Estimating alcohol by total weight during fermentation?

I tried doing a search but wasn't very fruitful given some of the common words. I'm curious if anyone has tried estimating alcohol content by weighing the entire fermenter?

As a bit of an experiment, I thought I'd sit the carboy on a fairly accurate (.05 lb resolution) digital scale during fermentation. The total weight of my 5.5 gallon batch (including the 6.5 gallon carboy) was 64.9 lbs right after I popped the airlock on. 24 hours later, the total weight is now 63.5 lbs.

I should have weighed my empty carboy to get a tare weight, but this idea didn't even come to me until after I started filling it. From what I've seen, a 6.5 gallon carboy weighs about 15 lbs. I'm using 8.33 lbs as the weight of water per gallon, which isn't exact, but just as a test I figured it was close enough. The temperature of the batch is within 2 degrees of when I first weighed it, but I haven't taken that into account either. I started playing with some calculations and this is what I came up with:

((64.9 - 15) / 5.5) / 8.33 = 1.08916294 (My OG was 1.084, so this is actually pretty close)

And now the batch is at 63.5 lbs, so:

((63.5 - 15) / 5.5) / 8.33 = 1.05860526

The difference of which should mean it's roughly about 4.0% right now. Sound correct? Or have I completely lost it?

I'm still obviously going to take some actual samples to measure with my hydrometer, but I thought I'd see how close the scale would be.

 

Well I did just find this thread, http://www.homebrewtalk.com/f13/monitor-sg-beer-fermenting-251137/, which mentions something I didn't really think of at the time... the added weight of the multiplying yeast cells. I suppose if anything, the alcohol content should actually be slightly higher than calculated based on weight then?

 

Extract is lost to the production of c02, yeast biomass, and ethanol (and some other stuff in small amounts). The proportions of each are estimated by Balling.

Your scale at .05 lb resolution is sorta crap compared to a hydrometer for this sort of thing. I am not sure I understand the point.

 

If you have to ask, you'll never know.

 

Nope

 

Conceptually you could do this. As fermentation proceeds the carboy will become lighter as CO2 is lost. Balling found that each 2.0665 grams of extract that is fermented is converted to 1 gram of alcohol, 0.9565 grams of CO2 and 0.11 grams of yeast. Thus for each 0.9565 gram loss in carboy weight you could assume you had 1 gram of alcohol. Subtract 0.11 grams of yeast weight for each 0.9565 gram of weight loss from the total net weight of the carboy and you have the net weight of the beer. Divide into the weight of the alcohol and you have an estimate of ABW. This is simply converted to ABV by multiplying by the specific gravity of the beer and dividing by the specific gravity of alcohol (0.0791). Thus you also need to know the volume of beer in the carboy in order to calculate the specific gravity. The volume estimate will be off because of the volume of the yeast cells, of course, and the other problem is that all the weight loss isn't from CO2 but includes the water vapor the CO2 sweeps out with it.

 

While the scale I used is accurate for most things (accurate to 8 tenths of an ounce), it's certainly not "perfect" for this. My scale that is accurate to 1 tenth of a gram is only good to a little over 1/4 lb, so clearly that won't work. It really was just an experiment... I should be able to "guesstimate" what % it is and also when it stops. Granted you can tell a bit just by looking at the activity. I already had the scale, so it's not like I paid anything to run the test. I'm still going to do hydrometer readings.

Thanks for the info, that gives me a bit more insight.

 

Uh, last time I checked, yeast cells can not break the second law of thermodynamics. The yeast can only multiply with stuff that's in the wort, and any added oxygen.

 

Chemical reactions take place within the yeast cells producing biomass, alcohol and CO2. These reactions all follow the second law - they continue until the entropy of the system is maximized i.e. until all the nutrients are consumed. There is heat flow from the yeast cells into the surrounding beer (and subsequently into the air surrounding the fermentor). Thus the entropy of the surroundings increases and, as the entropy of the system (chemicals in the cells) also increased the entropy of the universe increased. The second law is satisfied.

 

Yes, but they cannot "add" any weight to the wort.

 

Ah, so you meant to invoke conservation of mass rather than the second law. Yes, conservation of mass holds: 2.0665 grams of extract is converted to 1 gram of alcohol, 0.9565 grams of CO2 and 0.11 grams of yeast. This doesn't account for lost water (or alcohol) vapor or O2 dissolved in the wort nor for esters, higher alcohols, VDK's....

 

Actually I meant to invoke the first law of thermodynamics (which is essentially conservation of mass in a relativistic context)

Anyway, the point is, no mass is added by the yeast cells, it is simply the case that not 100% of the disappeared extract turns into alcohol.

(Also, your CO2 figure looks absurdly precise. Is it as accurate as it is precise, or is it just extrapolated from 1 gram of alcohol?)

 

OK but I don't notice much conversion of mass to energy even in my strongest brews (or the converse).

Roger that.

'Tisn't my figure. It's Balling's. How he got it I have no idea but I rather doubt that 5 significant digits is justified especially since the yeast value is given to two and is known to be quite variable. Nonetheless, if you convert extract loss to alcohol or alcohol to extract loss you are using his numbers.

 

There is, actually, to be fair. There are not nuclear reactions, for sure. However, the sugar molecules weigh more than the resulting alcohol/oxygen/CO2, the rest of the weight (just about infinitesimal) being either conversed to heat, or used in other chemical bindings between say, proteins in the yeast.

Vaguely reminds me of those hand thermometers they have at hospitals, measuring the body temperature by IR light towards the eardrum. They'll give the temp as 37.1, 37.3, despite the fact that they are nowhere near that accurate.

