pre-boil gravity?

i hear some people talk about a pre-boil gravity. would it be the same as after the boil and the wort is cooled? my pre-boil temp will be alot higher obviously. i know there is a conversion factor based on temp, but wasn't sure if it was comparable.
it would be nice to measure it "pre-boil", so i can add a little dry extract if i'm under my og.

NO.
When you boil, you cook off some water (Steam) and thusly concentrate the sugars in the wort. Your Pre-Boil Gravity will always (unless you add back some water) be lower than your post boil (OG) gravity.

it would be nice to measure it "pre-boil", so i can add a little dry extract if i'm under my og.

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steal a sample of your wort 10 minutes before the end of the boil....cool it in a plastic cup and ice water...then take your gravity reading....you still have time to add some extract, and there won't be a significant change in 10 minutes.

Nchomebrew
If you take a reeding before boiling it will give you a good idea as to where you are starting. . I like to take a small sip of it at this time all so.
Some times if the reeding is too low i will stop sparging early.

aahh. ok, so is there some kind of conversion chart somewhere? say i have 6 gallons of pre-boil at a specific gravity, what would it be after it boils down, to 5 gallons. stopping the sparge early is a good idea too, but i'd like to make a full 5 gallons.

It's simple math... like this:

6.5 gallons of preboil wort that you measure at 1.043 has only so much sugar or total gravity. It's figured as 6.5 x 43 = 280 points.

If you think you'll boil off about 1.5 gallons in a 60 minute boil (typical) leaving 5 gallons post boil, then you just take that total number and divide by 5. 280/5 = 56 or 1.056.

Bobby_M said:

It's simple math... like this:

6.5 gallons of preboil wort that you measure at 1.043 has only so much sugar or total gravity. It's figured as 6.5 x 43 = 280 points.

If you think you'll boil off about 1.5 gallons in a 60 minute boil (typical) leaving 5 gallons post boil, then you just take that total number and divide by 5. 280/5 = 56 or 1.056.

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In theory that should work but I've never been able to predict post boil gravity with any accuracy at all.

sweet, thanks bobby m. that'll help me. especially until i get my efficiency up.

if i do the pre-boil gravity, then i should still adjust for the temperature?
i believe it's .2 of a point per degree?

Take your sample and throw it in the freezer to cool it to 65...

are preboil and specific gravity the same thing?

No. Specific gravity is the density of any sample relative to that of water at some calibration temperature (60F for most hydrometers). Preboil gravity is the specific gravity at the start of the boil. OG is the specific gravity at the start of fermentation. FG is the specific gravity at the end of fermentation.

oberon567 said:

Take your sample and throw it in the freezer to cool it to 65...

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This. But I also take the time to put a piece of cling wrap over my sample to prevent ANY evaporation from occurring.
Especially during the winter when evap occurs more readily.
I have individual pipettes that I use for this purpose. No loss and it cools to ambient temps within about 5 minutes.

Thank you for clearing that up