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Buying a HP Pump

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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Unread postAuthor: MrCrowley » Thu May 15, 2008 11:54 pm

Me and ILTBTS think we know what's wrong.

The pump gauge has a checkvalve inside the gauge. So that the pump gauge shows you what the pump is producing, but when the air pressure goes into the chamber, since the volume is smaller, the pressure when equalized is less then what the gauge is showing.

And the check valve inside the gauge seems to stop the gauge pressure from equalizing so you only see how much pressure the pump has made, not the final pressure of the pump and the chamber.
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Unread postAuthor: JDP12 » Fri May 16, 2008 12:20 am

Good work MrC, let me know if you find any more results...i'll be interested to here them. i probably wont check the board again till later in the day because at the moment, i will resume work on my 10 page final english paper/exam thingy... its 12:20 in the morning here... hahaa
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Unread postAuthor: psycix » Fri May 16, 2008 5:53 am

Pump gauges are almost always a bit off. Always have one of your cannon and trust that one.

Because the pump gauge reads 260 psi they can sell it for a 260 psi.
My pump that claims to go up to 7 bar gives no more then 5,5 in real pressure.
It are cheap, crappy gauges wich mostly give an higher rating then usual.
That way they can sell pumps claiming they go to a higher pressure, and no more then just a few spudders will find out.
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Unread postAuthor: MrCrowley » Fri May 16, 2008 6:45 am

This is 40psi, not a few.

1.5 Bar is only 20-odd PSI.
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Unread postAuthor: jrrdw » Fri May 16, 2008 8:11 am

Does the gauge on the pump have it's own relief valve?
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Unread postAuthor: D_Hall » Fri May 16, 2008 9:39 am

psycix wrote:Gauges are almost always a bit off.

I fixed your post.

How much it's off... Well, you can buy a $5 gage that's probably only good to 10% or you can buy a $1500 gage that's good to 0.25%. Between those two extremes you pick your price/accuracy point and live with it.
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Unread postAuthor: MrCrowley » Fri May 16, 2008 4:30 pm

Well this is a $50 ($40USD) floor pump.
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Unread postAuthor: psycix » Sat May 17, 2008 2:55 pm

MrCrowley wrote:This is 40psi, not a few.

1.5 Bar is only 20-odd PSI.


But your 40 psi is 260 -> 220
I agree that 40 psi is a larger bit off then 20, but since your are on a way higher pressure, it is basically the same thing.
You should not say the gauge is 40 psi off, and thus is not as good as one only being 20 psi off.
It is relative: calculate it in percent of the total psi.
If you want to state things the right way to compare gauges:
A gauge is never xx psi off, it is xx % off.
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Unread postAuthor: MrCrowley » Sat May 17, 2008 3:54 pm

Yeah I know that they're X% off but all the chambers I was using had the same volume more or less. So I couldn't be arsed to change the fittings on my other cannons and then calculate a percentage when it really doesn't matter.

Anyway I fixed it now, the schrader clip thingy wasn't seating properly.
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