To ensure your cylinder is completely empty, attach the nozzle to the cylinder and twist until the gas is released. Ensure you perform this action outdoors in a well-ventilated area, making sure to keep the cylinder away from your body and others.
Warning: Some cylinders, such as N2O, may become extremely cold during gas release, so it's essential to shield your skin from the cylinder's surface and the gas.
Once the hissing sound of gas ceases, your cylinder is empty. At this point, the pin should be recessed by approximately 12mm.
As a helpful practice, consider marking the neck of the cylinder with a permanent marker to indicate that it has been emptied of gas.