Scimmia

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Ok, but the air doesn't "hold" water, and the only thing that matters here is the temperature of the water vapor, which is well above ambient.

Don't forget that we're also talking net evaporation rates here, as evaporation is always happening at a constant rate relative to the water temperature, the only thing that changes is the rate that moisture is recondensing on the surface.

Kaiser

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I don't think that humidity has as much of an affect on the boil-off as many brewers believe. The main effect on the boil-off is now much of the energy of the burner are you getting into your wort. The more energy that is available for evaporation, the more water you will loose. ambient temperature and humidity should not have a signigicant affect, but air pressure does.

You should build youself a shield for the burner and pot to make sure that the wind is not blowing away the flame. Once I did this I started saving on propane and had much more consistent boil-off rates.

Kai

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modenacart

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[QUOTE=Scimmia]Ok, but the air doesn't "hold" water, QUOTE]

You are arguing semantics with air not holding water. A colder ambient temperature would increase heat losses thus less energy going into the boil, thus less boil off. I am sure this is almost negligible anyways.

Also the moisture gradient and capacity of the air to hold water vapors does matter because it gives the water vapors a place to go allowing for more to boil off more readily. Any way you look at it, a warm ambient temperature will have more evaporation holding all else the same for both conditions.

Scimmia

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Call it semantics if you want, but the only effect the air has is it's ability to affect the temperature of the water vapor.

Well, only if you disregard the diffusive properties of gases. The air doesn't give the water vapor "a place to go", it's a vapor that will diffuse on it's own, regardless of the supposed "holding capacity" of the air. This is especially true when you consider that water vapor is actually lighter than air at the same temperature, so it will rise and diffuse even quicker at boiling temperatures.

Again, evaporation is a constant at a given temperature, it's the condensation that matters. You could make an argument that the cooler air will cool the water vapor quicker, making it recondense back into the kettle, but when you have rising steam from the boiling wort, I don't think it will be significant.

modenacart

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My master's project was modeling a peanut roaster and moisture gradients most definitely matter. There is no way you can argue that the moisture content of the air doesn't matter.

Scimmia

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Ok, then please, explain how. I'm always open to learning something new!

modenacart

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If moisture graidents didn't matter then rain would not condense out of the air and water vapor wound not just come from the surface.