Boil off calculation

[osprey12]

Hi, I’ve searched and have been unsuccessful finding a calculation like I’m looking for. I consistently reach my pre-boil gravity readings but my original gravity has been a little low. I’m assuming this means my boil off rate is being input incorrectly into BeerSmith. Is there a formula that takes preboil gravity, original gravity and time into consideration to arrive at a boil off rate? Such a thing must exist, I feel like I’m guessing too much and eyeballing when doing test boils with water and believe it would be much more accurate to take known data sets I have recorded and input it into a formula. Any ideas?

Thanks!

 

[Pkrd]

If your boil off volumes are correct then the gravities should be too. In BS you enter an hours boil off volume, tick the box and it will adjust for times.

Unless you're losing wort of adding sugar this will apply
V1*(1-G1)=V2*(1-G2)

Where V and G are gravities and volumes

 

[osprey12]

Thanks Pkrd, I understand that much. The problem is my boil off volume is not correct and would like to know exactly how much my boil off is by using preboil gravity, post boil gravity, and time. Once I have a formula that takes those factors into consideration I can plug them in and arrive at the correct boil off to put into BeerSmith. For example my last beer had the correct preboil gravity of 1.043 so the mash/sparge was not the issue, however my post boil gravity was 1.055 when I was expecting 1.06. I guess such a formula just may not exist and I may have to tinker around with some cheap beers to get it exact. It’s an electric brewery so there is ver little variables involved in the heat applied and should be consistent.

 

[kh54s10]

Work at it the other way around. See how much you collect in your fermenter. If you have the right volume there you know what your boil off is. If you have 7 gallons preboil and boil for an hour then have 5 gallons in the fermenter you boil off two gallons an hour. I forget how I figured out the first one. The second I made a small adjustment and the third another. Depending on ambient temperature and humidity I now get within a quarter gallon every time.

 

[kh54s10]

Gravity doesn't really come into play. It is volumes and evaporation. Everyone is going to have a different rate based on kettle shape and heat source.

If you are not hitting your gravity numbers that is a different issue. In which boil off rate is only a part of the issue.

 

[Qhrumphf]

Take volumes IMMEDIATELY before and after boil when the wort is 210-212 (or for whatever temp you boil at your elevation). Subract post-boil from pre-boil, divide by pre-boil.

Voila. Boiloff rate.

If you wait until after chilling thermal expansion botches the simple equation above (and BeerSmith factors this in). It's the same reason a gravity sample has to be chilled before reading or it reads low, the wort expands when hot.

If you're losing any volume at all to kettle, chiller, hoses, spillage, whatever then going by fermenter volume is will be even further off.

 

[khannon]

Boil off at a set temp is pretty constant with surface area.

What I did was measure out 5 gals of water into my boil kettle, and bring it to a boil with a lid on, boil for an hour, and measure what I got out. I then measured what was left by pouring it out (to figure out my boil kettle loss), added this together and subtracted the amount from 5gal to figure out my boil off rate.

Then when I do a boil, I can know how much I will boil off in a 60 min boil, and take my pre-boil gravity and know that I will go from 14 gal to ~12.5, do some math with the sugars, and approximately determine post boil gravity, allowing me to adjust the boil for my final numbers.

Sadly, I have had a few "brew days" that have been nothing but water, but on the upside the cleanup on those days is awesome.

(also, x-mas eve +beer+post on favorite forums, may lead to spelling or grammar mistakes.. no apologies for those)..

 

[osprey12]

Take volumes IMMEDIATELY before and after boil when the wort is 210-212 (or for whatever temp you boil at your elevation). Subract post-boil from pre-boil, divide by pre-boil.

Voila. Boiloff rate.

If you wait until after chilling thermal expansion botches the simple equation above (and BeerSmith factors this in). It's the same reason a gravity sample has to be chilled before reading or it reads low, the wort expands when hot.

If you're losing any volume at all to kettle, chiller, hoses, spillage, whatever then going by fermenter volume is will be even further off.

Because of all of those variables I was looking for a better and more accurate way than eyeballing. That’s why I was hoping that there was a set amount for % of gravity increase per gallon boiled or something that way a formula could be created. Guess I have to do it the hard way. Thanks guys!

 

[Qhrumphf]

Because of all of those variables I was looking for a better and more accurate way than eyeballing. That’s why I was hoping that there was a set amount for % of gravity increase per gallon boiled or something that way a formula could be created. Guess I have to do it the hard way. Thanks guys!

Alternatively as the (cooled) gravity won't actually change between BK post boil and fermenter, you can reverse engineer the post-boil with the equation above (G1*V1=G2*V2), and then use that volume to factor you boiloff rate (which would mathematically assume indentical temps). But it won't tell you anything about fermenter volume (unless you factor in ~5% cooling loss and 0% loss transfer to fermenter).

From there, you can calculate your boiloff rate.

Time won't be a factor apart from that being your boiloff rate for that particular boil. Actual boiloff rate over different lengths of time involves a LOT of variables. Some find x volume per hour accurate, some find y percent per hour accurate. Some may find neither entirely accurate. I've been more accurate with percentage.