total gravity does not change during boil

[beer today]

if total gravity = specific gravity x volume, why does total gravity not change as a result of evaporation.

[BeerEagle]

Edit: I'm a plonker.

[bigdave]

Im not really sure what total gravity is but as the volume reduces due to evaporation the specific gravity increases. So perhaps one offsets the other.

[beer today]

I am reading a piece about calculating the malt bill, and gravity calculations- Ray Daniels.
Also states Tot Gravity doesnt change as you boil or dilute your wort. The only way to change it is to add fermentable materials.

[barney]

The total number of gravity points cannot change in the wort. Evaporation will increase the potential strength and dilution will reduce it.

The total number of gravity "points" however is fixed.

This phenomenon. is very handy when calculating parti-gyle brew strengths.

[Martin G]

The total gravity does not change, so you have a direct relationship between specific gravity and volume as the water evaporates that is described by the formula.

[beer today]

simply as the volume decreases the specific gravity increases. ie boiling.

as the volume increases the specific gravity decreases, ie diluting.

it's linear hence the TG remains constant ?

[killer]

Pretty much.

Imagine doing a mash with about a kilo of grain.
Total gravity points correspond to the amount of sugars you get out.
So lets say you extract 300 total gravity points and end up with 5 Litres of wort.
You now have wort with a specific gravity of 1060 (300 points in 5 Litres so 300/5 = 60 points to give a SG of 1060)

If you boil off 1 Litre you now have 4 Litres but you still have 300 gravity points bacause you just removed the water
You now have a wort with a specific gravity of 1075 (300 points in 4 Litres so 300/4 = 75 points to give 1075)

[beer today]

thanks killer, thats what I was after.