Change In Sg Before And After Boil

[jadk42]

Love my new refracomometer. Thanks to the guys who helped a few weeks ago with advice. Can someone tell me what happens to gravity in the boil. Allowing for topping up to original volume after evaporation - does it change?
David

 

[Adamt]

During the boil, effectively only water is boiled off - this increases your specific gravity (the sugars are becoming more concentrated). If you add water after the boil you will reduce the specific gravity (you are diluting).

The temperature also affects specific gravity as water does expand at higher temperatures, though this won't be reflected (pun?) when using a refractometer.