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Zippo lighters are notorious for needing regular refilling. The fuel evaporates from the cotton regularly. Some people put a bit of rubber around the seal.
What atmospheric conditions keep fuel from evaporating? Temperature, humidity, etc. Should I keep the lighter near my body (for heat) or in an outside pocket?
Should I keep it topped off, or will fuel evaporate faster that way?
Higher temperature definitely increases evaporation rate. Keeping it away from body heat and not letting it heat up a lot from flame will keep the fuel from evaporating too fast.
Other than that, concentration gradient - a seal will reduce the evaporation around the container; won't stop evaporation through the wick. Also, it may make opening it for refilling more difficult.
Topping it off will increase the concentration (and its gradient) and so the initial evaporation will be faster - but the total time until "dried up" will be longer. (meaning, if you fill it with 2ml of fuel at once, it will last longer than if you filled it with 1ml, but shorter if you filled it with 1ml twice, the other refill after the first one evaporates). Considering minuscule amounts and low price of fuel there's really no reason to skimp on it (other than risk of spills if you overfill).
Year's ago I took out the inside of my Zippo and closed the top, then slid a tight piece of heat shrink over the lighter case. Heated it till the heat shrink was tight the took a Razer knife and cut along 3 side's leaving the hinged area in contact. Refilled the cartridge with fluid put back in the lighter lasted a lot longer on one fill up plus it was gripper look pretty cool and keep the lighter looking New the lid closed nicely never got wobbly all my friends caught on and did the same, I've use the same lighter for 30+ year's in till it got misplaced or someone wanted for them self. For real it really worked. (Remember to take the cartridge out before heating)
The big issue is that Zippos aren't that well sealed, especially the lid which allows fuels to evaporate through the wick and it's this wick to air interface which is the most critical thing as the whole function of the wick is to provide a big surface area for fuel to evaporate and thus burn.
Given that the fill level probably doesn't make a big difference as the wick will suck up fuel by capillary action as long as there is fuel to suck.
Temperature will make a difference as it will have a direct effect on the vapour pressure of the fuel humidity not so much as the air won't get saturated with fuel vapour in the same way as with moisture so they aren't competing as such.
also it is not easy to improve the sealing on a Zippo, at least not around the lid where is really matters as the interface between the lid and the body is fairly thin and there is no easy way to keep the lid firmly in place.
Compare to something like this which has a thread and O-ring.
From and engineering perspective the classic Zippo design is simple and convenient but the price you pay is wasting a bit of fuel through evaporation.
Probably the practical best solution is to carry a small screw-top container with extra fuel.
