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Old 10-28-2009, 07:14 PM   #1
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Default weight to determine SG?

Rather than use a thief to check SG during fermentation, and risk contamination . Is it possible to simply place the fermenter on an accurate scale and infer from the change in weight the change in SG?

Assume we are going to make a 25L batch of beer. If we started with pure water the weight would be 25Kg. If our OG was 1.050 we essentially have wort that is 5% heavier than pure water, it should weigh in at 26.25Kg (assuming you subtract the weight of carboy, airlock etc.)

As the wort ferments there will be a loss of mass due to the escape of CO2 out the airlock. The scale should show a slow decrease in weight. We should therefore be able to infer the SG from the change in weight. Assume after a few days the scale reads 25.5KG, the mass of the wort has fallen 3% and is now only 2% higher than pure water, which means our SG should now be 1.020

Obviously there is some room for error but does this work?

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Old 10-28-2009, 09:45 PM   #2
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Yes, but notice something...SG readings are out to 4 significant digits. Can you take a mass reading of your wort to 4 sig figs and also measure the volume of your wort to 4 sig figs? Do you also have a table of water densities out to 4 sig figs? If the answer is no (which it most certainly is), then calculating your SG from mass and volume will probably create so much variability in your answer that it will be useless.

Assume we are at 4C which means density of water is 0.9998 kg/L and we have 19L of wort. 19L of water should be 18.9962 kg. Imagine the real SG of your wort is 1.050 at 4C. It should be 19.94601 kg. If you make a 1% measurement error of 19.75 kg instead, then you will calculate your SG to be 19.75/(18.9962) = 1.040. Imagine if you also made a 1% mistake in the volume. Volume is probably what would kill you. Most people on these forums don't have the capability to even get 2 sig figs on volume readings.

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Old 10-28-2009, 09:48 PM   #3
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Certainly, I'm no scientist, I just have to ask, what about "other" stuff? Like yeast? You add the yeast and if it's liquid, there is a tiny bit of volume, and it multiplies. Surely that has weight, too? Would the yeast cake increase the weight as well? It seems to me that it would. If the yeast has 1000 times more cells after/during fermentation, it just seems reasonable to assume that the yeast will weigh more since there are more of them.

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Old 10-28-2009, 09:49 PM   #4
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How do you account for the mass of the yeast cake?

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Old 10-28-2009, 10:03 PM   #5
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The yeast are getting all the materials from the wort, so their multiplication wouldnt just add mass to the system, they are converting one form to another. The only mass lost from the wort/beer would be in gas form that escapes through the airlock. So mostly CO2...but Im sure H20 vapor and other gases escape too, further reducing the mass inside the fermenter. But I agree with Rocketman that it would be very difficult to measure with any accuracy the weight of the your beer/wort compared to a hydrometer reading.

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Old 10-28-2009, 10:35 PM   #6
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Originally Posted by JefeTheVol View Post
The yeast are getting all the materials from the wort, so their multiplication wouldnt just add mass to the system, they are converting one form to another. The only mass lost from the wort/beer would be in gas form that escapes through the airlock. So mostly CO2...but Im sure H20 vapor and other gases escape too, further reducing the mass inside the fermenter. But I agree with Rocketman that it would be very difficult to measure with any accuracy the weight of the your beer/wort compared to a hydrometer reading.
I don't get what you're saying about the yeast multiplication not adding mass. Because they eat sugar, and discharge ethanol and co2. The ethanol weighs less than water, and the co2 is dispersed. So, their increase in mass wouldn't be an "even" up exchange. Their products actually would reduce the weight (because the co2 is gone out of the airlock), but their mass would increase. Maybe I just don't understand the science of it, but if alcohol weighs less than water, and co2 is released, how does the weight of the increase of the mass of the yeast cake not cause a difference?
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Old 10-28-2009, 10:55 PM   #7
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Good question, yooper. I guess we're getting confused on terminology. Mass and weight are two different measurements. The mass inside the fermenter cannot be changed once the lid is on because, according to the law of conservation of mass, mass cannot be created or destroyed(assuming the fermenter is a closed system. Weight on the other hand is a vector measurement(while mass is a scalar) and is equal to mass x gravity(acceleration). Since mass is not being added to the system and gravity isnt changing, there is no way to increase the weight of the fermentor once you've added your starter. Growing yeast do not add mass, because adding mass to a close system breaks the laws of physics. The yeasties merely convert one chemical to another.

Mass inside the fermenter is constantly decreasing because of the escaping gas and that decrease in mass is measured by the decreasing density(hence the hydrometer readings) since volume roughly remains the same. So no matter how much the yeast grow inside the fermenter they will never add any mass or weight to the system because all of the mass was there to start with. Again assuming that the fermenter is a closed system. I hope i didnt nerd-out too much there, Im sorry if I did.

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Old 10-28-2009, 11:21 PM   #8
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1) It is not a closed system because the CO2 is escaping.
2) I do not claim (nor do I think yooper does) that mass (yeast) is being created. It is being converted (as you state). e.g. mass from the wort is being used for the multiplication of yeast.

However, the mass of the yeast cake must be taken into account (subtracted out) for an accurate reading. Take a hydrometer sample of the beer. Then shake the carboy to get the yeast suspended. If you took another hydro test (with yeast in suspension) your specific gravity would be higher. So, while yeasties multiplying does not add mass into the fermentor they do weigh something, and their weight is not typically included in a SG sample.

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Old 10-28-2009, 11:38 PM   #9
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Quote:
Originally Posted by RDWHAHB View Post
1) It is not a closed system because the CO2 is escaping.
2) I do not claim (nor do I think yooper does) that mass (yeast) is being created. It is being converted (as you state). e.g. mass from the wort is being used for the multiplication of yeast.

However, the mass of the yeast cake must be taken into account (subtracted out) for an accurate reading. Take a hydrometer sample of the beer. Then shake the carboy to get the yeast suspended. If you took another hydro test (with yeast in suspension) your specific gravity would be higher. So, while yeasties multiplying does not add mass into the fermentor they do weigh something, and their weight is not typically included in a SG sample.
Yeah, I think that is what I was trying to say in my non-science way. The co2 escapes as a waste product from the metabolization of the sugars by the yeast. So, it's gone out of the fermenter. Therefore, not a completely closed system. The other waste product, ethanol, weighs less (by gravity measurements, not by weight) than water and wort. So, weighing an item will NOT give you an accurate SG measurement.

In other words, weighing the fermenter before, during, and after fermentation will not give you any definitive proof of completed fermentation. The mass will remain the same, but the composition will not. A 5% ABV liquid in that instance may weigh the same as a 7.5% ABV liquid since the mass is still there. The specific gravity of the liquid, however, may change dramatically.

A gallon of vodka would weigh nearly the same as a gallon of water. The Specific Gravity however is different. I guess that's what I'm thinking here.
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Old 10-29-2009, 12:51 AM   #10
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Oh, now I see what you guys mean. Thanks

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