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another pump Q: 2 x 12g co2 in 700x23 = what pressure? nm(13 posts)

another pump Q: 2 x 12g co2 in 700x23 = what pressure? nmJS Haiku Shop
Jun 5, 2002 9:26 AM
re: PV=nRTcollinsc
Jun 5, 2002 9:44 AM
change n (number of moles, but units can easily be arbitrary) from 1 cartridge to 2, and P changes to 2P when volume and temperature remain constant.

if one 12g goes to 94psi (i think thats what they say isnt it?) then two would get 188psi.

I would imagine some tiny change in temperature, but Im not sure how much it would affect pressure.
1 x 16g co2 in 700x23 = 125 psi nmDougSloan
Jun 5, 2002 9:47 AM
re: another pump Q: 2 x 12g co2 in 700x23 = what pressure? nmDINOSAUR
Jun 5, 2002 9:48 AM
According to the Performance Bike catalog a 12g C02 will inflate a 700x23 tire to 94psi. 2x94=188psi, which would blow the tube. This is probably more than you asked, but I also pack a very small Torelli mini pump and mount my tire and inflate to 40 psi (about 50 strokes) and finish off with a 12g Co2 which inflates my tire to 120psi...I might just switch over to a 16g Air Chuck, which would be easier, as I seldom have "F's" anymore, the 16g are more expensive...
my possibly wrong opinionAllUpHill
Jun 5, 2002 10:13 AM
I think if using one cartridge gives you one pressure, using a second wouldn't quite double it. It would fall a bit shy, how much, I don't know. Too lazy to work out some numbers. Say you use one cartridge and it gets your tire up to 94 psi -- the cartridge'll have 94 left in it that you just shoot out into the air when you unscrew it. Then go at it with the second cartridge, which is also initially at a pretty high pressure, I've always wondered how much, but much more than 94 psi of course. It's going to give up it's moles of gas until the pressure between the cartridge and tube is equal, but that pressure is of course going to be higher than the 94 psi you had left in the first cartridge when you were done with it. So, after everything finishes out, the second one will have a higher pressure of gas left in it than the first that wouldn't go into the tube, and so more unusable moles of gas left over than the first (since the cartridges have the same volume).

Therefore, if she weighs less than a duck, then... ... She's a witch!!
Absolutely Correct [except for the Monty Python quote]MisJG
Jun 5, 2002 11:32 AM
The greatest pressure you could get from a cartridge would be to the point of eqalization between the tire/tube and the cartridge itself.

Now this is probably completely wrong, but follow my logic. . .
If cart 1 will get your tire to 94 psi, then there will be 94 psi left in cart 1, so was there 188 psi in cart 1 before you opened it?
If so, then the next cart would fill the tire until equalibrium was reached between the pressure of the tire [now 94] and the unopened cart [188?]. Therefore, cart 2 would inflate to 141 psi.
Cart 3 would work until 164.5
Cart 4 to 176.25
etc, etc, etc

Make sense? There are probably more complex factors that I am unaware of when dealing with compressed gas, such as volume and what not, so these numbers are probably not correct, but on the road with inflation issues, 2 catridges will more than likely get you home [and then some].

Now, for the Monty Python quote, "If she weighs the SAME as a duck. . .then she's a witch!"
Nope...Lon Norder
Jun 5, 2002 11:48 AM
"If cart 1 will get your tire to 94 psi, then there will be 94 psi left in cart 1, so was there 188 psi in cart 1 before you opened it?"

Only if the cartridge had the same volume as the tire - which it obviously doesn't. I think it should be:

initial cartridge pressure = (final tire pressure)x(cartridge volume + tire volume)/(cartridge volume)
Sounds good.AllUpHill
Jun 5, 2002 12:42 PM
P1V1 = P2V2 (from good old PV = nRT as mentioned, but assuming no leaks and no ultimate temperature change, nRT is the same all around).

Now if we can estimate figures for cartridge and 700x23 tire volume... hmm, I'll guess the inside of the cartridge is a cylinder of radius 1cm and length 5cm (just roughly, without running up to measure one), it's volume would be about 15.7 cc. Let's say the tire is a cylinder with a radius 23mm/2 = 1.15cm and a length 70cm, so a volume of 291 cc. so... if it inflates the thing to 94 psi, it starts at ... 94psi * (307cc/15.7cc) = 1838 psi

That sounds pretty high. maybe I'm missing something.

So, once you add the second cartridge, you should have final pressure = (1838psi*15.7cc + 94psi*291cc)/(307cc) = 183 psi
oopsAllUpHill
Jun 5, 2002 12:58 PM
the 700 in 23x700 refers to wheel diamter not circumference.

So, a tire volume of 914 cc, and a cartridge pressure of 94*(914cc+15.7cc)/15.7cc = 5566 psi (!!)

And a final pressure after the second cartridge of (5566psi*15.7cc + 94psi*914cc)/(914cc + 15.7cc) = 186 psi
the ultimate Monty Python siteTig
Jun 5, 2002 12:13 PM
Python scripts, sound waves, and cool products can be found at http://www.montypython.net/

Oh, here's the official script dialog for that scene:

BEDEVERE: Exactly! So, logically...,

VILLAGER #1: If... she.. weighs the same as a duck, she's made of wood.

BEDEVERE: And therefore--?

VILLAGER #1: A witch!
LBS snubbed me over CO2p chop
Jun 5, 2002 5:24 PM
Yup-- asked about CO2 inflaters and the guy weighed in his "I've never trusted anything like that..." and that was it, no help.

or rather...

CO2, asked me
I'd never use that, said he
He gazed from great height

I'd considered much:
Reviews, anecdotes; but his
Mantra cut the air

Ignoring his tune
I picked for my fockin self
Some lowly-ass gas
bravo! nmJS Haiku Shop
Jun 6, 2002 4:28 AM
Yup.Spoke Wrench
Jun 6, 2002 9:30 AM
I seen it done before. Happens when a guy spends too much time hanging around the bike shop and not enough time actually riding his bike. Might have to make a few changes in that regard myself.