I purge when refilling a completely empty lighter, but not when topping off a half-filled one when going out. Recently acquired another lighter with a better fuel window and found that the topping off resulted in little added fuel when not purging.
What's best way to top off a partly filled lighter to get the most fuel into the lighter? Read where chilling the lighter in the frige before topping works - ? True? :-)
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I always purge my lighters when filling, no matter the level. If you watch the purge nipple, you should be able to tell when it gets to the gas. For mine, there is more of a frost color as the lighter fluid will cool as released.
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Putting the lighter in the freezer might let you get 5 or 10% more in the lighter. It's not a huge effect.

When to purge probably depends a lot on how you fill. If you accidentally let a lot of air into the tank, you'll need to purge more. So, whatever works for you.
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Never purged a lighter. I figure the pressure in the can is greater than the pressure in the lighter. :-)
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I purge more than I don't. lately I added to two different lighters that i didn't purge.

Normally I have enough lighters on hand or with me on the go that I will run them to completely empty and purge to make sure.

I have noticed no difference in either way.
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I only purge when it seems that the lighter is holding less fluid each time I fill it, maybe every 15 refills or so.
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