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Measureing Combustion Pressure (PSI Gauges)

Post questions and info about combustion (flammable vapor) powered cannons here. This includes discussion about fuels, ratios, ignition systems, safety, and anything else relevant.
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Measureing Combustion Pressure (PSI Gauges)

Unread postAuthor: Deadeye » Thu Jul 24, 2008 6:20 pm

In an effort to perfect the best fuel mix I want to install a peak pressure gauge to basically record the combustion pressure of each shot.

My theory is that the gauge would be less anecdotal than my (not so) trained ear. I realize it will differ greatly based on ammo resistance so I will be repeating the same ammo for testing.

I have seen dial (analog) tire gauges you push onto the stem and then remove and it holds the pressure reading until you a push a little button and it releases the stored pressure. On most I think the button serves double duty as a bleeder valve so you can deflate to your target pressure while pressing down on the stem. On most (I found) this bleeder button is only that and there is no "memory".

Problem:
Most tire gauges are only 60 PSI - I want 100.
I would need a modification to eliminate the shraeder valve setup and get it to 1/8 or 1/4 NPT. Most unscrew to allow angled valves - I just don't know what threads -
I would prefer analog but might have to go digital.

Query:
Has anyone tried this; if so, was it worth doing.
Any rec's on where to get the right gauge. - (under $20 hopefully)
Any thoughts, perceived pitfalls, etc.
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Last edited by Deadeye on Thu Jul 24, 2008 10:15 pm, edited 1 time in total.

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Unread postAuthor: jimmy101 » Thu Jul 24, 2008 9:42 pm

I've fiddled with using a standard tire pressure gauge.

Turns out that the OD of the pressure gauge is very close to 1/4" (don't quote me on that. Take the gauge to the hardware store and find the correct compression fitting.) So you can rip off the normal fitting and install the gauge into a compression fitting. I had to hone out the inside of the ferrule a bit with a dremel. The image below show a tire pressure gauge (the chrome tube, not the generic dial gauge mounted behind it) fitted into a compression elbow then screwed into the chamber.

Image

This setup is not very accurate. The two problems I had were that (1) the scale has momentum and appears to overshoot the correct reading since it is accelerated to a pretty high velocity. (2) The piston in these cheap tire pressure gauges leak like crazy. You get a lot of blowby around the piston which tends to push the scale out farther than it should. (Only the piston should move the scale, not air flow.) My very crude solution to both problems was to just rest my finger on the scale during firing to increase the friction.

A better solution would probably be to spend more than the ~$1 I paid for my gauges. :roll: McMaster has much nicer gauges of the same basic type for $10 to $20.

There are other kinds of peak pressure recording gauges. I recently got a dial gauge with a peak recording needle at the hardware store. (This gauge wasn't for spudgunning, I suspect I'm getting ~100 PSI spikes on the household water line.) The gauge is designed to monitor the peak pressure in home water systems and has a female garden hose fitting. The gauge peaks out at 200 PSIG. I suspect that this type of gauge would also have inertial problems. Get the needle moving fast enough and it'll grossly overshoot the actual peak pressure. Check in the water heater section of your local hardware store.

Another type of gauge that has been suggested (not sure if anyone has ever actually tried it though) is an automotive cylinder compression tester. The pressure range is about right (typically 200 PSIG max?) and it is designed for fairly high speed pressure transients. Lets see, engine turning at 500 RPM on the starter motor, compression stroke is 1/2 turn of the crank, so the compression stroke lasts about 60mS. That's pretty close to the combustion time for a typical spudgun.

BTW, occasionally you find standard tire pressure gagues that go to 100 PSI. That should be enough for a 1x combustion gun.
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Unread postAuthor: Deadeye » Thu Jul 24, 2008 10:28 pm

Thanks for the detailed info and even a photo -
Looks like there is alot going on there with that chamber -- love the Cat 5 wiring the spark gaps.

Like you illustrate nicely, I pretty much eliminated the plunger types even though there was the inherent memory aspect. They just seem too Willy-Nilly on the readings and I agree about the blow by. You can take 5 different readings of a tire and get five different interpretational findings.

I've seen the water tester but not with a memory so I discounted it. Had not considered the combustion tester - your point is dead-on; it is more designed for this application than a tire tester - and that was one of my concerns of accuracy with a sudden milli-second peak if it can capture it that quickly - my hope was more if not accuracy then consistency which would accomplish my goal.

