Filling keg with a scale?

[Dmanshane]

Somewhere I have read a post about using a scale to measure how full your keg is. I am doing closed transfer from conical to keg. I do Not want to open keg to see how full it is. I know there is a mathematical equation on how to fill your keg by weight using specific gravity. can someone help me with this please?

 

[Robert65]

Water weighs 8.34 lbs/gallon. (This varies a little with temperature, but across the range of possible beer temperatures that's insignificant.) So a gallon of your beer weighs 8.34 x 1.0xx [whatever the specific gravity is at the time of transfer] lbs.

 

[Dland]

I put up to 40# of beer in C keg. Any more and you won't have any head space.

 

[eanema]

You should be able to assume your key is being filled with water. Water is very close to 0.997 kg/L, or for you southerners 8.3454 lbs/gallon. So if you put your empty key on a scale and zero it, then transfer in 41.75 lbs (8.35lbs * 5 gallons) you have just filled you keg with 5 gallons of beer.

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.

 

[eanema]

Lol, was that too much math? Haha

 

[eanema]

Also, looking at the previous responses, I was a little off base... Much more accurate using a final SG reading. Ignore my post ;-)

 

[Qhrumphf]

You should be able to assume your key is being filled with water. Water is very close to 0.997 kg/L, or for you southerners 8.3454 lbs/gallon. So if you put your empty key on a scale and zero it, then transfer in 41.75 lbs (8.35lbs * 5 gallons) you have just filled you keg with 5 gallons of beer.

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.

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.

 

[Vale71]

That would be 3.984°C or in short 4°C.

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...

 

[S-Met]

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.

Minor errors in logic here. Assumes pure 100% ethanol as you mention later:

especially because this doesn't take into account the non-fermentable proteins, hop oils, etc that are present in your beer.

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.

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).

 

[day_trippr]

For those using digital scales that have automatic power-down, I advise not taring the scale, but instead calculate the total mass (beer plus keg) and use that as the target weight. Should the scale power down in the middle of racking, you can simply lift the keg off the scale, power it back up, set the keg back down, and continue filling...

Cheers!

 

[grampamark]

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

 

[SEndorf]

Either I'm missing the point, or we have a bunch of talented and bored mathematicians on a Sunday afternoon.
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.

 

[S-Met]

Either I'm missing the point,...

This.
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.

 

[SEndorf]

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

Fair enough.
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?

 

[Robert65]

True, we answered particular the question the OP asked.

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.

 

[day_trippr]

I run a metered tap list and need to know starting volume anyway, but I cut all of my gas tubes down to under 1/2" so the end is barely proud of the riser. If I fill to overflowing the beer level will be well above the top of the sidewall which will slow carbonation...

Cheers!

 

[Robert65]

If I fill to overflowing the beer level will be well above the top of the sidewall which will slow carbonation...

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.

 

[day_trippr]

I'm firmly against cold side losses. I strive to have a "10 gallon batch" literally fill two cornys with nothing left in the glass - though it occasionally happens - but that's what liter soda bottles and carb caps are for

Cheers!

 

[Dmanshane]

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

 

[ba-brewer]

I fill thru the out post and put a thin piece of plywood under one side of the keg so the gas post is a bit lower. If it out flows while tipped it will be above the liquid when standing flat.

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/

 

[SEndorf]

OK. Obviously there are more complicated ways to skin this cat.
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.

 

[ba-brewer]

If you are multitasking knowing how close to full can be helpful to know when it is time to stay on the keg filling task.

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