to be able to get up to about 1000, maybe 1500 lbs air pressure to test lines and fittings, etc. I would turn it into a bicycle type pump, hand powered. Some math will tell me how much pressure out for how much push on the handle, something like that. Or I'll pick up a gauge to read the pressure. Can the seal in the strut take that kind of pressure, or is there an easier and better way- I don't need volume, just a few strokes of the handle and read the gauge, listen for leaks, or watch for bubbles.

09-25-2003, 06:40 PM
It depends on the strut. It can be up to 2500psi N2. Be most careful with those things.

09-25-2003, 06:59 PM
I don't think that is feasible anyway. If you have any appreciable volume to fill you'll be pumping till your arm falls off to get that kind of pressure. How about a scuba tank as the source?


When I say volume I only mean a few cubic inches.

Also note that on a gas spring the effective area of the piston is the piston rod only, not the ID of the gas spring. That is not a lot of area. Also, the gas spring piston has a damper disc fitted to the piston rod with a bore hole to regulate extension speed. This would make it completely ineffective as a pump.

[This message has been edited by Evan (edited 09-25-2003).]

09-25-2003, 10:06 PM
DArryl: if you have away to measure the force on the piston, the same formula you use to calculate pressure will give the internal pressure now in the cylinder.

But I suspect evan is right- unless you Need a very small volume, you will pump a long time at low pressure before you start building pressure. You might consider a two stage pump. How many cubic inches are in the final system to be tested?

Despite warnings to the contrary, I have drilled into both shocks and struts. Nothing exciting happened, but i get tense every time. I would not try to "burn" into one at all. I would never suggest any one else try the drilling even.

The inside surface of both shocks and struts is very good. Shocks have larger diameter, but as Evan says they too have by-passes unless modified.

Testing with air at high pressures scares me. Try to use a liquid. We had a man testing heat exchangers, they used air to test. The exchanger was in a tank of water, man leaned over the edge, the exchanger exploded, the concussion made paste of his liver. Nice thing about liquid is that the pressure drops very quickly with any leak, gases keep blowing a long time, depending on volume. I know you know all this already but think on it a while.


Edit comment: you are talking about 10 atm (more or less). The first strokes will pump in a full charge of air, the final strokes will go way down before they over come the pressure and inject more air. I suspect Evan had thought of that when he mentioned lots of pumping even on small volumes. I did not till after post. I should have thought more before hitting the send key.

[This message has been edited by docsteve66 (edited 09-25-2003).]

09-25-2003, 11:29 PM

1000 to 1500 psi? That is 75 to 100 bar. LOTS of pumping. Imagine how hard it is to compress a fully charged gas spring. That is how hard it will be to pump that pressure with just the itty bitty area of a gas spring rod. Not me, thanks. It won't work. Scuba tank if you must use air. Steve's water idea is way safer, that's why they do hydrostatic testing on gas cylinders. (everyone knows that)

09-26-2003, 01:02 AM
Don't screw with the gas accutators. They have high pressure Nitrogen and are extremely dangersous to cut into unless they have totally failed and no longer work. Some have oil in them for a dampening action - if you get sprayed by fluid at 100+Bar. it will cut through you like a hot knife through warm butter. This causes severe damage to tissues and often results in amputation.

So go ahead and become a "Darwin Award Winner" - or play it smart and DON'T SCREW WITH THEM!

Steve is talking about "Hydro-Testing", which involves filling the test chanmber with water while submerged in water. Never use air under water - you get the kind of fatal explosions Steve warned about.

[This message has been edited by Thrud (edited 09-26-2003).]

09-26-2003, 01:45 AM
Pressure testing of anything is only done with liquids by those that know what they are doing. A couple gallons of air at room pressure is a good bit smaller than a BB at 1000 psi. From this you can see how even a small container of compressed air can make for a very large and powerful explosion if something fails. All liquids are essentially incompressible which means that a failure usually only drips a small amount of liquid because it can not store much energy. Pressure gauges rated for pressures of thousands of pounds are calibrated with oil with the pressure essentially applied by a screw pushing on a hydraulic cylinder with the outlet hooked up to the guage to be calibrated and a master guage. You should be able to rig something like this up and do high pressure testing safely. Be absolutely sure that there is no air bubbles in the system as even a little air at thousands of psi can cause a pretty good bang if some thing fails. Low pressure guages are calibrated with a device called a "dead weight calibrator" which uses a carefully lapped rod in a precision cylinder as a piston and calibrated weights are applied to put on the required pressure. A hand operated pump is used to apply pressure to lift the weight. I think I have one of these test rigs that I will sell you if you are interested. Don't use air to pressure test anything if you value your life.

09-26-2003, 01:52 AM
Thrud, Cass,

Yes, good. Live longer.

09-26-2003, 05:35 AM
Thanks, guys. I should have seen it, the explosion potential using air. I figured a leak would show up easier with a gas than with a liquid, but who cares if you're dead. I'm instantly convinced that liquid is the only safe way, that's what I'll rig up. A bolt pushing a rod into a cylinder, some means of letting all air escape, or vacuuming the system of air, before filling with liquid, and a gauge. Got it, thanks again.
By the way, I have a gas strut that I cannibalized some time ago, that's why I wanted to use that. I don't recall how the pressure got out, but it's going back in the bin until I think of another use for it.
In addition to the danger of using air, I don't want to have to pump for an hour just to fill a line to pressure, let alone a container of some kind. I guess that was just a bad idea.

09-26-2003, 12:14 PM
Evan: I KNEW THAT!. I was sitting at table with coffee and it hit me!- came right in to correct and found myself way in the rear. Frankly, I have no "feel" for Atm ,bar what ever. But for this problem, it just seemed that PSI might have less meaning than ATm or bar. Thanks again. If I had been using a slide rule a factor of ten (jeeze, 10!!!) might be excusable for a complex problem. Oh, well thats how engineers kill people, and the Darwin factor says do your own thinking.

Thanks to all for helping keep my ass out of a sling. And glad Darryl is considering hydrostatic tests.

I embarrass myself.