Calculate SG by Weight?

[Rhumbline]

I've seen this mentioned before and the response is usually "it won't work" but I'm not going to let that stand in my way.

If specific gravity is just a ratio of the weight of one volume of liquid compared to the same volume of water it seems it should be easy to calculate SG by weighing the liquid in question.

If water weighs 8.35 pounds per gallon and I'm calculating for 5.5 gallons, then my reference weight = 45.925 pounds.

I weighed an empty primary and lid, 3.833 pounds so my reference is 49.758 pounds.

An OG of 1.108 at pitching would calculate to 54.717 pounds and an FG of 1.022 would yield 50.768. To put it another way, each 0.001 of specific gravity would equal 3/4 of an ounce.

I would need to have an accurate measure of how many gallons of wort I start with but I don't see why it won't work as long as I can build a circuit to measure weights that accurately.

I could leave the fermenter on the weighing scale and see easily how close I am to target.

It will be early next year before I can focus on this, I've got some other projects in the works.

 

[TexasWine]

Funny you bring this up. I use weight to calculate volume. Measure my specific gravity, weigh the wort, and calculate the precise volume.

The problem you're going to have is finding a way to measure volume with the same accuracy at which you measure weight. So what I would recommend is calculating your precise volume as I do by taking an OG reading. Then use your differential, x.xxx change in gravity points = xx ounces, to determine where your fermentation is.

Let me know if you give it a whirl!

BTW, I use a luggage scale suspended from my ratchet pulley to weigh my wort, making sure to tare out the bucket.

 

[flars]

Do you have a calculation for evaporation?

 

[Rhumbline]

Funny you bring this up...

The problem you're going to have is finding a way to measure volume with the same accuracy at which you measure weight.

Let me know if you give it a whirl!

BTW, I use a luggage scale suspended from my ratchet pulley to weigh my wort, making sure to tare out the bucket.

I was considering some kind of Arduino based system. But you are correct, crunching some more numbers tells me that even a 1 fl oz error could translate to a .001 difference in SG.

But I do think that it will be a useful way to get an idea of whether fermentation has stopped.

I didn't think evaporation would be a big deal, blow off might screw up the math.

I'll experiment with my next brew.

 

[Rhumbline]

I thought I'd post my experience with a brew I'm experimenting with.

I brewed a batch yesterday and did a partigyle. Ended up with a bit over 5.1 gallons of wort at a measured 1.048. If my math is correct, it should have weighed 49.33 pounds. I measured it on my digital bathroom scale at 49.2 pounds.

I would need to be able to measure volume and weight much more accurately, but I'm interested in seeing how accurately this can predict the change in SG.

 

[pricelessbrewing]

Doing this to determine your og is fine, but it won't really work with your fg. They are too many other variables like proteins, yeast, yeast by products, etc.

 

[Rhumbline]

I'll add a final update.

To calculate SG by weight I'd need a very accurate method of measuring volume, and that has its own issues.

What has worked very well is using weight to determine when fermentation has stopped. For the past several days the weight has remained the same and the hydrometer has verified that I'm stable.

So, I'll call it a partial win.

 

[pricelessbrewing]

Interesting. More info? Finding a quick and dirty way to determine when fg is reached would be awesome.

 

[Rhumbline]

Interesting. More info? Finding a quick and dirty way to determine when fg is reached would be awesome.

It's not a very sophisticated method. It won't calculate specific gravity, it just tells me when fermentation has stopped.

I use a digital bathroom scale and weigh the fermenter. On the brew in question, I started with an OG of 1.048 for a bit over 5.1 gallons. Starting weight was 49.2 pounds.

I was shooting for a FG of 1.020 and I calculated a total target weight including the fermenter of 47.95 pounds.

I ended up at 1.020 with a measured weight of 47.2 pounds, but that weight stayed steady for four days which I took to mean fermentation had ended.

I like the idea that I can monitor for fermentation end without opening the fermenter and messing with the brew.

I have a few other ideas to try, but this will work for the time being.

 

[rudylyon57]

I'm going to try this also. We were all taught about the conservation of mass in high school. So, if it is assumed that the only thing leaving the fermenter is CO2, AND, the volume of wort does not change (does it?) then it's an easy problem. 1) Get tare weight of vessel and whatever will be on it while taking weight measurements later. 2) Get an OG reading. 3) Weigh (W) the wort and vessel, subtract tare (T) weight and calculate volume (V) based on OG. 4) Monitor weight during fermentation and calculate SG using SG=(W-T)/(V X 8.34).

One member of our local brew club pointed out that the yeast cake has a different SG than the wort/beer...probably correct.

 

[pricelessbrewing]

Yeah there's a lot more going on then that. The best you can probably do is "well the weight stopped changing, probably a good time to check the fg"

 

[rudylyon57]

I'm going to try this also. We were all taught about the conservation of mass in high school. So, if it is assumed that the only thing leaving the fermenter is CO2, AND, the volume of wort does not change (does it?) then it's an easy problem. 1) Get tare weight of vessel and whatever will be on it while taking weight measurements later. 2) Get an OG reading. 3) Weigh (W) the wort and vessel, subtract tare (T) weight and calculate volume (V) based on OG. 4) Monitor weight during fermentation and calculate SG using SG=(W-T)/(V X 8.34).

One member of our local brew club pointed out that the yeast cake has a different SG than the wort/beer...probably correct.

Here are the results. I say that with an accurate scale, weighing can not only indicate terminal fermentation but also terminal gravity (FG).

View attachment SG by weight.pdf

 

[pricelessbrewing]

That's awesome. Quite interesting! Was this with 1 brew, or several? Like I said before, I'm much more interested in the "is it done fermenting?" vs "it's at 1.012 now!"

Care to provide the data set? (PM or email is cool if you don't want to post it)

 

[TexasWine]

Here are the results. I say that with an accurate scale, weighing can not only indicate terminal fermentation but also terminal gravity (FG).

The link appears to be broken on my end. I'm on my mobile, so perhaps that's the issue.

 

[pricelessbrewing]

The link appears to be broken on my end. I'm on my mobile, so perhaps that's the issue.

It appears to be a pdf of a chart from excel showing a plot of the weight vs time, hydro sg vs time, and a trend line to approximate the hydro as a function of weight. Seems to be accurate for most practices. I'll definitely look into this more on my side, would love to be able to just weigh the bucket and know when it's going to be done.

 

[rudylyon57]

That's awesome. Quite interesting! Was this with 1 brew, or several? Like I said before, I'm much more interested in the "is it done fermenting?" vs "it's at 1.012 now!"

Care to provide the data set? (PM or email is cool if you don't want to post it)

I have another brew that I'm tracking right now. I'll get that data into the chart and send you the excel file. I would be happy to share it. Here is a snapshot of progress on a Wookey Jack clone with only the OG hydrometer data point shown.

The day to day change in carboy weight is all that's needed to know when fermentation is complete. To get an accurate measure of SG you also need a fermentation that does not blow anything other than gas from the airlock.

View attachment Wookey_6Days.pdf

 

[Anyhowe]

If you want to pay the bucks Pico's Kegsmarts does just that with a scale and even graphs the results for you.

 

[pricelessbrewing]

If you want to pay the bucks Pico's Kegsmarts does just that with a scale and even graphs the results for you.

Hmmm not really.