Calculating boil off from gravity measurements

[TheDaysy]

I have just completed my first ever all grain brew but I made the mistake of using default boil off and efficiency numbers in my equipment profile and my 4.1% beer is looking like its going to be more like 3%.
I have calculated and updated my mash efficiency but I don't have a way to take any volume measurements after the initial mash water so my question is, has anybody had success calculating their boil off rate using their pre and post boil gravity readings?
What I have done is looked at the change in gravity points I got and compared this to the change brew father was telling me I should have had for a 3 litre per hour boil off. My actual numbers were 72% of the predicted so does it stand that my boil off rate was 72‰ of that 3L/h?

 

[foxy]

I have just completed my first ever all grain brew but I made the mistake of using default boil off and efficiency numbers in my equipment profile and my 4.1% beer is looking like its going to be more like 3%.
I have calculated and updated my mash efficiency but I don't have a way to take any volume measurements after the initial mash water so my question is, has anybody had success calculating their boil off rate using their pre and post boil gravity readings?
What I have done is looked at the change in gravity points I got and compared this to the change brew father was telling me I should have had for a 3 litre per hour boil off. My actual numbers were 72% of the predicted so does it stand that my boil off rate was 72‰ of that 3L/h?

Easy and more accurate way to do it, fill your kettle to 10 litres boil for 1 hour, when cooled back to starting temperature check the volume subtract from the 10 litres and you have your boil off rate.

 

[TheDaysy]

Easy and more accurate way to do it, fill your kettle to 10 litres boil for 1 hour, when cooled back to starting temperature check the volume subtract from the 10 litres and you have your boil off rate.

The problem I have is that I don't have any good way to measure 10L of water, I guess I could try doing it by weight on my bathroom scales but I doubt they would be either precise or accurate enough

 

[foxy]

The problem I have is that I don't have any good way to measure 10L of water, I guess I could try doing it by weight on my bathroom scales but I doubt they would be either precise or accurate enough

Weight or a jug, with a jug say 5 litre or whatever you have, if a jug make a dip stick, marking off every couple of litres. Personally the jug and kitchen scales would be more accurate

 

[TheDaysy]

Weight or a jug, with a jug say 5 litre or whatever you have, if a jug make a dip stick, marking off every couple of litres. Personally the jug and kitchen scales would be more accurate

I like the idea of creating a dip stick, I could then use that to take readings during my brew days which would be super handy, Thankyou I think you might have just solved my measuring dilemma

 

[strange-steve]

Yes you can use the simple dilution/concentration formula below:

Starting volume * starting gravity = finishing volume * finishing gravity

Just rearrange for finishing volume like so:

Finishing volume = starting volume * starting gravity / finishing gravity

For example if you started with 25L of wort with a gravity of 1.050 and after boiling for one hour the gravity was 1.055 then:

Finishing volume = 25 * 50 / 55 = 22.7L

Which means 2.3L or 9% per hour boil-off rate.

 

[foxbat]

I like the idea of creating a dip stick, I could then use that to take readings during my brew days which would be super handy, Thankyou I think you might have just solved my measuring dilemma

I use a dipstick, otherwise known as an ordinary stainless ruler that has markings right up to the end. Because the volume of wort in my pot is a cylinder I can use simple geometry to calculate the volume.

1. Precalculate the volume of your kettle to the top. You get to choose where "top" is.
2. Use the measurement from the wort to the top to calculate the volume of empty space as a cylinder.
3. Subtract (2) from (1) and the remainder is your wort volume.
4. Use a spreadsheet to automate the calculation for every millimeter measurement in the range you're going to use and you've got a handy lookup table to use for the future and the maths can go away.

Make sure your kettle is standing flat when you brew and remember that 100C wort is about 4% greater in volume than it is as 20C.

 

[TheDaysy]

Yes you can use the simple dilution/concentration formula below:

Starting volume * starting gravity = finishing volume * finishing gravity

Just rearrange for finishing volume like so:

Finishing volume = starting volume * starting gravity / finishing gravity

For example if you started with 25L of wort with a gravity of 1.050 and after boiling for one hour the gravity was 1.055 then:

Finishing volume = 25 * 50 / 55 = 22.7L

Which means 2.3L or 9% per hour boil-off rate.

Thankyou for the formula, i have just used it and it gave me a boil off rate of about 1L per hour which is about half of what i had come to with my attempt and a third of brewfather's default value which is probably a good part of how i ended up a full 1.1% under my target