How do I calculate wort density?

[Elysium]

I have just been asked for something called "wort density".
How do I calculate that?

 

[brewkinger]

Specific gravity


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[Elysium]

Specific gravity


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So....you are saying that wort density is the SG?

 

[Yooper]

So....you are saying that wort density is the SG?

Yes. That is probably what whoever asked you that meant.

 

[Elysium]

Yes. That is probably what whoever asked you that meant.

Yep. That is the situation.

A Hungarian trying to communicate with a Spanish speaker about beer brewing.

Thanks Yooper.

 

[brewkinger]

Damn... Second fiddle to Yooper again!
LOL
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[jake_humphries]

Nah, wort density is not SG, but they are related in that the density depends on the concentration (i.e. gravity/degrees Plato, Brix, etc.). you can find tables to convert between the 2 units.

this post was edited for correctness.

 

[doug293cz]

Nah, wort density is not SG, but they are related. density is the actually a concentration of dissolved solids measured in kg/L. you can find tables to convert between the 2 units.

No, density is not concentration. Density is the mass per unit volume. Density can apply to solids, liquids, and gases. For liquid solutions, density is the mass of solution divided by the volume of solution. Weight of solute comes in only indirectly. Density can be given in different systems of units, such as: kg/L, g/ml, lbs/gal, etc.

Specific gravity is the ratio of something's density to the density of water. Since the density of water is about 1 kg/L, 1 g/ml (this is exact only at 4°C), specific gravity equals density when using metric units for density, but not when using imperial units. Specific gravity is has no units, so it doesn't mater what system of units you are using.

°Plato and °Brix are units of concentration, specifically the weight percent of sucrose in water (i.e °Plato = °Brix = mass of sucrose / mass of solution = mass of sucrose / (mass of sucrose + mass of water.) In brewing it is assumed that all extract behaves the same as sucrose w.r.t. the effect of concentration on specific gravity.

When someone asks for the density of wort, they are asking for the specific gravity or the °Plato value. Professional brewers like to talk in Plato, even tho Plato isn't actually a measure of density. Plato values can be converted to specific gravity values, and vice versa, using empirically fitted equations.

Brew on

 

[Gnomebrewer]

SG is the density in Kg/L. An SG of 1.064 means it has a density of 1.064Kg/L (or 6.4% greater than the density of water). Unless I've been mistaken for many years, which is possible.

Edit: Beaten by Doug, and his answer is in much more detail!

 

[jake_humphries]

doug above is correct in that density is not the concentration. but he is wrong in that it is not the same as specific gravity, and as i stated, there are tables that will give you the wort density given a specific gravity, but these are 2 different measures.

i initially put concentration because i am doing the EdX course in brewing science, and it gave the units as kg/L and the chemist in me saw this, when given for a liquid solution, as a measure of concentration. i thus went searching for this as i'd never heard this used in brewing, and i stumbled upon this thread. but after posting above i realised it was actual density as the name says when i considered the example given in the EdX course of 1.0465 kg/L, which couldnt possibly be a wort concentration (you couldnt get 1.05kg of extract in 1 liter of wort). it means a liter of wort weights 1.0465 kg. this is not the same as specific gravity nor degrees Plato (or Brix or Balling).

a formula is given in the EdX course to calculate the grain bill given the wort density and concentration in degrees plato. thus, this density can't possibly be the same as specific gravity. whether brewers often use an incorrect term to refer to gravity is another thing. and yes, a table is given that gives gives wort density from SG/degrees Plato.

Sorry i did not correct my post above after realising that. i have now edited it.

 

[Gnomebrewer]

a formula is given in the EdX course to calculate the grain bill given the wort density and concentration in degrees plato. thus, this density can't possibly be the same as specific gravity.

Why not?

As far as I can see, the only difference between wort density and SG is due to temperature - the SG of water is measured as 1.000 @ 20C (or whatever the hydrometer is calibrated at), but is actually something like 0.998Kg/L at that temperature. Wort SG is measured in comparison to the 20C water, so the SG reading will be slightly higher than the actual density of the wort at 20C, but (assuming the effect of temperature on wort density is similar to the effect of temperature on water, which it may or may not) should tell us the density of wort at 4C (it's most dense).

 

[jake_humphries]

Why not?

i had a closer look at the table, and while they are not the same, they are very similar. As you said, for a given SG, the numerical value of the density is about 0.002 less than the SG, e.g. the density of a 1.05048 SG wort is 1.04860 kg/L. I would say that that the difference is likely largely due to the difference in densities of pure water at 4C compared to 20C but also probably due in part to the freezing point depression of impure water, i.e. water with dissolved sugars, etc. will likely have a maximal density at a temperature other than 4C.

 

[Gnomebrewer]

The density of water at 20C is 0.9982Kg/L.
1.05048 x 0.9982 = 1.04859
That's rather close to 1.04860!
So it looks like wort with a density of 1.05048Kg/L at 4C will have a density of 1.04859Kg/L (or 1.04860) at 20C and read 1.05048SG at 20C. Or very close to. Close enough for homebrew.

 

[jake_humphries]

Close enough for homebrew.

As i stated above, the "for a given SG, the numerical value of the density is about 0.002 less than the SG". I originally wrote but then revised before posting that it would be close enough for homebrewers. I revised because a difference of 0.002 would be meaningful to me. My hydrometer reads to an precision of 0.001 based on the tick marts alone (it has tick marks every 0.002, and i estimate if it is at or approximately equidistant from the ticks). the example i gave was not a great one because when rounded to 3 decimal places, the difference is only 0.001. But most all other values in the table have a difference of 0.002. Whether or not 0.002 is close enough for homebrew will differe between homebrewers and the purpose of the measurement. I am not hear to argue about that, i just posted to say that wort density is not the same as specific gravity, though it may be close enough for some brewers.

 

[doug293cz]

As i stated above, the "for a given SG, the numerical value of the density is about 0.002 less than the SG". I originally wrote but then revised before posting that it would be close enough for homebrewers. I revised because a difference of 0.002 would be meaningful to me. My hydrometer reads to an precision of 0.001 based on the tick marts alone (it has tick marks every 0.002, and i estimate if it is at or approximately equidistant from the ticks). the example i gave was not a great one because when rounded to 3 decimal places, the difference is only 0.001. But most all other values in the table have a difference of 0.002. Whether or not 0.002 is close enough for homebrew will differe between homebrewers and the purpose of the measurement. I am not hear to argue about that, i just posted to say that wort density is not the same as specific gravity, though it may be close enough for some brewers.

And SG is not even close to density if you measure density in lbs/gal. Water, which by definition has an SG of 1.000, has a density of 8.3304 lbs/gal @ 68°F (20°C.) SG is SG no mater what kinds of units you prefer to use.

Brew on

 

[schematix]

doug above is correct in that density is not the concentration. but he is wrong in that it is not the same as specific gravity, and as i stated, there are tables that will give you the wort density given a specific gravity, but these are 2 different measures.

Specific gravity is a measure of density relative to water at a specific temperature. You sir are the one that is incorrect.

Density is mass per unit volume.

S.G. is a ratio.

Carry on.