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Discrepancy between pump pressure gauge and Orbit gauge

Post questions and info about pneumatic (compressed gas) powered cannons here. This includes discussion about valves, pipe types, compressors, alternate gas setups, and anything else relevant.
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Discrepancy between pump pressure gauge and Orbit gauge

Unread postAuthor: Yamroll » Tue Apr 12, 2011 10:20 pm

Howdy all,
New to the forum, new to spudding (apart from a poorly executed venture about 10 years ago). Seems like a good place to ask about a problem (or not) I'm having on my new spud gun.

So I've got a sprinkler valve and it honks!

Kidding.


1.5" SCH 40 PVC air tank, 38" long. Actuated by a modded sprinkler valve.

I installed an Orbit brand pressure gauge down by the end of the tank, next to my schrader. Model # is 91130 if it makes a difference.

Now, what I'm noticing is the pressure reading on the Orbit gauge is always about 40% lower than the pressure reading on my pump (it's just a foot powered bike pump, nothing fancy).

When I designed the whole thing, I intended to run it at 100 PSI, from what I've read I probably COULD go higher, but I'd rather not wreck my parts too soon (whether they're made of PVC or meat...).

I suspect the reason for this is the Orbit gauge is calibrated for water pressure as opposed to air pressure, but, my fluid dynamics are a bit rusty. Anyone know why this would be?

(tldr)


Thanks
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Unread postAuthor: Technician1002 » Wed Apr 13, 2011 12:02 am

Sometimes in air cannons the very high rate of pressure change can cause the pointer to either slam aganst the stop bending something inside the gauge or slip on the shaft so it does not point right.

Water or air, the gauge should read the same.

I have destroyed a couple of gauges on my large cannon. I am looking into threading the inside of the gauge fitting to install an allen head screw to restrict the flow so the gauge is slow responding. It would not be a 100% seal, but enough blockage to prevent damage.
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Unread postAuthor: jackssmirkingrevenge » Wed Apr 13, 2011 1:29 am

Personally I don't see the point of having a gauge on the cannon unless it's a multishot, since most fill sources come with a gauge.

In any case, plenty of places where you can buy cheap dedicated air gauges.

Go on then, let's see your cannon ;) welcome to spudfiles!
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Unread postAuthor: Labtecpower » Wed Apr 13, 2011 2:08 am

I have the same problem. My gauge reads 7 bar, when it's already at 9 bar.

One shot I tried out (150 g electric motor, 130 psi) hurted pretty bad.
I remember technician1002 saying QDV cannons get scary past 100 psi.. he was right. (altough it was pretty impressive, never found the projectile)

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Unread postAuthor: Crna Legija » Wed Apr 13, 2011 4:38 am

possibly your schrader valve is acting like a pressure reducing fitting, you could try remove the spring holding it closed to see if it helps
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Unread postAuthor: Zeus » Wed Apr 13, 2011 4:46 am

To the best of my knowledge schrader valves pop around 50-75psi. If the valve stem isn't depressed that could cause such a discrepency.

I know I could say TTBOMK but then I wouldn't get as many spudbucks.

And I rather like that picture too.
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/sarcasm, /hyperbole
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Unread postAuthor: jackssmirkingrevenge » Wed Apr 13, 2011 4:48 am

Crna Legija wrote:possibly your schrader valve is acting like a pressure reducing fitting, you could try remove the spring holding it closed to see if it helps


Good point. Schrader valves typically "crack" at around 70 psi, if your pump connection isn't pushing it down then the pressure the pump is putting out is not necessarily that in the chamber.
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Unread postAuthor: Crna Legija » Wed Apr 13, 2011 4:50 am

lol copycats :lol:
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Unread postAuthor: Technician1002 » Wed Apr 13, 2011 5:27 am

jackssmirkingrevenge wrote:Personally I don't see the point of having a gauge on the cannon unless it's a multishot, since most fill sources come with a gauge.

I use a gauge as I'm often on the other end of a 50 foot air hose. A gauge on the cannon helps when you want adjustable power. Simply waiting till the compressor cycles off at 100 PSI is not always the desired power.
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Unread postAuthor: Yamroll » Wed Apr 13, 2011 10:37 am

Hmm, interesting thoughts on the schrader.


I'll have to play around with that when I get it fully up and running, thanks.


As for pics, those will be coming soon. I just got the modded valve into the system, I was kind of waiting on that before making the final adjustments on some of the accoutrement (pistol grip, barrel support, sights). Once I get the parts all cut down and assembled to my satisfaction I'll throw on a coat of paint and then upload some pics and put up a youtube video.

Might take a little while, last week of the semester here and I'm feeling the crunch. :(
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Unread postAuthor: Yamroll » Sun Apr 17, 2011 10:59 pm

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