Author Topic: Parti-gyle OG calculation  (Read 9078 times)

Offline Foodhead

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Parti-gyle OG calculation
« on: November 27, 2012, 04:49:46 AM »
Hi!

I've been reading this article on the BeerSmith blog: http://beersmith.com/blog/2011/10/07/parti-gyle-brewing-two-beers-from-one-mash/

Great article, Brad! I can't make sense of the equation, though...

It says: "For a 50-50 split by volume, with half of the wort in each batch we get a roughly 58% of the gravity points in the first batch.  So a 1.060 overall batch OG would translate to a 1.070 first runnings and 1.050 second runnings, with both of equal size."

With these values inserted in the equation (Number_points_ runnings = (Tot_points * Points_fraction / fractional_volume), I get (60 * 0.5 / 0.5) for both the first and the second runnings, i.e. 60.

Or have I misunderstood the concept of "points_fraction"?

Thanks in advance!

/kalle.

Offline durrettd

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Re: Parti-gyle OG calculation
« Reply #1 on: November 28, 2012, 12:10:44 PM »
kalle,

Here's how I do the arithmetic:

6 gal of wort in kettle at the end of boil with a specific gravity of 1.050
1.050 --> 50 points
5.5 gal delivered from kettle to fermenter
50 points * 6 gal = 300 total points
points delivered to fermenter = 5.5/6 * 300 total points = 275 points available
1/2 of wort = 2.75 gal
2/3 of recovered points = 2/3 * 275 points available = 183.3 points in first runnings
1st running points/gal = 183.3/2.75 = 66.7 --> 1.067

1/3 of recovered points = 1/3 * 275 = 91.67 points in 2nd runnings
2nd running points/gal = 91.67/2.75 = 33.3 --> 1.033

Hope that helps.
also, hope my arithmetic is correct. If I've screwed up, please let me know!

Dan


Offline Foodhead

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Re: Parti-gyle OG calculation
« Reply #2 on: November 28, 2012, 04:19:18 PM »
Thanks!

...but I'm afraid I still don't quite get it. This just assumes that 2/3 of the sugar is in the first half of the wort, right? I won't argue with that, but say I'd like to make a smaller batch of the smaller beer, splitting the wort 2/3 - 1/3 or even 3/4 - 1/4. How would that be calculated?

I'm aware that this isn't an exact science, but I always like to at least know the theory behind it all. That is, before I let Beersmith do the math for me. ;)

Best,

/kalle.

Offline durrettd

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Re: Parti-gyle OG calculation
« Reply #3 on: November 29, 2012, 11:57:09 AM »
"Assume" is an important part of the discussion, as is "approximate". Assumptions and approximations are an important part of the planning, but they are replaced by measurement during the execution.

Those with more experience can probably provide approximations for a 2/3 - 1/3 split, but until you get someone to provide those assumptions, try this:

As you run off your wort, stir it gently to ensure it's a homogenious mixture and check your specific gravity frequently. When you get the specific gravity you want for your first runnings, quit. You may or may not get the projected volume, but the gravity will be right for the beer you intend to brew. Repeat the process for the second runnings, again stopping based on specific gravity, not volume. After a few brews you can build a curve of fraction of run-off versus collected gravity. If you do this, please post it so the rest of us can benefit from your experience.

Offline Foodhead

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Re: Parti-gyle OG calculation
« Reply #4 on: November 29, 2012, 12:05:17 PM »
Thanks again!

And I guess this is the way I'll be going about it after all. And I hope you didn't take my questioning as disrespectful, I just simply got the impression from the quoted article that the equation should actually work (albeit of course approximately) for different kinds of splits.

Our planned first parti-gyle will be a larger batch of pilsner, with a smaller batch of some kind of session ale from the second runnings and some specialty grains. The OG of the smaller beer isn't all that important, as it's mostly experimental, but estimates are always both helpful and enlightening. :)

Best,

/kalle.

 

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