Adjusting Specific Gravity
Stop! Before pitching your yeast into your fermenter check that OG, original gravity, is what it should be,eg. 1040 to ensure a satisfactory ferment.
Oops, it reads 1050. Easily solved by diluting with cooled boiled water.
How much water?
Calculate with GU, gravity units, which is just the last two units of SG,ie 1040= 40GU
however our 20 litre batch is 20 x 50 or 1000 Gus
divide by 40 = 25 litres so an extra 5 litres dilution required.
You can also do this at the end of the ferment if the FG Final Gravity stubbornly sticks above your target. Usually because you started with a high OG. Some commercial breweries deliberately do this for transport purposes . Its called High Gravity Brewing.
On the other hand if you come up short, say 1035, you need to add more fermentables(sugars) as follows
GUs we have are 20L x 35 =700, ie. 800GU @sg40 less 700 GU @ 35 leave 100 points.
You divide the total GU points by the value of the malt you are using, in my case malt extract.
However bear in mind we are talking in metrics which are different (yes, really) to pounds and gallons which is what American authors use,eg.John Palmer
http://howtobrew.com/book/section-2/what-is-malted-grain/table-of-typical-malt-yields
on PPG Pounds per gallon.
With metrics we use LDK(Litre Degrees per Kilo) aka HBU (Home Brew Units) or HWE (Hot Water Equivalent). It has a conversion factor of 8.3454.
So you must multiply the ppg number,viz 36 for Malt Extract by 8.3454, which makes the LDK 300 ie Net GU is divided by LDK or100/300 = .333kg of malt extract required to bring GUs to correct SG for 20 Litres
Compare this with US pounds and Gallons (20litres=)5.28 US gallon x SG 40 = 211.2 GU less 5.28 at SG 35 =184.80.
So you are short by 26.4 GU?? divide by 36ppg = .7333 pounds malt extract, converted to kgs is 333kg, the same as above(whew!).
If you find that your FG is below your target and the beer is too thin use the same calculation to add lactose or malto dextrine but boil first
If that seems too much go to
cheers