Technical Forum
Effects of dilution on gravity and specific gravity
Introduction
When a solution is diluted the only thing that changes is the volume. The mass of materials dissolved in the must or wine remains the same.
The mass, volume and concentration are related by the formula:
mass = volume x concentration
As the mass does not change the initial concentration times initial volume equals the final concentration times final volume or:
c_{i} x V_{i}  =  c_{f} x V_{f} 
c_{i} is the intial concentration (g/L)
V_{i} is the initial volume (L)
c_{f} is the final concentration (g/L)
V_{f} is the final volume (L)
It happens that the formula holds if gravity (G) is substituted for concentration. Recall that gravity equals the Specific Gravity  1. If you double the volume you halve the gravity.
G_{i} x V_{i}  =  G_{f} x V_{f} 
G_{f} is the final gravity or SG_{f} 1
So if you have 3 L of must with an SG of 1.080 what happens if you make the volume up to 5 L?
G_{i} x V_{i}  =  G_{f} x V_{f}  
since G_{i}  =  SG_{i} 1  
G_{i}  =  1.080 1 or 80 

80 x 3  =  G_{f} x 5  
G_{f}  =  (80 x 3) / 5  
G_{f}  =  48  
since G_{f}  =  SG_{f} 1  
SG_{f}  =  1.048 
G_{i} x V_{i}  =  G_{f} x V_{f} 
How come the equation with G (dimensionless) instead of c (g/L) applies equally well. Curious to know?. Here is a proof in a round about way but its not for the feint hearted so feel free to ignore it and just use the above formula with confidence.
A Proof
SG_{i}  =  mass 
volume  
Let X be the volume of water added. Now numerically the volume and mass of water is the same since the density of water is 1  
SG_{f}  =  m + X 
V + X  
m + X  =  SG_{f} (V + X) 
SG_{i}V + X  =  SG_{f}V + SG_{f}X 
(SG_{i}  SG_{f})V  =  (SG_{f} 1)X 
X  =  (SG_{i}  SG_{f})V_{i} 
SG_{f}  1  
Now we can arrive at the same equation starting with:  
G_{i} x V_{i}  =  G_{f} x V_{f} 
V_{f}  V_{i}  =  G_{i}V_{i}/G_{f}  V_{i} 
G_{f}(V_{f}  V_{i})  =  G_{i}V_{i}  G_{f}V_{i} 
G_{f}(V_{f}V_{i})  =  V_{i}(G_{i}  G_{f}) 
(SG_{f}  1)(V_{f}  V_{i})  =  V_{i}((SG_{i}  1)  (SG_{2}  1)) 
V_{2}  V_{i}  =  X 
X  =  (SG_{i}  SG_{f})V_{i} 
SG_{f}  1 