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-   -   pre vs post boil gravity? (http://www.homebrewtalk.com/f36/pre-vs-post-boil-gravity-382014/)

rockytop714 01-16-2013 07:24 PM

pre vs post boil gravity?

I brewed my first all grain batch, last weekend. Making a 3 gallon batch, my pre-boil volume came out to around 4-4.5 gallons and had a gravity of around 1030-1035. The boil brought the volume down to right around 3.5 gallons but, like a D.A., I forget to take a gravity reading before transferring to the carboy and pitching the yeast.

is there any kind of formula I can use to get an idea of what the OG might've been, using the pre-boil volume/gravity compared to the post-boil volume?

I was shooting for around 1060. Do you think I'm at least close?

thanks :mug:

seabass07 01-16-2013 07:34 PM

Are you using a recipe formulation program like beersmith? The easiest way is to punch in your pre-boil volume and mess with efficiency numbers to make it hit your preboil gravity. Then change the volume to post boil. But if your preboil was 4.5gal at 1.035 and you boiled it down to 3.5, your post boil gravity was around 1.044.

Conan 01-16-2013 07:46 PM

BeerSmith has a boiloff function. Below are several scenarios based upon your data. All values assume a 60 minute boil.
(Starting Vol) (Evap Rate %) (Final Vol) (SG) (OG, projected)
4.5 19 3.5 1.035 1.045
4.0 9 3.5 1.035 1.040
4.0 9 3.49 1.030 1.034
4.5 19 3.5 1.033 1.042
4.5 9 3.49 1.033 1.038
4.5 39 2.64 1.035 1.060
In short, no- you probably didn't hit 1.060. The bottom line is a model of your required evaporation rate (39%) to achieve a 1.060 after a 60 minute boil. As a side note, can you be more specific with your volumes and gravities? There's quite a difference in a half gallon and .005 points of gravity, more so when compounded. Kyle

jflongo 01-16-2013 07:53 PM

Here is mine from yesterday.

PB Gravity 1043 AB Gravity 1058

So it basically went up by 15. However, I did a little over 6 gallons down to a little over 5 gallons. To get a 25 - 30 jump, you would definitely have to boil it longer.

billl 01-16-2013 10:48 PM

The basic formula is preboil gravity points x preboil volume / postboil volume = postboil gravity points.

So 35 x 4.5/3.5 = 45 or 1.045

Looks like you were a bit lower than intended.

kaz4121 01-17-2013 12:45 AM

Bill has it right, total gravity points are constant in the kettle, so just use proportions to find what you post boil gravity is using his equation above.

rockytop714 01-17-2013 02:28 AM

thanks everyone. I wasn't using any kind of software... that's why I'm asking questions, on here. haha...

Bill, that's basically what I'd expected though, for whatever reason, I just didn't think it could be that simple. haha... I assumed that I'd fallen short of the anticipated OG. Just because the volume was a lot higher than I'd originally intended.

Conan, I'd like to be more specific but I need to fess up to a few things: first and foremost, this was my first stab at all grain and I did a BIAB method. I felt like it was too much water to start with, but that's what was called for in the recipe I was following. I'll know, next time, to start with less water. Secondly, this is also the first time I've used the hydrometer that came with my kit. Until now, I've brewed extract kits. so checking the OG didn't seem as important. So I didn't know exactly what I was doing with that. I gave the range on the OG that I gave because it's as close as I was able to tell. It was above 1030 but not quite 1035.

I will say that, even though I didn't hit 1060, I'm still pretty pleased that I got what I did, from a first try and, if it tastes good, when it's done, then that's all that really matters, to me.

Thanks for all the help, folks!

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