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Peak pressure measurement.

Post questions and info about hybrid (compressed gas with fuel) powered cannons here. This includes discussion about fuels, ratios, ignition systems, build types, safety, and anything else relevant.
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Unread postAuthor: Hotwired » Sat Feb 23, 2008 5:31 pm

Digital tyre gauges, probably hacked to record the output.

Response time is questionable and max pressure usually no higher than 150psi. 99.5psi is the most common I'm seeing.

However they are cheap.
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Unread postAuthor: jimmy101 » Sat Feb 23, 2008 8:00 pm

Hotwired wrote:Digital tyre gauges, probably hacked to record the output.

Response time is questionable and max pressure usually no higher than 150psi. 99.5psi is the most common I'm seeing.

However they are cheap.

I've got a $2 one from Radioshack, not only digitial but also talks (and is bilingual). The problem is they only go to 100 PSIG.

Most use a piezo-resistive element so they should have sufficiently fast response times. Rip the sensor out of the gauge. Add a battery and record with a PC sound card.

Should work with a generic combuston gun, but not much use in a hybrid.
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Unread postAuthor: TurboSuper » Sat Feb 23, 2008 8:27 pm

Mabye just make your own with a pressure transducer and an A/D Converter?

Edit: I think this is what Jimmy was getting at, except he lets the sound card read the analog output. They sell ones that work up to 30k PSI, they ain't cheap though.
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Unread postAuthor: Hotwired » Sun Feb 24, 2008 12:31 pm

I was actually thinking along the line of recording a constant digital output rather than an audio signal.

There are large numbers of digital pressure gauges out there but you're looking at a hundreds of dollars for them, the cheapest devices are the mass produced ones intended for an average joe to use and tyre gauges are what turns up.
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Unread postAuthor: jimmy101 » Sun Feb 24, 2008 5:45 pm

Turbo & Hotwired:
Yep, you can buy suitable gauges but who's got at least a couple hundred bucks that they cost?

Regarding data acquisition, the soundcard is an A/D system and does record a constant digital signal. The soundcard is really the way to go. You don't need sample rates above the 48KHz the sound card will do, or more than the 16 bit precision and two channels is enough for a heck of a lot of things. Besides, an A/D system with specs like a soundcard will cost muchos bucks, might as well take advantage of the huge price break you get on a consumer device.
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Unread postAuthor: boilingleadbath » Sun Feb 24, 2008 10:02 pm

What type of effort would it take to direct a piston down onto a load cell/strain gauge?

I don't know the typical frequency response times of these things, but I thought I'd throw it out there.

(of course, this is probably how they make commercial "digital" pressure gauges)
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Unread postAuthor: chartreusesnot » Sun Feb 24, 2008 10:15 pm

Can someone tell me what the difference between a pressure transducer and a pressure gauge is? And why does the response time matter? (these things are physical mechanisms, the delay in them is only the in the inertia of the moving parts inside... I can't imagine the delay would be very long...)
Also, couldn't you angle a video camera to the pressure gauge? video cameras take 29.7 frames per second. (or around there). That type of speed would definitely allow you to see the gauge pressure at its highest.

I'm asking because I might need peak pressure for a project in physics.
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Unread postAuthor: D_Hall » Mon Feb 25, 2008 1:23 am

chartreusesnot wrote:Can someone tell me what the difference between a pressure transducer and a pressure gauge is?

A pressure transducer is a (often piezoelectric) gizmo that produces an electrical current or voltage in response to pressure. What you do with that current/voltage is up to you.

A pressure gauge is a mechanical gizmo that (usually) spins a needle to indicate a pressure. You know, a pressure gauge! :)

And why does the response time matter?

If it takes 0.05 s for your gauge's needle to spin from 0 to 100 psi when hit with a 100 psi event, that needle will only register events that last at least 0.05 s. If the event lasts for (say) 0.005 s, you're never going to know that you got hit with 100 psi. You may know you got hit with something, but you're not going to know what you got hit with. You may THINK you know, but you don't.

(these things are physical mechanisms, the delay in them is only the in the inertia of the moving parts inside... I can't imagine the delay would be very long...)

And that's where you'd be wrong.

