# What is the pressure on an empty pneumatic tube?

At the residence I'm constructing a kitchen digester. The gas is to be collected into an old bicycle/scooter tube - initially as empty as may be after wringing it out..

Will the digester (total volume 50Litres, I plan to use no more than 2/3 of the total volume) i.e. about 7.5L gas at any time - be able to fill the tube against atmospheric pressure? Is it correct to assume the gas in the tube will be at 1 atm. pressure?

EDIT: The photograph of the link shown is a very similar system; the digester I'm looking to construct is larger in volume (50L as against 1Gallon in the figure), and collects the gas over water.

-
A picture of the system would help me a lot. I don't understand the system from your text. – Bernhard Oct 22 '12 at 13:24
The construction isn't complete yet; I'll toss a photograph/sketch in once it's done. – Everyone Oct 22 '12 at 19:09

## 2 Answers

Is it correct to assume the gas in the tube will be at 1 atm. pressure?

Only if you don't stretch the tube at all.

The inner tube is essentially a toroidal rubber balloon with relatively thick walls (compared to typical party balloons) So your digester would have to produce considerably more pressure to be able to store much gas in the inner tube.

On road bikes, inner tubes are pressurised to perhaps 8 x atmospheric pressure, but that's inside the road-tire casing.

See The Design and Theory of a Basic Anaerobic Digester for an alternative design of gas collector.

-

The pressure inside an empty inner tube is zero. It should be "easy" to fill it to one atmosphere.
I would recommend using very large rubber tubes so that you can collect a large volume of gas at low pressure, which you can then change to high pressure via a pump, if needed or desired.

-
It is worth distinguishing carefully between gauge and absolute pressure here. The gauge pressure is zero, but unless you are able to evacuate the very effectively the absolute pressure is like the same as the surrounding air. – dmckee Dec 28 '13 at 17:15