- Thread starter Dmanshane
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So, mass of beer = density * volume of beer

Mass (lbs) = 8.34 lbs/gallon * gallons

Or

Mass (kg) = 0.997 kg/L * liters

Now, I don't know how accurate you are trying to be, but alcohol weighs about 78.9% of an equal volume of water, so technically you are over filling your keg ever so slightly if you follow the above formula but the error is likely not an issue. But I suppose that decision depends on context.

If you are concerned about such things (which I doubt you should be) this formula will calculate mass of beer based in volume and alcohol percentage:

Mass (lbs) = 8.34 lbs/gallon * gallons * (1 - Etoh%/100) + 6.5845 lbs / gallon * gallons * Etoh%/100

Mass (kg) = 0.997 kg/L * liters * (1-Etoh%/100) + 0.789 kg/L * liters * Etoh%/100

So for example, to transfer exactly 5 gallons of 5% beer to your keg using this formula:

Mass = 8.34*5*(1-0.05) + 6.5845*5*0.05

Mass = 41.31 lbs

So like I said, not worth the extra effort, especially because this doesn't take into account the non-fermentable proteins, hop oils, etc that are present in your beer.

Specific gravity tells you all you need to know. Weight in water * SG - tare weight of keg.

So, mass of beer = density * volume of beer

Mass (lbs) = 8.34 lbs/gallon * gallons

Or

Mass (kg) = 0.997 kg/L * liters

Now, I don't know how accurate you are trying to be, but alcohol weighs about 78.9% of an equal volume of water, so technically you are over filling your keg ever so slightly if you follow the above formula but the error is likely not an issue. But I suppose that decision depends on context.

If you are concerned about such things (which I doubt you should be) this formula will calculate mass of beer based in volume and alcohol percentage:

Mass (lbs) = 8.34 lbs/gallon * gallons * (1 - Etoh%/100) + 6.5845 lbs / gallon * gallons * Etoh%/100

Mass (kg) = 0.997 kg/L * liters * (1-Etoh%/100) + 0.789 kg/L * liters * Etoh%/100

So for example, to transfer exactly 5 gallons of 5% beer to your keg using this formula:

Mass = 8.34*5*(1-0.05) + 6.5845*5*0.05

Mass = 41.31 lbs

So like I said, not worth the extra effort, especially because this doesn't take into account the non-fermentable proteins, hop oils, etc that are present in your beer.

And I thought at appropriate reference temp (specific temp escapes me) one milliliter of pure water weighed exactly one gram by definition, and thereby one liter was exactly one kilogram.

At the standard temperature of 25°C density drops to 0.997 kg/l

Beer SG changes with temperature too so if one wants to be ultra-accurate SG should be measured at the temperature the beer has at the time of transfer but we're really talking about erring by as little as 0.1% here, so...

Minor errors in logic here. Assumes pure 100% ethanol as you mention later:Now, I don't know how accurate you are trying to be, but alcohol weighs about 78.9% of an equal volume of water, so technically you are over filling your keg ever so slightly if you follow the above formula but the error is likely not an issue.

Specific gravity of 100%pure distilled h20 at specified temp is 1.000. I am unfamiliar with beer being dry @ 1.000 though don't know with long-term sours.especially because this doesn't take into account the non-fermentable proteins, hop oils, etc that are present in your beer.

Regardless, when dealing with a 5gal batch, the variance between 1.015 and 0.990is negligible, especially without lab grade temperature control. 8lb/gal or 40lbs is close enough for me. Though to be fair, I use feel. The minor temp change causes the slightest condensation on the outside of my keg and I feel for the temp gradient/condensation coupled with approx volume from my fermonster. (You have a conical (I assume stainless so visual volume is not an option without a sight glass).

Cheers!

I don’t use a scale when filling my kegs but I always weigh the kegs after filling, mostly just to have a “volume into fermenter” number to put into my brewing app.

I have pinlocks and all of them weigh 10 lbs (+/- 1oz) empty. I don’t worry about the ounce. Filled to the rim with water they hold just under 47 lbs, which works out to 5 gal and 5 pints. So, if the keg weighs between 51 and 52lbs I’m close enough to call it 5 gal. YMMV

I have pinlocks and all of them weigh 10 lbs (+/- 1oz) empty. I don’t worry about the ounce. Filled to the rim with water they hold just under 47 lbs, which works out to 5 gal and 5 pints. So, if the keg weighs between 51 and 52lbs I’m close enough to call it 5 gal. YMMV

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The OP wishes to close transfer from conical to keg and wants to know when his keg is full. What is the point of a scale when you simply attach an exhaust line from the gas port into a bucket of starsan.

When you see beer, it's at the bottom tip of the gas tube and you're done...

The previous thread re scales centered around determining how much beer was left in the middle of serving. In that application, I can see the value of a scale as one of many alternatives in determining the remaining volume of your keg.

This.Either I'm missing the point,...

See 1st post. OP asks for math equation from SG to determine volume from weight of keg.

From the OP:

Somewhere I have read a post about using a scale to measure how full your keg is.

...

I know there is a mathematical equation on how to fill your keg by weight using specific gravity.

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Fair enough.This.

See 1st post. OP asks for math equation from SG to determine volume from weight of keg.

From the OP:

It would be nice to hear from the OP.

Are you seriously looking for a mathematical equation? Or are you simply trying to determine when your keg is full without opening it?

Also true, there are simpler ways to do this.

I put a picnic tap on the gas post and periodically bleed pressure to keep the flow going. It won't overflow if I walk away, and maintains positive pressure for a more truly closed transfer than a simple exhaust tube. This can even be used to do a CP transfer of fully carbonated beer, though it's a bit labor intensive.

Cheers!

I too have my gas tubes trimmed, recessed from the inner keg surface. I fill to overflowing, and then dispense some beer from the liquid side so I have a known amount of head space. Works as well as stopping a known amount short of overflowing, and I just figure this loss, as well as many other losses (samples, harvesting slurry, etc.) into my brew length. Choice of methods depends on priorities and limitations in your particular system.If I fill to overflowing the beer level will be well above the top of the sidewall which will slow carbonation...

Cheers!

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Lol okay first answer was what I was looking for I knew what weight of water was just couldn’t remember how you added the sg. And I don’t want my keg so full there is beer to the dip tube on gas side. Hate getting beer in co2 lines

I have been using this calculator to determine the volume in the keg after filling, simple easy spreadsheet.

https://www.homebrewtalk.com/forum/threads/keg-volume-calculator.633022/

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Or.. you could fill until the exhaust line bubbles beer. Then pull 2 or 3 glasses to clear the murk. Then your gas tube is free and clear. Even though it will carbonate just the same.

I tend to be exacting in my processes, but this one intrigues me. I've never felt compelled to use "calculators" in simply filling a keg.

I have found that if you closed system gravity fill the liquid will stop when level.