I had this crazy thought this morning. I was thinking about specific gravity and what that measurement actually means mathematically. Scientifically, it’s a measure of the density of a liquid, but it’s actually just a ratio of the weight of a liquid divided by its volume. So if you have a big enough fermenter to contain all the krauzen so that there is no blow off, you can get a ball park FG at the end of fermentation just by weighing the carboy. As wort ferments, the sugar is converted to CO2 (which is blown off), and alcohol which has a lighter density than water. My thought for an experiment is to take a digital scale with a big enough weight capacity to weigh out an entire carboy full of wort. Put the carboy on the scale and just leave it there for the duration of primary fermentation. Take an initial weight. Subtracting the weight of the carboy wouldn’t be necessary for this experiment. When the active fermentation slows, start measuring the weight. When there are no longer changes in the weight, fermentation would be over. Of course, it will be difficult to measure the actual volume and weight of the finished beer because of the trub, but since we’re looking for a change in weight, this could just as effectively gauge completion of fermentation as taking hydrometer samples, correct? Then you could actually take that hydrometer reading to get your actual FG. Of course, a change in weight would be easier to gauge with a 5 gallon vs 1 gallon fermenter.
A lot of the mass in the wort is converted to biomass of the yeast, which is going to remain in the fermenter. Also, I think you would have a very difficult time discriminating between measurements at the tail end of fermentation, where gravity is dropping just a few final points. Lots of variables effect how much carbon dioxide is discharged from the wort, as well… temperature, atmospheric pressure, etc. I think it would be about as reliable as gauging fermentation progress via airlock activity.
Just playing devil’s advocate - the theory is certainly sound, though.
Perhaps if someone had a conical fermenter and could remove the grub to weigh it.
OK, I’ll play along with you, then put some in a one gallon jug and monitor it as you check on yer gravity on the other 4 gallons to see IF you can find a reasonable answer… Sneezles61
Rather than dedicate a scale to your fermenter, wouldn’t it be easier to just dedicate a hydrometer to the fermenter??
OK, so that’s tough for a narrow neck carboy/better bottle, but totally feasible for a bucket or bigmouth.
That’s just crazy talk, Crazy I say. I’ll bet you even aced the Calculus and Physics in pre-vet. Me, I was lucky my profs graded on a steep curve. 
I think it’s potentially valid enough to run an exBeeriment.
[quote=“jimrmaine, post:6, topic:20853, full:true”] I’ll bet you even aced the Calculus and Physics in pre-vet.
[/quote]
What’s calculus and physics? 
What’s calculus and physics?
[/quote]
You use a calculus to add numbers and physics is just a fancy name for gym class.
Great points! Didn’t factor in the yeast. Since production and loss of CO2 is the only way the carboy would lose weight (if all blow off is controlled), and since you should be able to see that in the airlock, the weight isn’t going to tell us anything more. I thought it sounded too easy. Back to the drawing board. Thanks for all the responses.
I start getting ready to bottle when the water balloons in my fermenters sink. 
I would like to hear more about this…
I assumed he meant when the balloons on his fermenter deflate
I wonder if its a front facing camera?
Sneezles61
I fill the balloons with 1.0 specific gravity water and ensure no air bubbles before tying off. From experience, water balloons in a bucket of water have essentially neutral buoyancy so any offset from the balloon material itself can be neglected. As the gravity in the fermenter begins to decrease, the balloons float lower on the surface. When the balloons descend below the surface of the ferment I know that the gravity in the fermenter is less than 1. I realize that there are other factors, including carbonation and suspended solids, but this method is not meant to be a rigorous analysis.
Thats clever
That’s what I told Archimedes
I’ve not been to his brewery
Hahah! Genius
