Evaporation loss and gravity readings

Daniel Cook

Daniel Cook

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If I have a pre-boil gravity of 1.054 and a post-boil gravity (OG) of 1.061, then by the following formula I have an approximate 12.5% rise in gravity due to boiling: (pre - 1.0) / (post - 1.0).

Is it safe to assume that I also had a 12.5% drop in wort volume as a result of boiling? In other words is gravity as a function of evaporation losses a purely linear relationship? If so then I could probably get more accurate at knowing my overall brewhouse efficiency. My biggest current limitation is that I don't have a decent way to measure my "brew day complete" volume (stainless fermenter, no sight glass, no flow meter).
 
You may be able to use a dipstick to measure volume in your fermentor , I don't know what kind of stainless fermentor you are using, but if there is enough space to get a stick in (of course something that can be sanitized ) and read it that might do the trick for you. The dilution and boil off calculator on this site might be able to answer your question regarding the relationship between boil off and gravity, but measuring volume by gravity alone would not tell you anything about any losses due to trub in the kettle.
 
The specific formula is V1/G1=V2/G2. It's a simple proportion. So V1*G2/G1=V2. I get 1.13 for the ratio of the gravities. And yes, for a solution this dilute, you can approximate the change in gravity as a function of evaporation as linear.

And as mentioned elsewhere in the last few days, you could make a dipstick for your fermentor....
 
At one point or another your going to have to measure volume to get a accurate idea of efficiency. I usually measure my wort volume after lauter/sparge and calculate my efficiency from that volume, markings on the kettle or a dip stick would work for that. Once it's in the boil I monitor gravity only and I'm only concerned with volume unless it's way off and I will know by then because my numbers fall out of the normal range for my mash efficiency.

If you monitor your mash efficiency, you can get an idea if your going to nail your target gravity. Knowing what to expect out of your mash is important if you are have any chance of executing a recipe accurately. Once you start boiling, you can monitor your gravity and adjust your heat and/or boil time to achieve the desired gravity (you may have to add some water as well). Checking efficiency at the fermenter is more on the lines with keeping tabs on expenses verses finished product, something a pro would be concerned with who has to make a profit.

By time it hits my fermenter, I've had enough beers in me not to care about efficiency.

BTW, a refractometer works great for monitoring boil gravity. Just strain the wort through a paper towel to remove any suspended hops, etc to get a clearer reading. It only takes a few drops and is super fast.
 
^^^ Agreed. The formula needs at least one volume.
 
HighVoltageMan! said:
BTW, a refractometer works great for monitoring boil gravity. Just strain the wort through a paper towel to remove any suspended hops, etc to get a clearer reading. It only takes a few drops and is super fast.
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make sure you stir the wort thoroughly before taking a reading and don't use the foam, I've taken 3 samples and all were different from wort not stirred up enough
 
Ozarks Mountain Brew said:
make sure you stir the wort thoroughly before taking a reading and don't use the foam, I've taken 3 samples and all were different from wort not stirred up enough
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Yes! Thanks for pointing that out, I tend to assume too much.
 
Go to the "Tools" drop down and select "Calculators" and you'll get to this:
https://www.brewersfriend.com/dilution-and-boiloff-gravity-calculator/
You can easily make an observation of post-boil volume if you make marks in your boil pot. If you leave anything behind, you can measure that. The difference is what made it to the fermenter. The kettle volume before transfer and the OG reading before pitching will give you your kettle efficiency. That fermenter volume and the OG reading will give you your brewhouse efficiency.
 
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Unfortunately my fermenter barely fits inside an upright freezer, and there is not enough clearance to get a solid dipstick inserted after filling. However I can sort of peer inside the top (or take a pic w/phone) and see where the top of the wort is, and I've previously noted how high various volumes of room temp water come up the side of the fermenter. So I have a rough idea of final wort volume, e.g. within a half-gallon. That will probably have to do. I am also keen to get an idea of my evaporation rate and use that for planning. For example, the BrewersFriend boil-off calculator indicates that I boil between 3 and 4 gallons per hour, based on previous brew days. That's a decent variance, but I am still fine-tuning exactly how high to set the boil elements to achieve the boil I like (and since I brew outdoors there is temp/humidity variability too).
 