Thanks again, at least you illustrate that there may be a benefit to the info - I'll let you know what I come up with.
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Unread postAuthor: psycix » Fri Jul 25, 2008 10:34 am

What about this setup:

CHAMBER-->CHECKVALVE>--T-piece**

**BALLVALVE--QUICK CONNECT/SCHRADER
**PRESSURE GAUGE

1-Pump up the area in the tee to a pressure, say, 70psi.
2-Fire the gun. Watch the pressure. Estimate the max pressure it had.
3-Pump it up to that pressure. Fire again.
4-Did it have a new max, or didn't it move?
-In case of the new max pressure, pump up some more and repeat from step 3.
-In case of no movement, this means you are above the maximum pressure. You can let some air out and try again. Continue until you think you are on the right pressure.

Using this methode you can accurately find the combustion pressure. Note that the pressure is not contained as the hot gases cool down and the pressure drops again.

You could also start on say, 130 psi, and then letting some pressure out every shot until the needle moves on firing.
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Unread postAuthor: jimmy101 » Fri Jul 25, 2008 1:52 pm

Psycix, that should work but it is a heck of lot of work to get the chamber pressure for a single set of firing conditions.

Deadeye, actually, the cat 5 cable supplies power to the chamber fan, returns the signal from a piezo transducer inside the chamber, returns a signal derived from the stungun ignition system and returns the signals from 15 phototransistors mounted in the barrel.

The wiring to the three spark gaps are via the black, yellow and dark green wires with aligator clips. Here is a picture of the entire setup (the barrel is dismounted from the chamber);
Image
The black 35mm film container contains the spark detector; a neon bulb + phototransistor with the neon bulb wired in series with the spark gaps.

The whole thing uses both stereo channels of a sound card as the data recording system.

http://www.inpharmix.com/jps/Tech_Gun.html (incomplete)
http://www.inpharmix.com/jps/Closed_Cha ... udies.html
http://www.inpharmix.com/jps/Chamber_Temperature.html

The dial gauge is really just for leak testing purposes though I might try the peak recording dial gauge.

BTW, the peak recording water gauge I picked up at Home Depot for $9 is similar to the one shown at http://www.plumbingsupply.com/testing.html, scroll down to the "lazy hand" gauge. The gauge has a 1/4" NPT male thread and a fitting to convert that to 3/4" female garden hose thread. The one at Plumbing Supply looks a lot better than the one I got. Mine has 10 PSI increments, the plumbing supply one looks like it has 5 PSI increments on the 300 PSI gauge shown.
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Unread postAuthor: Deadeye » Mon Jul 28, 2008 7:07 pm

Thanks for the info guys. - Some I understood, some went over my head.

I think I found what I was looking for though.

Here is the add:

Accu-Gage® S... Series Tire Gauges
Tire gauge holds pressure reading until released. Pushbutton release bleeds air to desired tire pressure. Steel case. Optional rubber guard protects gauge. 15*, 30, 60, 100, or 160 max psi tire gauge

I am going to look for it local so I can spy the threads and then order if I can't find, and take my chances.

Once I get it working I'll post some results.
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Unread postAuthor: jimmy101 » Tue Jul 29, 2008 11:50 am

Deadeye:

I'm not sure if that gauge will give you the information you want. The problem is that when it pressurizes the gases in the gauge will heat up, even if none of the combustion gases actually make it into the gauge. The check valve then closes. The hot gases cool. The pressure drops.

You may hve to fire the gun repetively, without reseting the gauge, to get an accurate reading. Basically, the protocol that psycix suggest.
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Unread postAuthor: psycix » Wed Jul 30, 2008 12:21 pm

You will have to fire it multiple times anyways since one particular reading is never enough to be sure. Always use the average from multiple readings.
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Unread postAuthor: Hubb » Wed Jul 30, 2008 1:04 pm

What about using a liquid to measure the pressure? Here is a simple sketch of what I mean.

The hot gases from the chamber will put pressure on the liquid and force it into the gauge. The check valves are there to keep the pressure on the gauge.

Of course there would need to be a way of bleeding the gauge, but...well, if needed, I can put more thought into this for bleeding purposes and such.
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