Is a Helium cylinder heavier when full or when empty?

helium is in the cylinder under presuure. Gas is released under pressure when empty it has less helium in side and therefore lighter

http://www.balloonhq.com/faq/whyfly.html

I checked the referenced website Toughynutter and found the infomation even more confusing, I guess my problem is.. since Helium is lighter than air then if a cylinder is filled with helium, as the helium is used then it would be replaced(?) with air..and wouldn't the air be heavier than what it is replacing (helium)thus making the cylinder heavier empty than full??.....Less helium= more air? Or am I just way overthinking this whole thing?

If there were a small amount of helium in the tank (not pressurized) the tank would weigh the tiniest bit less due to the buoyancy factor of the helium. When the helium is compressed it becomes more dense and heavier than the air outside the tank so it weighs more.

When a gas cylinder is emptied, NO air gets into it. If it did, there could be serious problems! The only time air gets into a cylinder is if some idiot takes the top off and lets the contents out. 'Lighter than air' gasses will be relaced quickly by air (assuming the cylinder is upright), 'heavier than air' ones like carbon dioxide will be replaced far slower. Bottles of liquid like orange juice are different. Remove liquid, and air enters to replace it. (Fizzy pop is slightly different, by containing pressurised gas, but as the pressure drops the bottle rule applies.) For a cylinder to weigh less when full than when empty, the contents would have to have negative mass. Not be antimatter, by the way. Antimatter weighs the same as normal matter - it's just built differently. They haven't found any substance with negative mass since they abandoned phlogiston.

Thanks to all, I believe I have it sorted out now,the simpliest explanation as I "now" understand it is that the compressed helium does have mass at whatever weight and as the helium is used/expended the mass becomes less therefore the cylinder thus becomes lighter,as I stated before I was indeed overthinking the problem, sometimes I feel so dumb it's not even funny. Again thanks to all of you for your explanations.

You weren't being dumb. You were just suffering the confusion produced by 'lighter than air' as a term, and the common use now of squirty things with pumps rather than compressed gas powered aerosol cans. In the pumped ones, air does replace the ejected contents, but in the aerosol (and gas cylinders) it doesn't.

Oy! And then there are the internal bladder propulsion systems used with products such as "Edge Shaving Cream". That is a whole different system!