Accounting for extreme relative humidity

[Sooperhonk]

Had an extreme all grain brew session in Hendersonville NC a few days ago in my unheated barn brewery. It was 3 degrees outside. Not sure what the RH was, but the air was abnormally dry. I planned on a 6.5 gallon batch, so used 6.5 gallons of sparge water. After a 90 minute boil, I only had around 4 gallons left in the kettle. I figure this is due to the extreme low RH outside and higher evaporation rate but have no idea how to account for this in future brews. I've seen a couple boil calculators out there but don't see an input for humidity. Anybody know how to adjust boil volumes for low humidity/high evap brew days?

:fro:

 

[blizzard]

It takes a few tries to dial in the amount of boil-off you'll have. I have never had to make adjustments for humidity, though I live in a climate that is generally always very dry.

If you want to get 6.5 gallons into the fermenter, you have to start with a larger volume than that because you will evaporate some during boiling. For 6.5 gallons in the fermenter, I would shoot for 9 or so in the kettle to start. Maybe a little more for a 90 minute boil.

Also, you just need a gentle rolling boil. I'm guessing you are boiling at a much higher intensity than needed.

 

[bobby4]

I find there is a huge difference when the humidity changes. In the dry winter it evaporates like crazy, on a muggy summer day it is much, much slower. I was caught once with too low evaporation and the extra long boil I needed to get to target volume took me forever. Now I have a rough idea of how my equipment performs at different humidities, but to be safe I intentionally start the boil with a lower volume. It is easy to top it up near the end.
I am not sure how well a calculator would work due to the characteristics of your particular equipment.

 

[Sooperhonk]

Good input. Sounds like this will come with experience but I should try to estimate how much volume is actually in the kettle after sarging vs. just measuring the amount of sparge water I add. For example, I probably had 6.5 gallons of sparge water, but 7.5 or so after sparging off what was trapped in the grain bed after the hour steep, I'm guessing? And probably should have started boil with closer to 9 gallons vs. 7.5. ? Live and learn.

 

[bobby4]

Just remember to boil a bit less volume wise, it is easy to add water but a PITA to boil longer.


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[TopherM]

We have high humidity in the summer here in FL, and pretty low humidity in the winter. I have never noticed a difference in my boil off levels in the different weather. I can follow the argument that humidity would be somewhat of a factor, but it is definitely pretty subtle. I can't imagine it is more than a few ounces difference, as I end up very consistent with my final volumes despite the seasons.

 

[bobby4]

How different is your humidity? We go from near 0% in the winter to over 90% some summer days, and I find a very noticeable difference. I am open to there being other factors, but it is definitely higher for me in winter than in summer. I assumed this is humidity, but is there another explanation? I started with my new burner last winter and I was consistent at 1.5 gallons per hour. In the summer I ended up at 1/2 that rate. I thought maybe it was equipment, but this winter I m back to 1.5. Perhaps there is another factor?


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[DSorenson]

When I brew in cold weather (below freezing) I usually add another half gallon to my sparge water. It comes out pretty close here in CT. (Of course keep in mind that my outside water is turned off for the season and it takes a while longer than normal to cool off.)