Boil off rate vs humidity

HumulusHead · Jun 1, 2014

So I'm trying to nail down my boil off rate so that I do not need to use any top off water after I chill.

My last batch I did before yesterday's brew needed about a quarter gallon top off after chilling. So I decided to re evaluate my system and I believe that I had my mash tun dead space measured wrong. I checked it and sure enough I was off by about a quarter gallon.

Now when I brewed yesterday I had about an extra quarter gallon after chilling!

The only thing I can thing of is the humidity. Would that affect my boil off rate? My last brew happened when it was still quite cold and dry outside. Yesterday was hot and muggy as hell.

In the end the quarter gallon either way doesn't negatively affect my beer, it just bothers me that I can't nail down my boil off rate.

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Pie_Man · Jun 1, 2014

In my experience, boil off rate is more of an estimate. The environment can certainly alter the boil off rate, as can your propane's regulator setting, even the fullness of the propane tank. I noticed when a propane tank nears empty, I have to increase the throttle on my regulator to keep up a good.

zwiller · Jun 3, 2014

Pie_Man said:
In my experience, boil off rate is more of an estimate. The environment can certainly alter the boil off rate, as can your propane's regulator setting, even the fullness of the propane tank. I noticed when a propane tank nears empty, I have to increase the throttle on my regulator to keep up a good.

+1 I brew in the garage in 30F to 90F temps and can attest the environment plays a role in the brew, and boil off rate specifically. I would estimate the boil off rate could be 20% or more in the winter and 10% in the summer. I have a wooden spoon with makings on it and it helps me target the final KO volume. I might shorten or extend the boil around halfway if boiloff rate looks low or high. I pay more attention to the wort level than the clock. Once it looks like I am on track (halfway), I proceed with late hop additions and start being time critical. Very scientific...

To me, the spoon is the key. My spoon is marked every gallon and I make a double mark at 5.5G as it is my goal 90% of the time. I use it to fill kettle too.

HumulusHead · Jun 3, 2014

I have been noticing the same thing you are. I brew in the garage too. When it's real cold and dry my boil off seems more than when it's warm and muggy.

I like the constant monitoring method with the spoon. I will have I try that! I just want to get my boil off closer.

This last batch I ended up with more than expected and I didn't want to extend my boil due to time constraints and ibu issues

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zwiller · Jun 3, 2014

Meant to tell you being off a quart (quarter gallon) is actually decent accuracy and doable IMO.

HumulusHead · Jun 3, 2014

I do agree that it is close enough.

I was just curious cuz I was a quart low last brew and a quart high this brew.

So I had a half gal difference without doin anything different!

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HumulusHead · Jun 3, 2014

I can def live with a quart tho

oh well, more beer!!!

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troglodytes · Aug 14, 2014

zwiller said:
+1 I brew in the garage in 30F to 90F temps and can attest the environment plays a role in the brew, and boil off rate specifically... I might shorten or extend the boil around halfway if boiloff rate looks low or high. I pay more attention to the wort level than the clock. Once it looks like I am on track (halfway), I proceed with late hop additions and start being time critical. Very scientific...

I probably should have used this method last night when brewing outside over a propane burner for the first time. I calculated boil off rate to be around 13% due to the fact that it was 77F, raining & 95% humidity. In a 75 minute boil, I ended up boiling off 2.4G off a preboil volume of 7.4G. Looks like I'll be topping off.

BTW, is it just me or is that calculated boil off rate of 25.9%/hr a ridiculously high amount? Did I have way too vigorous of a boil using that propane burner for the first time?

Yooper · Aug 14, 2014

troglodytes said:
I probably should have used this method last night when brewing outside over a propane burner for the first time. I calculated boil off rate to be around 13% due to the fact that it was 77F, raining & 95% humidity. In a 75 minute boil, I ended up boiling off 2.4G off a preboil volume of 7.4G. Looks like I'll be topping off.

BTW, is it just me or is that calculated boil off rate of 25.9%/hr a ridiculously high amount? Did I have way too vigorous of a boil using that propane burner for the first time?

No- the problem is calculating boil off rate by percentage in the first place.

Here's why. I have a keg for a boil kettle. In the winter, when it's dry here, I boil off 2 gallons per hour. Well, if I make a 5 gallon batch and start with 7 gallons- that's 28.5% boil off. But, if I am making a 10 gallon batch, and start with 12 gallons (again- 2 gallon per hour boil off), that is 16.6% boil off.

They are both correct.

That's why calculating % of boil off is silly. In the same pot, in the same environment, with the same burner- your boil off will not change. Whether I make a 3 gallon batch or a 12 gallon batch- I boil off 2 gallons per hour.

In the summer, when it's a bit more humid here, my boil off rate is a tad under 1.5 gallons per hour.

It's a fixed amount, and not at all a function of the amount in the boil kettle.

troglodytes · Aug 14, 2014

Thanks Yooper, that makes complete sense. I'll note in beersmith the update boil off by gallon and set and forget it for future brews.

For this one in particular, I'll top off until I hit the OG I was expecting. As of right now the wort's a bit concentrated.

Boil off rate vs humidity

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