Pre boil gravity?

Is it necessary to get a pre boil gravity. And also how does it help with ur ABV?

 

Not necessary.......just helps you calculate your brewhouse efficiency.

 

It is not necessary but it is useful in calculating brewhouse efficiency, which is how much sugar you extracted from the grain. I take this reading instead of before going into the primary solely for sanitation. Anything I measure before the boil will get a 60 minute boil to kill off any nastys that were on my testing equipment.

 

The pre-boil gravity can also be useful in predicting what your OG will be after the boil. I know what my boil off rate is so I can use the pre-boil gravity to see if I need to add any extra water, or if I am good to go. Programs such as BeerSmith have calculators for this.

 

It is only important to measure pre-boil gravity if you mash grains, (i.e. all-grain, or maybe partial mash). Even then it's not "necessary" to measure it. Extract brewers, or extract brewers that steep grains, don't need to worry about measuring their pre-boil gravity; it can be calculated if it is needed for whatever reason.

For mashing, when have completed your mash (conversion of starch in grain to sugar), then your sparge (rinse of the grains), you'll have a specific volume at a specific gravity (SG); that's your pre-boil volume and pre-boil gravity and it tells you how much of the potential sugar you got out of the amount of grain you used. The pre-boil volume/pre-boil gravity together can tell you whether you're on target with making the beer you intended to make (including helping determine what the eventual ABV will be). Sometimes, for a variety of reasons, your pre-boil gravity when you mash can be lower or higher than you expected; these would include how well the grain was crushed, whether complete conversion of starches occurred, how well the sparge rinsed the grain, how much water was used to sparge.

 

I plan on doing a BIAB next weekend and pre-boil gravity will be real important for me. Not sure how efficient my process will be so I can use the pre-boil gravity (and a little bit of math) to determine if I want to add any DME to reach my target gravity.

 

Pre-boil gravity is a good data point but I usually hold off on corrections until I am very close to the end of my boil. This way, I can pull my hops and make any volume/SG corrections that I may need at what is essentially my batch size (not accounting for shrinkage during cooling).

EDIT: The pre-boil numbers clue me in to what adjustments I will most likely need. I use a refractometer so I can take many readings easily.

 

I disagree.
I am a BIAB brewer. Your pre-boil gravity is dependent on how thick you mash.

What goes into the fermentor is the main thing.
If you have too much water, boil off the extra.

The only reason I can see pre-boil gravity being interesting is if you always mash at the same thickness, and want a benchmark as to how you did.

 

I like the way you think.