# Propane/Oxygen - from Cylinder to Oxy/Fuel cutting torch

• martijn_tholen
In summary: The tank pressure is the same for all the gas in the tank. You would not be able to tell how much propane is in a tank by looking at the pressure.

#### martijn_tholen

TL;DR Summary
How to calculate total gas volume for each cylinder based on the consumption ratio of oxy/fuel for cutting torches
For one of our projects we are at present using quite large amounts of gas & oxygen.
What I was trying to figure out was how to calculate the total available amount of gas in a cylinder (be that either Propane/Oxygen or Propylene (Propene)) and then to determine whether or not -given a certain number of cylinders of each- do we have sufficient gas in stock based on their relative consumption ratio.

For example:
we may have 1 oxygen quad (i.e. interconnected cylinders) with 9 cylinders, type: G/G2, with a 50ltr water-volume @ 150bar filling pressure.
we have propane cylinders, type: S/SE, with a 108ltr water-volume @ 10bar filling pressure.
Given that the oxy/fuel consumption ratio when cutting steel for oxygen vs propane is ± 4:1, how many cylinders of propane would I need to end up with an exact ratio between oxygen and propane?

Can I simply use normal gas laws here?
I.e. oxygen, 9 cylinders, each with a volume 50ltr @ 150bar = 67500ltr of oxygen?
and, subsequently for propane, volume 108ltr @ 10 bar = 1080ltr of propane?
i.e. in that case I would need 16 cylinders of propane?
(9 x (150 x 50)) = 67500ltr @ 4:1 = 16875ltr of propane required => 16875 / 1080 = 15.6 = 16 cylinders?

Would that be a correct approach or would I need to take into account a different volume for each gas based on the fact that the propane and oxygen go through a phase transition in the cylinder (i.e. they go from liquid to gas)
Any help would be appreciated.

Propane is stored as a liquid. The amount of propane is measured by weight in small tanks, typically less than 20 lbs or so. Larger tanks, at least in the US, are measured in gallons. A good search term to better understand the relationship is propane temperature pressure. Note that the density of liquid propane can vary by 10% over a typical range of ambient temperatures. The gas volume is calculated from the liquid volume.

You can measure the amount of propane by weighing the tanks if you know the tare (empty) weight of the tank. You can also measure the amount by pouring some hot water down the side of the tank. The tank surface temperature changes at the liquid surface level.

Pressurized oxygen tanks have a stated volume, with typical sizes being 80 or 122 cubic feet for welding oxygen. Oxygen at 150 bar is stored as a pressurized gas. The tank pressure tells how much is in the tank.

• berkeman