nitrous refilling and waste gases?

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I'm new to nitrous...im sure my title is confusing.

I'm wondering about nitrous filling...and if you ever need to empty the bottle out completely to get out "waste gases" like compressed air or whatever, to make sure you have all nitrous.

Last week i filled my bottle with the last bit of the mother tank.
Now when i ran last night, it ran good for a decent amount of runs, but then i started to run more rich, and lose some time as the night went on. even though everything was the same including bottle pressure. I also only made about five full nitrous passes, so i know there is still plenty in the bottle.

So i'm guessing i need to empty the last 5 lbs or so of stuff out of this bottle and fill it up again with our new mother bottle. I just hate to waste it...it could be something else completely with the car, but all the factors seem to be the same.

I did notice when i let the last bit out of the mother tanks it acted like it was just compressed air....it wasn't liquidy and freezing like nitrous.

Anyway, thanks for the advice as always guys.