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Question on Boyle's law and how to use it for pumps/psi

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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Question on Boyle's law and how to use it for pumps/psi

Unread postAuthor: lockmanslammin » Tue Sep 23, 2014 11:26 am

Hello all,
It's been quite a while since I've been here, and I'm suprised to see how many of the names I remember. I have probably a bit of a pnoob question. The question is kind of two fold.

Question one: what is the formula for figuring pressure increase in the chamber per pump, when knowing the exact volume of the chamber, and the exact volume of the pump?

Question two: So that I have a better grasp of how Boyles law works, if your pump volume was , say 5 cubic inches and your chamber was 5 cubic inches as well, would you essentially after one pump be placing one volume of air at 14.7 psi into another volume of 14.7 psi and end up with 29.4 psi in the chamber? Which would show on a gauge as 14.7psi due to the surrounding atmospheric pressure? And after one more pump would the pressure be 44.1 and the gauge read 29.4?

Tad confused here. Any help appreciated.

Chris
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Re: Question on Boyle's law and how to use it for pumps/psi

Unread postAuthor: Technician1002 » Tue Sep 23, 2014 5:59 pm

You are on track. Remember that the pressure when working with gases has to be Absolute, so after one pump your final pressure is close in theory. 3 things to consider when moving theory to the real world.

1 The pump is not 100% effecient. Some of the pump stroke will remain in the pump and re-expand in the pump. This volume varies with pressure. The higher the pressure, the less air exits the pump. A totally blocked pump will simply bounce on air not exiting.

2 Compression causes heating. The hot mixture will be higher in pressure until it cools back off.

3 Your common pressure gauges are not Absolute, but relative to atmospheric presssure, so after one stroke a tire gauge will read about 14 PSI.
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Re: Question on Boyle's law and how to use it for pumps/psi

Unread postAuthor: lockmanslammin » Tue Sep 23, 2014 7:02 pm

Thanx tech,

I will be attempting to make the system as efficient as possible, but it of course will still have those problems to an extent as you pointed out.

So to keep the example going then, taking in consideration the inefficiencies, and they're increasing nature as pressure rises, will the second pump bring the pressure up to around 27.6, and the third pump maybe up to 40.8?

The other thing I was wondering is how to use this method when the pump volume, and the chamber volume are different. In my situation it might be around 5.8"3 pump, and 5.1"3 in the chamber.

Either way it seems that it will get the pressure up quickly. Also, it will be an on board pump with a diameter of just over an inch. It also has no lever for mechanical advantage so 40psi will likely be the feasible, comfortable limit.

It will be firing mini marshmallows from a gravity fed tube magazine via a piston qev, and will likely get handed over to some kiddos, so it really doesn't need a lot of power. I'm hoping that they will be to weak to pump it to dangerous levels if left to they're own vices... Lol.

Thanx for the help.

Chris
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Re: Question on Boyle's law and how to use it for pumps/psi

Unread postAuthor: Technician1002 » Wed Sep 24, 2014 11:12 am

In Boyl's law the pressure is directly ralated to volume unless a gas condences.

For example if you have a pump 1/2 the volume of a chamber, at atmospheric pressure, after one stroke the chamber will contain 1.5 chamber volumes of air for 1.5 Atm of air pressure. If the chamber and pump are the same size, the volume would be 2 volumes or 2 Atm of absolute pressure. If the chamber is 1/2 the size of the pump, then a pump stroke would place 2 chamber volumes in with the existing volume for 3 chamber volumes of air or 3atm absolute. Above assuming a perfect pump for example.
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