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Digital PSI gauge?

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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Digital PSI gauge?

Unread postAuthor: BowerR64 » Sat Apr 12, 2014 7:04 pm

Can you convert a cheap tire PSI gauge to work on our cannons some how? Im thinking about putting one on one of my carbide cannons to gauge the chamber when it fires. I have a few of the tire gauges, not sure how high they go though but i thought it would be smaller and look a little more high tech then a big round analog one with a needle that everyone else uses.

I have a few extra 4" caps i could cut the valve stem off and then cut it down and just glue the stem right to the meter then screw it on and fire it.

I may not even need the valve stem though.
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Re: Digital PSI gauge?

Unread postAuthor: BowerR64 » Sat Apr 12, 2014 8:29 pm

ok never mind im just going to take one for the team and try it.

I had this old slime PSI gauge the battery quit so i tore it open and found the battery was soldered in, i managed to rig in a new one. So now it works.

A little research found that this gauge is rated for 5-150 PSI perfect, im either going to be under 150 or error it out wich i dont think will happen my cannon wouldnt even still be working if i hit that high of PSI.

I drilled a little hole in an extra 4" cap i have, pushed the stem from an old wheelchair tire ive since replaced and cut it down. Then i used shoegoo (because i love that stuff, that and liquid nails) and i shoegooed the gauge right to the back of the cap over the stem. I used the PVC pipe cement on the inside between the PVC and the valve so its sealed on the inside with PVC cement and shoegoo on the outside to the gauge.

Anyone want to take some predictions on how much PSI im pushing with my 4" carbide cannon? Im going to guess 80psi using a loose fitting tennis ball. Im going to figure 60 without any projectile.

If it doesnt rain tomorrow ill go try it. Everything should be dry by then.
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Attachments
Digital pressure 1.JPG
Inside the cap back side of the tire valve stem
Digital pressure 2.JPG
The cut down valve stem and the gauge air inlet
Digital pressure 2.JPG (36.65 KiB) Viewed 1010 times
Digital pressure 3.JPG
gauge mounted with the cap as some extra support
Digital pressure 3.JPG (38.43 KiB) Viewed 1010 times
Digital pressure 4.JPG
one of my favorite types of glue
Digital pressure 6.JPG
The gauge mounted and drying
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Re: Digital PSI gauge?

Unread postAuthor: POLAND_SPUD » Sat Apr 12, 2014 11:30 pm

Ohh speaking of this, has anyone knows any fairly inexpensive pressure transducers ??
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Re: Digital PSI gauge?

Unread postAuthor: Boomer58cal » Sat Apr 12, 2014 11:57 pm

Ha ha... I hope it works! I've been eyeballing a digital gauge at Harbor Freight, but its three times as much as their analog gauge. I guess I'll have to test it my self. Good luck
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Re: Digital PSI gauge?

Unread postAuthor: BowerR64 » Sun Apr 13, 2014 1:57 am

I think this is like a powder scale, as you first apply the gauge to the stem it reads the pressure till the change in the pressure stops then it locks on that number. If you touch it again any change in the pressure value will readjust and display the new value.

I thought there was a sort of button in the gauge that not only pressed the valve but then pushed a button on the gauge. Its just a hole right into the pressure sensor. It should work reguardless of leaks as long as im not over 150 psi

Ill find out though

Here is my setup
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Cheap Slime digital tire gauge from walmart
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Re: Digital PSI gauge?

Unread postAuthor: jackssmirkingrevenge » Sun Apr 13, 2014 8:59 am

I predict you will destroy it :)
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Re: Digital PSI gauge?

Unread postAuthor: Technician1002 » Sun Apr 13, 2014 9:19 am

One of the problems you may run into with a digital gauge is the same thing you run into with digital bathroom scales. Sample and settling time. A combustion cannon will have the same measurement issue you will have trying the measure the impact force of a basket ball on a digital bathroom scale. I doubt you will get any semblence of a valid reading. It is an interesting concept though.
A higher speed strain gauge applicaiton may hold promise of a more valid reading. A gauge designed to take a dynamic rapidly changing value may work much better. G sensors in a Wii remote, or strain gauges in a Wii Ballance board are designed to dynamically update values. Whether that sample rate if fast enough for a combustion event is unknown. It may be possible to combine the pressure sensor analog (if it has one) output into the analog input of a Wii remote to graph the pressure.

At the rate of rise and fall of a combustion, you may be able to get relative pressure measurements by finding the analog (if it has one) output from the strain gauge sensor and simply feeding it into the microphone input of a PC. Then you can compair relative combustion curves.

Good luck.

Update, some in physics use the Wiimote as a cheap input device. Due to this community, I could look up the sample rate of a remote. The update rate is sent the sensor data at 97 Hz (10.3 ms between samples or "reports").
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Re: Digital PSI gauge?

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Re: Digital PSI gauge?

Unread postAuthor: jimmy101 » Sun Apr 13, 2014 12:28 pm

What tech said. The problem is that the digital gauge is designed to respond to a static pressure that persists for some fairly large fraction of a second. In a combustion gun everything is done in about 1/20th of a second, which is too fast for the gauge's electronics. And, in a static sensor sound (which a pressure gauge is sensitive to) is removed from the signal by filtering. But in a combustion chamber sound and the chamber's pressure changes are occurring in the same time domain.

