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Gas Cylinders: Exchange when Empty?


Sep 28, 2002
Salinas, CA USA
I was told early in my career never to let an acetylene cylinder go completely empty before exchanging it, otherwise the last bit of gas would be full of acetone. I never heard a pressure setting for that rule though.

Are there similar rules for any other gases? e.g. Argon? I noticed my weld quality went way down yesterday while I was at the end of a cylinder of Argon. Could have been the regulator needing adjustment due to lower cylinder pressure...
I remember being taught years ago that it is best to leave a small amount of pressure in the bottles when exchanging them, this was to keep positive pressure in them so that air (contamination) could not get in. When you have to turn up the reg due to low bottle pressure that is about the right time to swap out the bottle. I usually rum the bottle closer to E though but there is not really much to gain.
I wouldn’t worry about air contamination. The gas house vacuums the bottle before refilling. You do get bad gases on occasion- co2 is the most common. Regulators freeze up with bad bottle.
At work we go thru lots of bottles a day, so I get to see large sample size.

Leave a bottle open with a small leak so it goes empty or 3/4 full- you pay the same price for exchange.
I've had a few bad welds from contamination in argon bottles when they are almost empty, and have heard the same thing from other welders. It must be air or a gas heavier than argon that stays at the bottom until the pressure goes very low. The fix is a fresh full bottle.
The supplier is supposed to pull a vacuum on a bottle that is not pressurized before refilling it. I'm told that if they crack the valve and get gas out they assume it has some of the original gas left and that it's OK to fill, that assumption might not always be correct.
I know the gas companies hate it when a cyl valve is left open. I think it means they have to do additional steps when filling it. But from the user's perspective I am not aware that shielding gasses somehow magically get adulterated when the tank pressure drops below some threshold.
As others have noted I get bad welds from the bottom of Argon tanks with TIG. I simply run shielding gas tanks until they stop producing a good weld, and can't think of any reason to stop before then.
High pressure bottles are vacuumed down every fill. We tried 90/10 for welding once and realized we didn't like it and couldn't get a rebate for the 90% of the gas remaining until they found someone who wanted to buy the remaining gas. They won't top them up and they obviously can't deliver a partially full bottle.

Dewars are just refilled. We had some that an operator left open to the air and I tagged it and told the welding gas supplier it needed to be pumped down before it got filled. Highly unlikely for air to not get displaced during filling but I didn't want to be the source of someone's contamination. The same operator was trying very hard to connect an oxygen bottle to a nitrogen dewar, but fortunately couldn.t
Whether a vacuum is pulled every time probably depends on where you get your gas bottles. A fly-by-night operation likely will take shortcuts. Somewhere like Praxair will likely not.