Originally Posted by Hedley
I have just started using the hydrometer and think I understand it.
However, in another thread someone mentioned that I should always top up my demijohn after racking so that it is nearly full (to stop oxidisation). That is all well and good, but I am now confused how this will affect the hydrometer reading.
I usually end up with about four and a half bottle's worth of wine at the end of the process. So I lose a bottle and a half through racking.
Is there a bit of maths I could do to adjust the SG reading depending on how much I have to top up with water? For example, say after the first racking I have lost half a bottle's worth. I add same amount of water to top up. Now I take an SG reading. Can I apply some formula to that to get an accurate reading based on the volume of wine I have effectively replaced with water?
Any help most appreciated.
So, I had to delve into my chemistry roots to get this formula, but so long as you use the same unit of measure throughout the whole thing, it works like a charm!
To start off, you need to measure 4 things:
1. The Specific Gravity of what you already have in your fermentor, we'll call it G1
2. The Volume of what you already have in your fermentor, V1
3. The Specific Gravity of what you are topping off with, G2
4. The Volume of what you are topping off with, V2
What ever you measure your volume with (gallons, liters, quarts, etc.) needs to be the same for both your V1 and V2 for this to work. You can't say your V1 is 3 gallons and your V2 is 1 quart. You would instead use 3 gallons and 0.25 gallons. Same goes for metric volume.
So here is the magic formula, keeping in mind that * is multiply and / is divide and anything in () you do first before going on to the next portion!
Final Gravity= ((G1*V1)+(G2*V2))/(V1+V2)
Might be confusing
but let's try an example
before you give up!
Say you have a must that, after you rack, is only 4
gallons. It's in a 5 gallon carboy, so naturally you need to top up with 1
more gallon. The S.G of your 4 gallon must is 1.07
, and you're topping up with 1 gallon of 1.02
must from a previous batch.
Your measurements would then be:
and V1= 4
and V2= 1
Now plug it all in!
Then we start simplifying: Multiply 1.07 and 4, then 1.02 and 1
Do it again: 4.28 plus 1.02, then 4 plus 1
Finish it off by dividing 5.30 by 5 and....
Your Final Gravity= 1.06!
I love this formula because even if you don't use water to top up, it still works!
I like making cysers (love child of mead and cider) so I always top up with more apple juice, which does not have the same specific gravity as water (which is 1.000).
If anybody needs anymore clarification, or just wants help with their particular batch they are topping up, feel free to drop me a message!