Also, couldn't you angle a video camera to the pressure gauge? video cameras take 29.7 frames per second. (or around there). That type of speed would definitely allow you to see the gauge pressure at its highest.

29.7 frames per second is pathetic where high speed events are concerned. It simply isn't fast enough.

Put it this way: At the office our "standard" data rate is 1000 data points per second. That's for "mundane" tests. For tests that are expected to experience high speed events we go much faster. Do we record at those rates just because we like producing lots of numbers? No. We record at those rates because experience tells us that's what you NEED in order to capture the events.
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Last edited by D_Hall on Mon Feb 25, 2008 2:16 am, edited 1 time in total.
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Unread postAuthor: D_Hall » Mon Feb 25, 2008 1:27 am

boilingleadbath wrote:What type of effort would it take to direct a piston down onto a load cell/strain gauge?

Huh? You lost me.
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Unread postAuthor: SpudMonster » Mon Feb 25, 2008 3:37 am

I think he's saying why not ghetto-fab a digital gauge by connecting a piston to the chamber and having it push against some sort of scale?


What about a digital pressure gauge/check valve combination, but add in a U-pipe filled with light oil? The oil will not compress, but is still fluid enough to flow easily through the check valve. The problem with conventional bourdon tube pressure gauges is that even the glycerin filled versions can get seriously screwed up after a few shots. Perhaps a heavier damping fluid would work? I'm thinking something like Astroglide or KY Jelly maybe?
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Unread postAuthor: D_Hall » Mon Feb 25, 2008 9:42 am

Sure, the amateur rocketry crowd does that a lot. THey use automotive oil pressure transducers hooked to hydraulic cylinders (selecting the cylinder size to give them the pressure range they want). The downside is that once again frequency response sucks. There are also historysis effects.

But it can't be beat for low cost flexibility.
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Unread postAuthor: jimmy101 » Wed Feb 27, 2008 12:00 pm

Like D_HAll said, standard video rate is way too slow. 30 FPS is a frame every 33mSec. Most spud guns fire, trigger to spud gone, in 50mSec. So the entire firing process is less than 2 video frames. (But I'll post some videos of combustion in a semi-open clear tube.)

The other problem with mechanical gauges is that they have momentum. Slam the needle from zero to 100 PSIG in 20 or so mSec and the needle will continue well past the peak pressure. The needle has mass, at the typical dP/dt rate of a spudgun it can't/won't stop at the actual peak pressure. So, even with high speed video equipment it is likely that the gauge will read much higher than it should.

From what I can gather, most high speed electronic gauges are piezo resistive and are essentially strain gauges. Pizeoelectric has too low a frequency response.
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Unread postAuthor: jimmy101 » Thu Feb 28, 2008 12:46 pm

Here is how I fitted a tire pressure gauge to a chamber. The brass fitting is an elbow with 3/8" copper swage (compression) fitting on one end and a 1/4" NPT male thread on the other. I had to hone out the ferrule and nut of the compression fitting a bit with a dremel to get it to slide over the gauge body.

The dial pressure gauge is also a 1/4" NPT male fitting.
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Tire pressure gauge (peak pressure recoding gauge) fitted to a chamber using a compression fitting. The two gauges are tapped into a triple layer of PVC.
tg_fitting.jpg
Tire pressure gauge (peak pressure recoding gauge) fitted to a chamber using a compression fitting.
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Unread postAuthor: willarddaniels » Sat Mar 08, 2008 9:13 am

I picked one up a few days ago for $125 on eBay. It normally cost $950 because it a really nice one.

I have researched a lot on these and you can expect to pay at least $225 retail for one, usually $260. I have looked locally, contacted many vendors across the US and even negotiated a lot with an Italian company.

They are darn useful and acurrate- I will be using mine on a variety of projects and experimentation. I think they are a must when using hybrids. Plus, I used the benefits of having one to persuade my wife to allow it.

Hopefully when we have constant 73 degree weather I can post some results of digital gauge/chrony tests.
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Unread postAuthor: jimmy101 » Sun Mar 09, 2008 3:03 pm

willarddaniels wrote:I picked one up a few days ago for $125 on eBay. It normally cost $950 because it a really nice one.

Do you have a part number or specs (or web link)? Is it a peak pressure recorder or does it gives a continous output?
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