Taking a hydrometer reading

When I read the guides, they instruct to take a hydrometer reading before you start your boil so you know if you need to make any adjustments. Since your boil volume will be significantly higher than your volume after boil wouldn't it be an inaccurate reading? Or is the SG expressed in recipes not based on what actually goes into the fermenter, but a measurement that's based on a larger volume?


Thanks

You are correct in that the measurement will be lower than what you will get after the boil.

I guess you could do that and just dilute the recipe. ie- say you are going to boil 6 gallons down to 5, then go to beersmith (or whatever you are using) and change the batch size from 5 to 6 gallons.

I say rdwhahb, if you are doing extract then its pretty hard the mess up a SG.

You're correct that the pre-boil volume will be different from the post boil volume. Recipes suggested final gravities are based on post boil volume. Once you take a pre-boil volume their are online calculators you can use to figure out what your post boil volume will be then if its too low you can add some extract to the boil to get the final gravity up to the goal gravity.
One problem with these calculators is you need to know your boil off rate to determine post boil gravity based on pre boil gravity.

You are somewhat correct; pre-boil SG will be different than post-boil SG, but it will tell you what your post-boil will be. Remember, SG is a measurement of the amount of sugar in the water; if you boil the water, you are proportionally going to concentrate the sugars in the remaining.

Here is how you do the calcualtion:

take your hydrometer reading. Let is assume it is 1.060 and you have 6 gallons, and you expect to get down to 5 gallons after the boil.

For calculations, the SG of 1.060 will be using the value of 60. 1.036 would be 36, etc. Get the pattern?

The equations is as follows:

[Preboil SG] * [Preboil volume] / [Postboil volume] = [Postboil SG]

So, in the example above we would have 60*6/5 = 72, giving a post-boil SG of 1.072.


So, if you are aiming for a specific SG after the boil you can back-calculate what your pre-boil SG should be. Not so tough now, is it?

Here's a couple hydrometer questions after brewing my first batch.

Is it OK to get the post boil SG after pitching the yeast?

And, is it OK to get the post fermentation SG after adding the priming sugar?

Do measurment before yeast pitching to avoid losing yeast in the sample. I have forgotten and measured after, no issues, but not ideal.

Do not measure after adding priming sugar, as it will be higher than what the beer was due to the extra sugar. Measure before you add it.

Sorry, on ipad, not typing elloquently.

Tall_Yotie said:

Do measurment before yeast pitching...
Do not measure after adding priming sugar...

Click to expand...

Thanks. Forgetfulness was my problem too. Lots to think about especially on a first brew.

Yeah, thanks for the great info guys!

You all are welcome! Remember, the hydrometer is your friend!

Take a preboil SG.
Get your post boil OG.
After a few days of the brew fermenting take a SG. Learn how the trend goes for fermentation. Taste the sample. Repeat every couple days. Learn how the beer matures so you can taste samples of future brews to get an idea of how it is going to turn out. I lose about a bottle's worth to sampling, but it is well worth it.
Get your FG before priming it.

Rinse. Repeat. Enjoy.