Evaporation - Volume Question

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Evaporation - Volume Question

Postby JackSosebee » Sun Feb 11, 2007 4:59 pm

My question has to do with what would appear to be really simple math. The example in BeerTools Pro Wiki is a good example. If the starting Kettle Volume is 6.74 gallons and there is an evaporation loss of 1.01 gallons, with no water added, shouldn't the final volume be 6.74 gallons - 1.01 gallons = 5.73 gallons? The example shows 5.5 gallons.

This happens with my recipes as well. Kettle Volume - Evaporation Loss + Water Volume Added doesn't equal Final Volume. What am I missing?

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Contraction as it cools??

Postby billvelek » Sun Feb 11, 2007 5:52 pm

I haven't looked at the Wiki example, but I'll do that in a minute. But my intuition is that your question is the same one I had asked a couple of months ago, and it turns out that the difference, at least in my instance, was due to contraction of the water as it cools. Probably the same for you, but I'll check.

Cheers.

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Postby just-cj » Sun Feb 11, 2007 6:04 pm

From boiling to room temp, the volume of the wort will shrink by 4%. If you multiply that post-boil figure by .96, it should give you an accurate volume of cooled wort. BTP does that automatically.
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Postby billvelek » Sun Feb 11, 2007 7:04 pm

Using my calculator and your adjustment, it come out exactly. :D

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Postby rrosa » Tue Apr 03, 2007 8:27 am

I noticed the same thing yesterday and found this topic explaining the reason for the diference. But then the software assumes the kettle volume is measured near boiling temperature... Well, that is not usually the case. I usually measure the volume when I transfer from the mash/lauter-tun to the kettle, when the wort is between 60C or 70C (140F and 158F). I guess it would be too much to ask for temperature inputs to fill out next to the volume data, so I guess we have some more math to worry about by hand...
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Postby slothrob » Tue Apr 03, 2007 9:12 am

rrosa wrote:I noticed the same thing yesterday and found this topic explaining the reason for the diference. But then the software assumes the kettle volume is measured near boiling temperature... Well, that is not usually the case. I usually measure the volume when I transfer from the mash/lauter-tun to the kettle, when the wort is between 60C or 70C (140F and 158F). I guess it would be too much to ask for temperature inputs to fill out next to the volume data, so I guess we have some more math to worry about by hand...

If you're measuring on transfer from the tun to the kettle, then that's the Pre-Boil Volume, and the 4% isn't applied, so this won't affect you. The 4% is applied to the Post-Boil Volume, and really only affects the Boil-Off Volume, in a way...

If you enter your Boil-Off Volume based on a measurement during the boil, as many do to know when to stop the boil, then the program is set up fine for you. If you determine your Boil-Off Volume by your Final Volume, post-cooling, then enter the Final Volume and let the program calculate your boil-off, or subtract 4% from your "Boil-Off Volume" measured at Room Temp to correct for shrinking.

Either way, the Boil-Off Volume is actual boil-off, not boil-off + shrinkage.
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Postby rrosa » Tue Apr 03, 2007 8:04 pm

Sorry to continue on this, but I am trying to get this straight. You mention pre-boil and post-boil volumes, but what I see in BeerTools Pro is Kettle Volume and Final Volume. I am then assuming that Kettle Volume is the volume in the kettle before the boil but at nearly boiling temperature, while the Final Volume is the volume in the kettle after boiling and after cooling, so at "room" temperature (approximately 20C, or 68F, to get the 4%). Is that correct?

So, if I measure the volume when transferring the wort to the kettle somewhere near say 60C (140F), then I would have to add 2.5% to get the volume to fill in the Kettle Volume, right? Okay, I could start measuring the volume near boiling point to avoid this computation but measuring before that gives me more time to plan the boil time and hop additions.

But I am puzzled by your last sentence: "If you determine your Boil-Off Volume by your Final Volume, post-cooling, then enter the Final Volume and let the program calculate your boil-off, or subtract 4% from your "Boil-Off Volume" measured at Room Temp to correct for shrinking." You sound like there is a place to enter or read the Boil-Off Volume, before the shrinkage, is that so? I don't see this in BeerTools Pro. What am I missing?

Thanks for the explanations.

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volumes

Postby slothrob » Wed Apr 04, 2007 9:07 am

Sorry to be confusing, and I wouldn't bother adding 2.5% to anything.

Here's the most straight forward way to record the volumes:

Enter you volume pre-boil into the "Kettle Volume" field. Don't correct this number for temperature, but be sure to measure this at a similar temperature each time, if you want the program to accurately predict your final volume. Lock this volume with the tick-box.

Enter your boil-time into "Wort Boil Duration".

Enter boil-off volume, pre-shrinkage, into "Evaporation Loss". Determine this as:
(volume at end of boil, at boiling temp) - (Kettle Volume) = Evaporation Loss
or,
(Final Volume x 1.04) - (Kettle Volume) = Evaporation Loss
The 1.04 accounts for the 4% shrinkage.
or,
Simply enter the Final Volume, post-cooling, and let the program calculate your Evaporation Loss.

A useful number to remember is 5.2 gallons as your target volume. This is the volume at the end of boil, but before cooling, that will give you a Final Volume of 5 gallons post-cooling. 5.7 gallons, if your target is 5.5 gallons post-cooling.
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Postby rrosa » Wed Apr 04, 2007 10:02 am

I see! By Boil-Off Volume you mean the Evaporation Loss. Of course. Bad English from my part; I misinterpreted what you meant. I think I understood now. Thanks indeed!
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Postby slothrob » Wed Apr 04, 2007 2:14 pm

I'm sure it helps when I used the same terms as BTP.
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