Bug?

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Bug?

Postby billvelek » Tue Sep 08, 2009 6:27 pm

I realize that Jeff is probably sick and tired of my nit-picking and a few 'false alarms', but I've discovered something in BTP that mystifies me.

I was trying to use BTP to determine the gravity of a sugar-water solution for the purpose of furthering a discussion in another forum regarding the change in volume of water as sugar is added. I came upon a source which indicates that the total volume of a 20% sucrose-water solution is actually less than the volume of the water before the sucrose is added, For anyone interested, here is the link: http://www.madsci.org/posts/archives/19 ... .Ch.r.html

Anyway, I started with a blank recipe, set the final volume to 8 liters at 68F (which IIRC should be close enough to count as 8,000 grams of water) and added as my only ingredient 2kg of sucrose, thus making a 20% solution. There are NO other ingredients; although it appears unnecessary to do so, I set my efficiency to 100%, and also my attenuation to 100%. Now here are the problems, looking under the 'Analysis' tab:

The O.G. indicates 1.095 which I presume is correct, while the T.G. (terminal gravity indicates 1.000 which is the gravity of pure water. But with the alcohol added (which has a gravity of less than 1.000), then the resulting T.G. should be less than one ... shouldn't it? The tab also shows the presence of 'carbs', but there shouldn't be any with 100% attenuation, correct?

Maybe this has to do with the 'real' versus 'apparent' attenuation. I then tricked BTP by raising my attenuation setting to 122% which causes the data under the 'Analysis' tab to show a T.G. of 0.979 and a real attenuation of 100%. What is confusing to me is that if it is doing it with this, then when I enter a value in my attenuation field based on the information about a particular variety of yeast (I usually use the middle of any range that is given), how do I know that the data under the tab is correct until I enter an actual final gravity reading when fermentation is complete?

It just seems strange to need to put a value exceeding 100% in any field.

Cheers.

Bill Velek
Visit www.tinyurl.com/bvelek - portal to my brewing sites: 3,100+ members on 'Grow-Hops', and 1,350+ brewers on my 'BrewingEquip' group.
Running BTP v1.5.3 on WinXP 2005 SP3 w/AMD Athlon 64@3800+, 1GigRam, Res 1024x768
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Re: Bug?

Postby jeff » Thu Sep 10, 2009 7:15 pm

billvelek wrote:It just seems strange to need to put a value exceeding 100% in any field.


The answer lies in the definition of apparent attenuation versus real attenuation. Apparent attenuation is "apparent" because it does not factor in the change in density caused by the presence of alcohol after fermentation. Real attenuation reflects the true degree to which fermentable sugars are metabolized by the yeast.

Apparent attenuation will always be greater than real attenuation since alcohol exaggerates the difference between original and terminal gravities. Thus a real attenuation of 100% will result in an apparent attenuation that is greater than 100%.
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Re: Bug?

Postby slothrob » Fri Sep 11, 2009 5:22 pm

billvelek wrote:I came upon a source which indicates that the total volume of a 20% sucrose-water solution is actually less than the volume of the water before the sucrose is added, For anyone interested, here is the link: http://www.madsci.org/posts/archives/19 ... .Ch.r.html

Bill, you may be misunderstanding what he's saying in that link.
The total volume (92.34 mL) isn't less than it started out as (80 mL), it's just less than the total volume of water (80 mL) plus the total volume of sugar (12.65 mL), which predicts a volume of 92.65.

This is essentially due to the disruption of the slightly bulky water structure by the dissolved substance, allowing the water to pack a bit more tightly together. This is pretty typical of most things dissolved in water.

This predicts a actual volume increase of 0.617 mL/g (or 0.358 gallon/lb), instead of the expected 0.633 mL/g. The math I used in that other thread was off slightly because it didn't take this slight shrinkage into account. Because of this my volume increase was about 2.5% more than it would actually be, I believe.


Concerning the published attenuation values for different yeast strains, I suspect those values are for apparent attenuation, not real attenuation, since few brewers concern themselves with real attenuation.
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