Question about Gravity

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Just a quick comment on "Dipsticks".

My advice is to make up an "Ullage Stick" for whatever pot you will be using to boil up the wort.

The Ullage Stick is brought down to the top level of the wort and stops me having to introduce something into a hot sticky mass of wort.

An Ullage Stick basically measures what isn't there as you can see from the photograph. :thumb:

The dotted line (at 27 litres) is where I expect the wort to be after boiling from 30 litres for an hour.

Ullage Stick.jpg
 
Not meaning to be pedantic, but the specific gravity of a liquid is a measure of its density; how much it weighs per unit volume. A brewers hydrometer is calibrated to read 1.000 in pure water at 20C. This is very close to the actual density of water, which is 0.998 kg/L at 20C. What this means is that 1 litre of liquid with a gravity of 1.000 will weight about 1kg, and 1 litre of a liquid with gravity 1.040 will weigh about 1.04kg. Before you ferment you have a mixture of sugars and water, which is denser than water on its own, so you have a high gravity. Fermentation converts some or all of the sugars into alcohol, which lowers the density. For wine and cider all the sugar may be consumed and you end up with a solution that is less dense than water, so you might get a gravity reading of 0.995 for instance. For beer not all the sugars can be fermented, so you usually finish above 1.000, but this depends on factors such as mash temperature (lower temperatures produce more fermentable wort and a lower finishing gravity), what yeast you use (some strains can consume sugars that others can not), and how healthy your yeast is (stressed or nutritionally deficient yeast won't bother fermenting some of the harder to ferment sugars in wort).

How the gravity reading actually relates to alcohol and sugar content is not straightforward. While it is true that all the drop in gravity that happens during fermentation can be attributed to the loss of density due to the removal of sugars and the addition of alcohol, the relationship between gravity and density isn't linear in either alcohol or sugars. For instance, if you took 0.5L of ethanol and mixed it with 0.5L of water then the solution you end up with would be considerably less than 1L (in technical terms, you can say that a solution of alcohol and water has negative excess partial volume). The upshot of this is that you can't say "a drop in gravity of x means an increase in abv of y", although you can get a reasonable ballpark figure using this approximation for beers with starting gravity < 1.060 with the assumption that you gain 1.3% abv for each 0.01 drop in gravity. For higher gravity beers there are more complex formulas available that take into account not just the drop in gravity but also the specific starting and finishing gravities. There is no deep theory behind them, they are just formulas that, if you plug OG and FG in them, then you'll get an abv out of them in the end that is about the same as the result you would get if you sent your beer off to a lab for analysis. They have the additional property that the higher the OG and FG, then the higher the gain in abv you get from any given drop in gravity.

To your next question, how to diagnose what went wrong if you don't hit your target gravities, and what you can do about, there are some great responses already. A high starting gravity means that either your efficiency is higher than expected or your volume is too small, and you can dilute with water using an online dilution calculator. A low starting gravity means lower efficiency or your volume is too large. You can add DME to correct this, and a ballpark way of working out how much to add (in grams) is to multiply the amount you want to raise the gravity by, by the volume you have (in liters), and by 2860. So if you wanted to raise 20L of 1.040 wort by 0.01 to 1.050, then the amount of DME you need to add is 0.01 x 20 x 2860 = 572g of DME.

:thumb:
 
Not meaning to be pedantic, but the specific gravity of a liquid is a measure of its density; how much it weighs per unit volume. A brewers hydrometer is calibrated to read 1.000 in pure water at 20C. This is very close to the actual density of water, which is 0.998 kg/L at 20C. What this means is that 1 litre of liquid with a gravity of 1.000 will weight about 1kg, and 1 litre of a liquid with gravity 1.040 will weigh about 1.04kg. Before you ferment you have a mixture of sugars and water, which is denser than water on its own, so you have a high gravity. Fermentation converts some or all of the sugars into alcohol, which lowers the density. For wine and cider all the sugar may be consumed and you end up with a solution that is less dense than water, so you might get a gravity reading of 0.995 for instance. For beer not all the sugars can be fermented, so you usually finish above 1.000, but this depends on factors such as mash temperature (lower temperatures produce more fermentable wort and a lower finishing gravity), what yeast you use (some strains can consume sugars that others can not), and how healthy your yeast is (stressed or nutritionally deficient yeast won't bother fermenting some of the harder to ferment sugars in wort).

How the gravity reading actually relates to alcohol and sugar content is not straightforward. While it is true that all the drop in gravity that happens during fermentation can be attributed to the loss of density due to the removal of sugars and the addition of alcohol, the relationship between gravity and density isn't linear in either alcohol or sugars. For instance, if you took 0.5L of ethanol and mixed it with 0.5L of water then the solution you end up with would be considerably less than 1L (in technical terms, you can say that a solution of alcohol and water has negative excess partial volume). The upshot of this is that you can't say "a drop in gravity of x means an increase in abv of y", although you can get a reasonable ballpark figure using this approximation for beers with starting gravity < 1.060 with the assumption that you gain 1.3% abv for each 0.01 drop in gravity. For higher gravity beers there are more complex formulas available that take into account not just the drop in gravity but also the specific starting and finishing gravities. There is no deep theory behind them, they are just formulas that, if you plug OG and FG in them, then you'll get an abv out of them in the end that is about the same as the result you would get if you sent your beer off to a lab for analysis. They have the additional property that the higher the OG and FG, then the higher the gain in abv you get from any given drop in gravity.

To your next question, how to diagnose what went wrong if you don't hit your target gravities, and what you can do about, there are some great responses already. A high starting gravity means that either your efficiency is higher than expected or your volume is too small, and you can dilute with water using an online dilution calculator. A low starting gravity means lower efficiency or your volume is too large. You can add DME to correct this, and a ballpark way of working out how much to add (in grams) is to multiply the amount you want to raise the gravity by, by the volume you have (in liters), and by 2860. So if you wanted to raise 20L of 1.040 wort by 0.01 to 1.050, then the amount of DME you need to add is 0.01 x 20 x 2860 = 572g of DME.

:thumb:

Wow, thank you. This along with the other comments have done a great job of getting me a firm understanding. This is actually making sense now.

Thanks again guys :) I'll keep you all in the loop as to how my brew goes :)
 
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