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- Thread starter OldWorldBlues
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BvU

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Doesn't help you much at this point, but nevertheless.

With a bike pump you compress the contents of your pump and (if the pressure in the container is lower) send it to the container.

Suppose your pump piston has an area of 10 cm

In a container of 1 dm

May I suggest you build a pet bottle rocket instead of an air cannon ? You'll still need the bike pump ...

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For air you can use the ideal gas law .

With a bike pump you compress the contents of your pump and (if the pressure in the container is lower) send it to the container.

Suppose your pump piston has an area of 10 cm^{2}and your can compress it from 40 to 5 cm. Assume the temperature doesn't change (in fact it does!) with pV = nRT you calculate (nRT the same, V 8 times smaller) that you can increase the pressure to 8 bar. 8 bar (800000 N/m^{2}) times 0.001 m^{2}means the force you need at the end of the stroke is 800 N -- rather hefty !

In a container of 1 dm^{3}(0.001 m^{3}) at 8 bar (800000 N/m^{2}) and 300 K you have pV = 800000 * 1/1000 and PV / RT = 0.32 mol of air (molweight 29 g/mol) , so 9.3 grams.

Interesting; From some some "back of the envelope" math:

(please excuse my terrible and convoluted science/math skills -

Estimating, the dimensions of my little pump are 2" wide by some 4" long. From the cylinder volume formula that makes about 12.57"

Then we take this "Ideal gas law" (I'd heard of it but never bothered to learn ) pV = nRT and rearrange it to get:

p = nRT / V

now I looked at that wiki page you gave me, and from what I can see of that.. oh. Now that's a wild goose chase if I ever saw one. OK from what I gather:

p = pressure; that'll be my solution

n = "absolute substance in moles" -> ??

R = "Universal gas constant" -> Ya' know what, I'll use my calculator for that

T = "Absolute temperature of the gas" -> I googled it and from what I understand absol. temp. is just in Kelvin. You said this changed, but I'll ignore that for now?

...So far I have:

p = ? x R x T / V

Soooo for the pump itself, I'd have:

p = ? x R x T / 12.56, and I'll substitute T for the average room temperature (295 Kelvin, per Google) to get p = ? x R x 295 / 12.56

But I still don't know what n is.. I do have this equation though so I'll try "calibrating" it:

(the wiki page you gave me said that p should be in pascals -> 1 atm = 101325 pascals)

(the page also said V should be in meters

101325 = n x R x 295 x 0.000016387

we use some algebra to get:

n = RT / V * P -> n = R(295) / 0.000016387 * 101325

So assuming I did ALL that right, for 1 in

Golly, that's a large number! But now, armed with that unit rate, I can find the value of n for my pump system with a proportion:

(1/15,166,097,710,000) = (12.56 / X) -> n = 1.904861872e14

So after ALL THAT, we can finally find the quasi-answer- first I find the pressure (in pascals) when the pump is fully "open":

p = 190,486,187,200,000 * R * 295 / 12.56 = 3.719893795e16 pascals which means 5.395249806366171875e12 psi

but that doesn't make sense, as I doubt anything in the known universe short of a nuclear warhead can make pressures that high

So if you've read all the way to down here, thank you, and I'm sorry you wasted your valuable time for a wrong answer.

I should just not post this, but hopefully someone can help me out?

Thanks again!

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Oops, you switch from / to x !

n is the number of moles. 1 mole of air weighs 29 gram (main component is N_{2}).

Kudos for the attempt to use SI units !

Thanks!

I always tend to make silly mistakes like that. I wont't dump a ton of math here again, but once again I totally get it if you don't want to read it:

Hey...it might be easier if I used metric units to start with: 12.56 in

switching like you suggested to P = nRTV and using @scottdave 's suggestion

(1mole = 22.4L @ 1atm and 273K - > convert Liter to in

I'll get the following for the "open" pump (P is atm converted to pascals):

101325 = 0.0092 x R x 273 x 205.82152 -> prove it by: 0.0092 x R x 273 x 205.82152 = 4298.094211 ≠ 101325

Wrong again..

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