Author Topic: Gravity error after top off  (Read 328 times)

Offline louruscetta

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Gravity error after top off
« on: July 17, 2022, 08:39:15 PM »
Hi all--
We did a BIAB brew today and I am confused with the OG I got and the OG BeerSmith told me to expect. I would like some thoughts on this scenario.

We did a 10 gal batch for a wheat beer. We had 18.81 lbs of grain and the software told us we needed 13.97 gal of total volume. But since the mash tun volume needed was more than what we can hols, we decided to mash with 10 gal of water and top up the kettle with the remaining ~4 gal (all of this is reflected in the water volume tab.

Expected post mash gravity was 1.042 and we came close with 1.041. We then added the 4 gal to top up and reach pre boil volume.

Here is the issue. BeerSmith has an expected gravity of 1.05 after the boil. BUT--we added 4 gal of water after the mash; lost 2 gal of water to boil-off and trub. So I finish the boil with 2 gal more than what I finished the mash with--my SG should not go up, it should decrease right?? By the way, it did. I had an SG of 1.034 compared to an expected SG of 1.05. Can someone help me? I have faith in the software, but cant find where I might be missing the boat here.


Offline Oginme

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Re: Gravity error after top off
« Reply #1 on: July 18, 2022, 06:04:52 PM »
OK, you set up the recipe with a given set of assumptions regarding the process.  This was reflected in your equipment profile. 

Then you changed you process.  The software has no clue that you changed anything, as it is just a set of equations.

From your description, it assumes that the full amount of water used touches the grains and is used to extract sugars from them.  When you bypass this and add straight water to the kettle, you are bypassing the process you described in your equipment profile.  The effect is diluting the wort and not adding to the sugar extracted.

Trying to make sense of this given the above changes would really not give any insight into your future brews.  Since you are doing BIAB you have options and need to modify your equipment and mash profiles according to how you want to do your next brew.
In short, you can do any of the following:

A) you can repeat what you did and add water to dilute the wort.  You can move your water addition to the section labeled "Top off Water to Kettle".  Enter in the number you actually acheived from your first brew into the recipe and on the session tab under the Brewhouse Efficiency section BeerSmith will give you a calculation of your actual brewhouse efficiency which you can use to update that number in your equipment profile.  This tells the software how much of the sugar in the grains will make it to the fermenter.

B) you can do a pour through sparge on your grains with the top off water.  You will need to set up your mash profile to be a standard profile and not a BIAB profile as the software assumes you use all your water in the mash when the BIAB box is selected.  When you are done mashing, lift the bag out of the water and suspend it above the kettle or (easier) use a colander to hold the bag over the kettle.  Now pour the water overthe bag and allow the draining from the bag to flow back into the kettle.  Doing this will create less dilution of the wort because the water draining through the grains will extract more sugars.  You would probably be best served by keeping the original brewhouse efficiency from your first attempt and then adjusting it after the brew based upon your actual measurement and the calculated actual brewhouse efficiency.  Personally, I have found the 'pour through' technique to be more variable and highly dependent upon how fast your pour.  Note that you do not need heated water to do this, cold water works as well.  Heated water will drain better (slightly) and will not reduce the temperature of the wort, allowing you to reach boil faster.

C) Similar to B) above, but using another kettle or stock pot, put the water used for rinsing the grains in the stockpot/kettle and the when you raise the bag out of the mash, put the bag (after allowing much of the wort to drain out) into the stock pot with the water.  Open the bag and stir to allow the water to reach all the grain for better extraction.  Then pull the bag, I recommend at least 5 minutes of stirring with the rinse water and when I used this technique often did 15 minutes for greater consistency.  Then add the wort from the stockpot into the kettle and proceed to boil.  Again, your efficiency will probably be closer to your original estimated number but feed back to the program your actual volumes and gravity measurement to get the brewhouse efficiency calculation to do the heavy math lifting for you.  Likewise this this, you do not need to heat the water, but it does help with drainage and just a bit more extraction.

Feel free to ask any other question.  THere is an option D) -- Get a bigger kettle, but I am guessing that it is not in the near future for you.

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Offline louruscetta

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Re: Gravity error after top off
« Reply #2 on: July 18, 2022, 06:26:27 PM »
Thanks for this.

I actually did put all of this in the original design in the software. So under the water volume tab, I added 4 gal to the "top off water to kettle. As I did that, the "total mash water water" decreased by the 4 gal (as it should have).

After going through this last night, I feel I followed all the parameters as I entered it into the s/w.  In doing so, I would expect that the "estimated post mash gravity" would reflect the gravity with the lower mash volume amount, thus there would be a different expected gravity when comparing "post mash gravity" and "pre-boil gravity" given that I added 4 gal of water between the steps.

Here are pics of the water vol tab and the session tab showing that i input the set conditions, but there is no difference in expected gravity values after the mash and pre-boil.

I realize we did not come close to the efficiency expected--I think we may not have crushed the wheat enough. But still just trying to understand if the software would calculate difference in post mash gravity and pre boil gravity if the volumes are different and if it knows it.