Daniel Cook said:
For example, the BrewersFriend boil-off calculator indicates that I boil between 3 and 4 gallons per hour, based on previous brew days..
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That's a lot of boil off! My boil off is about 1.3 gal/hour with a relatively wide pot (at least for a 10 gallon pot).

Edit: That's a lot of boil off for 5 gallons.
 
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Daniel Cook said:
Unfortunately my fermenter barely fits inside an upright freezer, and there is not enough clearance to get a solid dipstick inserted after filling. However I can sort of peer inside the top (or take a pic w/phone) and see where the top of the wort is, and I've previously noted how high various volumes of room temp water come up the side of the fermenter. So I have a rough idea of final wort volume, e.g. within a half-gallon. That will probably have to do. I am also keen to get an idea of my evaporation rate and use that for planning. For example, the BrewersFriend boil-off calculator indicates that I boil between 3 and 4 gallons per hour, based on previous brew days. That's a decent variance, but I am still fine-tuning exactly how high to set the boil elements to achieve the boil I like (and since I brew outdoors there is temp/humidity variability too).
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You can't see inside your fermenter but, as I noted you can see inside your boil kettle. You can see what you have when you start and you can see what you have after you stop. The difference is your actual boil off. Pre-boil gravity readings can be glitchy in my experience, so I don't depend on them for critical analysis. Get a final volume from the kettle, before you transfer, and you can measure whatever you have left in the boil pot after tranfer. That's your fermenter volume. As long as you have an accurate post-boil kettle volume reading and a good gravity before fermentation commences, you can accurately track things from there.
Unless you have a big system, you aren't likely to be boiling off 3 to 4 gallons per hour.
Describe your system so we're not just guessing about things and you can get some more accurate feedback.
 
My system is 1-bbl. I usually target 36-37 gallons pre-boil, and 32-33 gallons post-boil. There are some losses in the kettle since it has a dished bottom to trap trub, but there are also losses in getting the beer over to the fermenter roughly 30 feet away: hoses, pumps, etc. I do a "bucket brigade", 5 gallons at a time, from the kettle to a 30-gal pot which sits near my fermenter. Since that pot can't hold all of the boiled wort, the final few gallons are in the last bucket. When I start pumping from the pot into the fermenter I pour that bucket into the 30g pot while it's pumping out. The pickup tube is pretty low on that pot but there are still losses there. In the BrewersFriend Equipment Profile I've set miscellaneous losses as 1qt but that's probably optimistic.
 
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I have what I term a "first-world problem": too much beer. I prefer to brew roughly once a month but obviously can't consume a gallon per day so I enlist friends into a growler club, take kegs to work and to charity / school events, etc. It's a good way to get invited to parties! With about 30 people involved I can definitely get through six 1/6th-bbl kegs in a month. And life gets in the way so sometimes it's more like brewing every 2.5 months.
 
Daniel Cook said:
I have what I term a "first-world problem": too much beer. I prefer to brew roughly once a month but obviously can't consume a gallon per day so I enlist friends into a growler club, take kegs to work and to charity / school events, etc. It's a good way to get invited to parties! With about 30 people involved I can definitely get through six 1/6th-bbl kegs in a month. And life gets in the way so sometimes it's more like brewing every 2.5 months.
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That, sir, is why I went to three-gallon brews! The guys at Ruby Street would love to sell me one of their rigs but I can't justify it, even the 10-gallon variety, because I don't go through enough beer!
 
Daniel Cook said:
In the BrewersFriend Equipment Profile I've set miscellaneous losses as 1qt but that's probably optimistic.
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Yeah... :)
I have a quart misc. loss on my 10-gallon system. :)
Since your boil pot is large, your 4 gallons boil off seems reasonable. Sounds like a nice system.
There's bound to be a way to measure specific losses here and there like kettle trub and what's left in the fermenter after you fill 5 kegs. If you can nail down a couple of solid measurements, you can figure out what actually makes it into the fermenter.
 
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