Brewsmith

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That sounds about right. It's basic science that hotter air holds more moisture, and the lower the relative humidity, the more potential there is for evaporation. This would explain why I hate Florida in the summer...

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weizen

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What about putting a lid on it before the end of your boil but when you reach the level you should be at i.e. I lose about 1 gal/hr in the summer but 2 weeks ago I lost 2.5/gal in 90 minutes. What would be the down falls of putting a cover on if any?

boyurboy

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When you lid the pot during the boil, you have to monitor more closely for boil overs depending on the size of the pot. The lid will also allow you to maintain a good boil with a lower flame on the burner and force more condensate to fall back into the mix, so less evaporation.

knowltonm

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DMS not being boiled off... = cooked cabbage or corn flavor. Not sure how long it takes to get rid of the sulfur compounds that create it and to boil it off, but I think it's a better idea just to start with more if possible. I know for some of us it's hard, my pot is always at max capacity when I brew.

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knarfks

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http://www.homebrewtalk.com/f36/boil-off-48883/index2.html

uwmgdman had a great explaination of this. Cold air = more boil off.

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Joetuo

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Thanks that post really cleared things up....I did a search before posting, but i must have been using the wrong key words.

menschmaschine

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What uwmgdman seems to be getting at there is "Humidity Ratio". HR is a measurement of humidity in the air that excludes temperature as a factor. Here is the calculation for humidity ratio (for degrees Fahrenheit):

HR =15432.3584*0.62*10^-5*0.01*Relative Humidity*(610.78*EXP(17.2694*((Degrees Fahrenheit-32)/1.8)/(((Degrees Fahrenheit-32)/1.8)+238.3))/2.2046).

You can plug that into an excel spreadsheet replacing Degrees Fahrenheit and Relative Humidity with cell addresses in which you've entered values for dF and RH.

This explanation still does not indicate temperature alone as a factor. In most cases, the HR of cold air is lower than the HR of warm air, hence the higher evaporation in cold air. But, if you compare the following 2 scenarios:

90dF, with 20% relative humidity = 41.47 humidity ratio
60dF, with 80% relative humidity = 61.10 humidity ratio

In this case, the warmer air has a lower humidity ratio and will allow for less condensation at the boundary layer (less water falling back into the pot) due to the lesser difference in boil temp vs. air temp. So, the 90dF air temp will result in a higher evaporation rate.

Like I said before, in most cases, colder air has a lower relative humidity and humidity ratio and will result in a higher evaporation rate, but colder air temperature alone does not result in a higher evaporation rate.

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cwhill

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+1 on HR =15432.3584*0.62*10^-5*0.01*Relative Humidity*(610.78*EXP(17.2694*((Degrees Fahrenheit-32)/1.8)/(((Degrees Fahrenheit-32)/1.8)+238.3))/2.2046) except I would use f(x)=22/7 - radical 2 for this particular calculation...

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