When a cylinder containing pressured gas is empty, it must be safely returned to the manufacturer you rented the gas cylinder from. It's important to know how to tell when your cylinder is empty. You'll notice on your pressure gauge when it is low but a cylinder is never entirely empty due to the atmospheric pressure left in the tank. A cylinder gauge reads pressure in pounds-force per square-inch gauge (psig) measurement and the atmospheric pressure is often left at 6.8 Kilogram-force per square-inch abolute (psia).
Attach a regulator that properly fits to the cylinder valve and make sure the secondary valve is closed. Attach the tube of a portable gauge to the regulator and slowly open the valve. You don't need to attach a gauge if one is already connected to your cylinder.
Check the gauge and, if it is at 25 psig or lower, then your gas cylinder is almost empty and you should stop usage. The atmospheric pressure is left containing a small amount of gas that is still flammable or toxic.
Look over the cylinder and make sure it is in perfect condition. Make sure the valve is fully closed and that there are no signs of leakage.
Make sure the cap nut or outlet plug for the valve is securely fit on the cylinder before returning it. Secure the protection device for the container valve.
Return the nearly empty cylinder to the manufacturer. Contact your local Office of Environmental Health and Safety if you can't return the cylinder to the manufacturer.