 

Perhaps not accurate, but precise

 

The heat comes from bond energy which derives from a combination of coulombic and Van der Waals forces. There is no conversion of mass to energy in these reactions. What would convert? Certainly not protons or neutrons or the the subatomic particles that bind them together as the nuclei stay intact. The electron configurations do change and take up energy in the form of photons (photosynthesis) and release it as heat (fermentation) but photons have no mass so no mass is anihilated there.

If there is indeed mass loss it appears it would take a Feynman diagram, bra's or ket's to explain it and that's way over my head and I suspect yours.

 

Gravity acts on all forms of energy, this includes bonding energy.

 

Gravity is one of the 4 forces of nature the others being electromagnetic (which we covered when we mentioned coulombic and Van der Waals forces and photons), strong and weak. Anticipating that your next post will mention one or the other of these latter 2 as there seems to be no limit as to how silly you are willing to get, I will mention that they aren't relevant to this discussion either.

Nor is gravity. The atoms in a molecule do have mass, off course, and so, when in proximity, are subject to mutual gravitational attraction. But it is so small, relative to the other forces, as to be completely insignificant. For example, the hydrogen bond between two water molecules has an energy of about 18 kJ/mol. The work required to overcome the gravitational force between two water molecules separated by the length of a hydrogen bond and to move them infinitely far apart is about 3E-21 kJ/mol - that's almost 22 orders of magnitude!

 

I never said the amount was significant, only that you were wrong.

 

Where exactly was I wrong?

 

^This is wrong, for one. The "mass" that we measure is only a measure of the amount of energy in a substance. Mass-energy equivalence, if you will recall. Nuclear reactions are NOT the only way mass and energy can be converted into each other - indeed, mass is a FORM or a measure, or perhaps a property, of energy.

And while it is true that photons have no mass in the relativistic sense, they do have energy, and gravity acts upon them (which is why black holes are, well, black). A warm solution, the same way, is heavier than a cold one, because there is more for gravity to act on. This however trips into the area of general relativity, which I'm terrible at.

For further reading: http://en.wikipedia.org/wiki/Mass-energy_equivalence

 

I just can't think of one. Do you have an example?

What does this have to do with the heat of a chemical reaction.

If the gravitational field is so intense the space time tensor shows curvature weird stuff happens but I'm still calling BS on this one. In my lab warming or cooling a solution does not change it's mass. Again, how is this relevant to the question of energy released/absorbed in a chemical reaction.



If you really believe that mass is being converted into energy when a chemical bond is formed broken and wish to convince someone of this then I think it is incumbent on you to state which mass is being converted. I've gone through all the particles (except the sub atomic ones) and shown that none of them lose mass. Perhaps you think gravitons exist (they may) and that they have mass which is anihilated.

E = m*c^2 means that when m converts to energy the amount of that energy is m*c^2. It does not mean that it does convert or that the 2 are interchangeable in any other sense any more than the second law says a chemical reaction will take place if its application indicates an increase in entropy.

Apparently you have been exposed to some of this at least at the Wikipedia level but clearly misinterpret much of it. Thermodynamics is a macroscopic science. It is not concerned with forces that are 20 orders of magnitude less than anything that can be measured in the laboratory. In fact thermodynamics is a science of measurement.

If you read up on the fist law you will see that it is a conservation of mass/energy law and that the rest mass energy of electrons and nuclei are considered in the balance. But as this does not change during a reaction it cancels (thermo is concerned with changes in state variables - not their absolute values) and the first law is written dU = dq +dw. This simplified form ignores rest mass energy, electric or gravitational potentials, the fact that the carboy may be on it's way to the floor (kinetic energy) and so on because these are not relevant to the questions as to whether a reaction is thermodynamically feasible or whether it is exo or endothermic.

I'm not your P-chem teacher. If you want to understand this stuff I would suggest a good physical chemistry text. At least you'll know the difference between the 0th, 1st, 2nd and 3rd laws when you are finished.

 

Some of these posts are a bit rude and/or unnecessary. While I don't have the scientific background to fully answer the OP, I'm positive that detailed discussions of relativity, gravitational fields, and space/time are irrelevant in order to provide the OP an answer. Should you want to continue those arguments, please post in another thread or continue via private message.

ajdelange's first post in this thread appears to sum up the concepts involved very nicely without running off on tangents that may be remotely relevant but aren't exactly of interest to the OP. Let's get back to that sort of discussion, please.

 

Let us use Wikipedia again.

http://en.wikipedia.org/wiki/Binding_energy#Mass_deficit

Thank you and have a nice day.


Edited to add: Yuri: Right. Sorry.

 

Again, not relevant, but I agree with Yuri.

And if I've been rude I also apologize.

 

I'm glad I found this thread and it was recent because I was wondering the same thing as the op.

Obviously it would be used to supplement hydrometer measurements, but I think it would be convenient to track the progress of the fermentation just by tossing the carboy on a scale and not having to expose the beer.

Also team ajdelange.

 

Sorry to resurrect this old thread, but I think I have something of value to add to this discussion.

I recently did this with a $25 shipping scale I bought off Amazon and a 5 gallon batch of doppelbock in a carboy. I interpolated the results using Balling's numbers and compared the results to my hydrometer readings. It appears pretty conclusive that this can in fact be done to a great degree of accuracy. I saw a study done by the Brewer's Institute back in 1990 on the feasibility of measuring CO2 production to determine ABV and fermentation progress, and they concluded it can be done to 99% efficiency. So there is nothing new under the sun, and yes, this can be done.

My next step will be to hack the shipping scale and feed the output into a microcontroller to monitor the fermentation progress in real time.