A digital gauge might work for measuring pressure while charging a hybrid. For fueling a 1x gun the change in pressure is probably too small for these digital gauges to accurately measure. You would be looking for a (0.04*14.7)=0.6 PSI rise during fueling. Most of these digital gauges I've seen are precise (which doesn't actually say how accurate they are) to +/- 0.5 PSI.

I've been looking for years for a cheap enough pressure transducer. For a spud gun you need something that'll respond in a couple milliseconds. Most pressure transducers are (1) expensive at more than $50 and (2) fairly slow.

For measuring the chamber pressure during firing a strain gauge is probably the way to go. You can get surface mount strain gauges pretty cheap and a sound card and pretty minimal electronics might be enough for the data collection system. You would have to calibrate the system though and that probably requires that the barrel can be sealed and the whole gun pressurized with compressed air to calibrate the sensor.

Ammo reloaders often use strain gauges to measure firing pressures. The basic approach might work in a spud gun even though the pressures are hugely different. See for example:
http://www.ktgunsmith.com/straingauge.htm
Note that the sensor's response time is very fast, looks like it is much less than 1 millisecond. And if the sensor is fast enough for a real gun then it is more than fast enough for a typical spud gun.

There might be other ways to measure chamber pressure that we haven't thought of. For chamber temperature you can use accoustics. I wonder if there is a way to get pressure based on acoustics. Unfortunately the speed of sound is pretty insensitive to pressure (but very sensitive to temperature). See for example:
http://www.inpharmix.com/jps/Chamber_Temperature.html

A Piezo transducer inside the chamber can be hooked up to a laptop via the MIC input. You can record a signal and there is useful information in the signal but I've never been able to convert that signal into an estimate of the pressure in the chamber.
http://www.inpharmix.com/jps/Piezo_Spud.html

Mechanical gauges respond too slowly to be useful for accurately measuring chamber pressure during firing (see the above page).
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Re: Digital PSI gauge?

Unread postAuthor: BowerR64 » Sun Apr 13, 2014 3:26 pm

Can you calculate pressure by the projectile velocity? Or get it close?

There is an app for smart phones that uses sound to give a rough estimate of velocity and i could maybe shoot a tennis ball at a brick wall and i think it could record the 2 sounds fromt he blast to the impact and give an estimat eon velocity.

Technician1002 wrote:One of the problems you may run into with a digital gauge is the same thing you run into with digital bathroom scales. Sample and settling time.


DANG! i never thought of that. lol you just ruined my day :cry:

Can you calculate pressure by the projectile velocity? Or get it close?

There is an app for smart phones that uses sound to give a rough estimate of velocity and i could maybe shoot a tennis ball at a brick wall and i think it could record the 2 sounds fromt he blast to the impact and give an estimat eon velocity.
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Last edited by jrrdw on Mon Apr 14, 2014 3:52 am, edited 1 time in total.
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Re: Digital PSI gauge?

Unread postAuthor: jimmy101 » Sun Apr 13, 2014 4:26 pm

You can go from velocity (and firing at a hard target is a good way to do that) to a rough estimate of pressure but...
1. You must know the geometry of the barrel (the ID and length which is trivial)
b. You must know the mass of the projectile (slightly trickier but still not that hard)
iii. You must know the static and dynamic friction between the ammo and barrel (pretty tricky but not impossible)

What you would still be missing is the pressure versus time data, which is tricky for a combustion but much more straight forward in a pneumatic. So it is possible and mathematically not that hard if you have all the needed parameters and if an exact answer for pressure versus time isn't needed.
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Re: Digital PSI gauge?

Unread postAuthor: jackssmirkingrevenge » Mon Apr 14, 2014 5:33 am

Some old discussion on the matter here: combustion-cannons/topic7808.html
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Re: Digital PSI gauge?

Unread postAuthor: BowerR64 » Mon Apr 14, 2014 3:57 pm

jackssmirkingrevenge wrote:Some old discussion on the matter here: combustion-cannons/topic7808.html


Would that style work better then the digital one i have mounted now?

Those static ones are fairly cheap enough to ruin and ive already drilled the hole an dmounted the stem.

Ill try the digital one first then try the static one just for fun. Give me a reason to go out and fire it. :D
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Re: Digital PSI gauge?

Unread postAuthor: jimmy101 » Mon Apr 14, 2014 4:43 pm

I've tried the static ones before and they don't quite work. The problem is that the pressure rise is very fast (compared to what the gauge was designed for) and the scale (and the piston and the spring) in the gauge have mass but very little friction. The net result is that the scale overshoots the peak pressure by a fair amount. You can tell the chamber fired but really can't get an accurate measure of the peak pressure in the chamber.
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Re: Digital PSI gauge?

Unread postAuthor: BowerR64 » Mon Apr 14, 2014 6:49 pm

jimmy101 wrote:I've tried the static ones before and they don't quite work. The problem is that the pressure rise is very fast (compared to what the gauge was designed for) and the scale (and the piston and the spring) in the gauge have mass but very little friction. The net result is that the scale overshoots the peak pressure by a fair amount. You can tell the chamber fired but really can't get an accurate measure of the peak pressure in the chamber.


Wow never thought of that either but that makes sence it would be so quick the momentumof the slider would over shoot the actual reading.

Ive already ruined the cap so i may as well try it.
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