weight to determine SG?

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metaldwarf

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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? :confused:
 
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.
 
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.
 
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.
 
JefeTheVol said:
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.
Click to expand...

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?
 
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.
 
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.
 
RDWHAHB said:
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.
Click to expand...

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.
 
A better way to do it would be to monitor the CO2 gas flow rate out of the fermenter. That's a bit tougher than using a scale, though.
 
I tried this. See details here: https://www.homebrewtalk.com/f11/weight-based-gravity-measurement-88821/

Mostly, I stopped because it was impractical. Moving the fermenter on and off of the scale was a pain. The surface of my scale was not big enough for a carboy so I had to use a piece of plywood and balance it on the scale.

That thread has a spreadsheet that you might find useful with all the math done for you. I accepted that there would probably be error for every batch, and that the error probably wouldn't be the same for every batch (e.g. 0.003 high) so my plan was to document the error for a variety of batches and then graph error vs OG, error vs batch volume, and anything else you can think of. If any of those graphs actually looked like anything, fit an equation to it and use that to predict an adjustment.
 
Not sure about estimating SG, but if you can assume that the only reduction in mass is escaping CO2, then you could estimate that there is then about 1.045 (molecular weight of ethanol/mol weight of CO2) times the reduction in mass in ethanol in the solution. There's a lot of assumptions there, but I'd be curious to see how close you could get. IOW if the mass dropped by 1kg bc/of escaping CO2 from fermentation, then there should be 1.045 kg of ethanol in the solution from there you could estimate SG if you knew the OG, i think.
 
shek said:
Not sure about estimating SG, but if you can assume that the only reduction in mass is escaping CO2, then you could estimate that there is then about 1.045 (molecular weight of ethanol/mol weight of CO2) times the reduction in mass in ethanol in the solution. There's a lot of assumptions there, but I'd be curious to see how close you could get. IOW if the mass dropped by 1kg bc/of escaping CO2 from fermentation, then there should be 1.045 kg of ethanol in the solution from there you could estimate SG if you knew the OG, i think.
Click to expand...

That's what I concluded. Mine was even simpler really, I decided that 1.045 was close enough to 1 for me. I just decided that their molecular weights were close enough to be considered equal.
 
See this thread for a great discussion on this very topic.

I have been taking very accurate weight measurements on my last five or six batches with an accurate shipping scale. Using the spreadsheet that Costarine provided in the linked thread, I got good results comparing the post fermentation gavities to the predicted gravities given the final weight. Then I looked into this more and derived an even more accurate formula that has given me great results (+/- 0.001 from what my hydrometer read). I was going to try to get a few more batches in to further boost my confidence in this method before posting my findings here. This post would have been the catalyst to post them now, except I'm away on travel and don't have that spreadsheet with me, so It's going to have to wait anyway. But to answer your question, Yes, it can be done. BTW, I use a bucket fermenter with an airlock. I think a setup with a blow-off tube will be much more difficult to get accurate results.

I was a tad concerned about the yeast also, but the formula works without having to account for it. I still do not understand this, but it is something I need to understand. Perhaps Kaiser can chime in on this? :confused:

I'll try to upload my spreadsheet when I get back home in a couple weeks.
 
earlytimes said:
See this thread for a great discussion on this very topic.

I have been taking very accurate weight measurements on my last five or six batches with an accurate shipping scale. Using the spreadsheet that Costarine provided in the linked thread, I got good results comparing the post fermentation gavities to the predicted gravities given the final weight. Then I looked into this more and derived an even more accurate formula that has given me great results (+/- 0.001 from what my hydrometer read). I was going to try to get a few more batches in to further boost my confidence in this method before posting my findings here. This post would have been the catalyst to post them now, except I'm away on travel and don't have that spreadsheet with me, so It's going to have to wait anyway. But to answer your question, Yes, it can be done. BTW, I use a bucket fermenter with an airlock. I think a setup with a blow-off tube will be much more difficult to get accurate results.

I was a tad concerned about the yeast also, but the formula works without having to account for it. I still do not understand this, but it is something I need to understand. Perhaps Kaiser can chime in on this? :confused:

I'll try to upload my spreadsheet when I get back home in a couple weeks.
Click to expand...


Think about it this way, the yeast are mostly just empty space filled with wort/beer/liquid. Dry yeast contains about 20 billion cells per gram, so even if you have a few hundred billion cells in your batch, the actual weight in yeast solids is only a few grams.
 
I agree that if you assume all weight lost is CO2 produced, and then run the molar mass calcs, then you can back out ethanol production & SG. Possible errors I see are H20 vapor loss, and CO2 left in solution. With a bit of luck those could balance each other out. Apparently those that have done report great results.

This is a different method than the OP though. I defend my yeast & changing liquid volume with that method. Yeast my be full of beer, but they are heavier. That is why they sink. Maybe not heavy enough to matter. I bet that they would introduce an error for the significant digits we measure SG to.

This takes "counting bubbles" to a whole new level. Now you just need to calibrate the volume of a bubble on your airlock setup. Sit there & count each bubble & run the calcs!
 
RDWHAHB said:
I agree that if you assume all weight lost is CO2 produced, and then run the molar mass calcs, then you can back out ethanol production & SG. Possible errors I see are H20 vapor loss, and CO2 left in solution. With a bit of luck those could balance each other out. Apparently those that have done report great results.

This is a different method than the OP though. I defend my yeast & changing liquid volume with that method. Yeast my be full of beer, but they are heavier. That is why they sink. Maybe not heavy enough to matter. I bet that they would introduce an error for the significant digits we measure SG to.

This takes "counting bubbles" to a whole new level. Now you just need to calibrate the volume of a bubble on your airlock setup. Sit there & count each bubble & run the calcs!
Click to expand...


A few months ago I bought an electronic gas flow meter to precisely measure the amount of CO2 exhaust during my fermentations and correlate the data to gravity readings. I haven't got it set up yet, but I should have it ready in the near future.
 
In layman's terms, the reaction is as follows:

Glucose => CO2 + Ethanol + Yeast

When I say yeast, I mean yeast biomass. The ratios are...

1 g glucose => 0.46285 g CO2 + 0.48391 g ethanol + 0.053 g yeast
(taken from wikipedia's entry on the plato scale)

Notice we don't lose mass/energy here, it just get's converted into other crap, some of which escapes (CO2) and makes the finished beer weigh less than the original wort. For example, your typical 5 gal batch has about 2.4 kg of equivalent glucose, and about 75% of that (1.8 kg) will be eaten by yeast to produce...

0.83313 kg CO2, 0.871038 kg Ethanol, 0.0954 kg Yeast
 

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