evaporation water loss

I want to calculate how much water I lose for 1 hour of boil.

I thought of boiling 1 gallon of water for one hour, and than just measure how much it is left...

If I use this percentage of loss for 7 gallons of my boiling wort, am I going to make a mistake?

I'm not so good with math or physics but it seems to me that something is wrong with my approach...

Make it realistic.

Put as much water in your kettle as you normally start with for a full boil, once it reaches a boil, start the clock. After the hour, see what has boiled off.

willynilly said:

Make it realistic.

Put as much water in your kettle as you normally start with for a full boil, once it reaches a boil, start the clock. After the hour, see what has boiled off.

Click to expand...

I don't want to waste time and energy... I'm sure there must be a proper calculation for this, without boiling whole volume as I would normally when I brew

400d said:

I don't want to waste time and energy... I'm sure there must be a proper calculation for this, without boiling whole volume as I would normally when I brew

Click to expand...

There is, but you have to know your BTU output.

1btu will raise 1 pound of water 1F

Once you get to boiling (210 ish) you need 970BTUs for each Pound of water vapor. So if you want to boil off 1.5 gallons, that will take just over 12,000 BTUs.

If you want to boil off 1.5 gallons per hour, you need to get about 12,000 BTUs into the kettle to do so once it reaches a boil.

There is really no replacement for a practical test, because you probably have no idea how many BTUs are going into your wort.

willynilly said:

Make it realistic.

Put as much water in your kettle as you normally start with for a full boil, once it reaches a boil, start the clock. After the hour, see what has boiled off.

Click to expand...

why do you need the full volume? i've always read that the boil off rate is pretty constant for a given "vigor" of boil in a given pot. in other words, if you boil off one gallon an hour, that'll happen whether your boiling 6 gallons or 11. not true?

i realize that 1. boil vigor is subjective, but if one could be relatively close from one boil to the next on this factor, would the rest of of this not hold true? and 2. that environmental things like temp and esp humidity play a role, but let's ignore those for now!

I can eyeball my boil vigor and when it's boiling the way I like it I will boil off 1.48 gallons of 1.060 FG wort in 1 hour. Why don't you just try to overshoot a bit and then calculate your evap rate on brew day? The brew gods won't frown if you boil a little longer.

You also need to take into account ambient air temp. and relative humidity. Cold air can hold far less moisture than warm air. I boil off a lot less in the winter than I do in the summer.

BuzzCraft said:

why do you need the full volume? i've always read that the boil off rate is pretty constant for a given "vigor" of boil in a given pot. in other words, if you boil off one gallon an hour, that'll happen whether your boiling 6 gallons or 11. not true?

i realize that 1. boil vigor is subjective, but if one could be relatively close from one boil to the next on this factor, would the rest of of this not hold true? and 2. that environmental things like temp and esp humidity play a role, but let's ignore those for now!

Click to expand...

Because IMHO, if you are going to go through the trouble and time posting to see how to do it, you might as well take that time and do a real world test. Then you can be confident in your results.

I dont care what people do, that was just my opinion. Sorry mine differs from yours.

willynilly said:

Because IMHO, if you are going to go through the trouble and time posting to see how to do it, you might as well take that time and do a real world test. Then you can be confident in your results.

I dont care what people do, that was just my opinion. Sorry mine differs from yours.

Click to expand...

no need to take offense, man. i was actually asking your opinion on the statements i posted (which are purely based upon what i've read and not my opinion).

since your posts implied that you have a good grasp of btu input, boiling, etc., i thought that you might be able to point out whether what i've previously read on this forum about boil off is right or wrong.

BuzzCraft said:

why do you need the full volume? i've always read that the boil off rate is pretty constant for a given "vigor" of boil in a given pot. in other words, if you boil off one gallon an hour, that'll happen whether your boiling 6 gallons or 11. not true?

i realize that 1. boil vigor is subjective, but if one could be relatively close from one boil to the next on this factor, would the rest of of this not hold true? and 2. that environmental things like temp and esp humidity play a role, but let's ignore those for now!

Click to expand...

I think you are some what in the ball park...

But I think you have to start with enough volume to have the bottom of the vessel covered when you are finished. The OP mentioned starting with ONE GALLON and boiling for an hour. I think most boil off about 1.5g/hr.

If your beginning volume is too low, I don't think you will get a representative "roll" in the boil for the full duration.

Another thing... will water evaporate out of wort at the same rate as just simply boiling water?

If, I were doing the test... I'd just add some extract, hops, yeast, and end up with with good data (and some beer). Oh... and measure how much you boil off.

Also, don't forget expansion and contraction... you will loose about 4% volume when you cool from boiling to 68f.

Good luck.

Ed