How to know when inhaler is empty?

Hi Some days take more ventolin than others so cannot work out easily. When ventolin has gone there are still inactive gases in the canister giving impression there is still some drug inside. I have read about placing canister in a bowl of water and checking that way. Anyone know any other way? I don't want to waste any medication. Thanks

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no idea about working out how much is left that way.

I devised my own system but you have to do it from when you start using it

With a sharpie marker i put a dot on the front of the plastic case for every 2 puff i use i then join 4 dots into a line to represent 10 puffs. it does mean you have to have a sharpie with your inhaler but i like knowing how much i've got left and feel more confident knowing i've definitely got some medicine left

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Thanks for your tip and taking time to reply. By the way,that Anatole France quote is so true and oner of my favpurites!