OG complication

[medievophile]

I am following the advice of someone regarding (relatively) high gravity extract beers who said to cook up 3 gallons with at least 3/5 of the malt, then to wait 2-3 days and add the additional 2 gallons with the rest of the malt (boiled for just 10 mins).

Regardless of whether or not others agree with this technique, how would I go about calculating the OG of the beer? The OG will obviously be different for the first 3 gallons and the last 2 gallons. Is there a way to calculate the combined OG? Testing the gravity after adding the last 2 gallons seems like it might be inaccurate since the yeast have already been going to town on some of the malt of the first 3 gallons.

 

[maffew222]

It's all relative in this case. But to calculate the OG, all you need to take in to consideration is the total volume of water and the total extract added.

And possibly yes, your original 3/5 added is the same as your last 2/5 if added to the same ratio of water.

How much extract and water were used initially? How much later?

 

[JR_Brewer]

I'm not an expert, but I think I understand the basics of gravity-math. Here it goes...

We normally express gravity as 1.000 with 3 decimal places. Starting with that, get rid of the "1" and keep only the 3 digits to the right of the decimal place. Then multiply by the number of gallons to get something we call "gravity points."

So, let's say your 3 gallons had an OG of 1.080. Take 80 (from the 1.080) and multiply by 3 (# of gallons) to get 240.

Do the same with the second batch, let's assume 2 gallons at 1.090. 90 x 2 = 180.

Now add the 2 numbers, and then divide by the TOTAL volume. In this example, 240 points + 180 points = 420 points. 420 / 5 gallons = 84. That means the theoretical OG had the beer been brewed at one time would be 1.084.

Make sense?

 

[Calder]

With exteact you can just calculate it.

Another way could be to take both an hydrometer reading and a refractometer reading once fermentation is complete and back calculate it.

Seems to me to be a bad practice. The yeast will only grow to sufficient amount for the initial 3/5 ths. You will not get any more growth when you add the rest. Result = stressed yeast.

 

[WoodlandBrew]

That's a smart way to brew high gravity beer. It's similar to the way a brewery would do a Single Step Propagation in the fermentation vessel. Greg Doss of Wyeast did a presentation on this.
here are the slides from the presentation:
http://www.mbaa.com/meetings/distri...S, Yeast Propogation A Practical Apporach.pdf

I would do it a little different, but with the same idea. Using dry extract instead of a mash means you can skip the boil. Use all of the water and enough extract to yield the cell count for the remainder of the extract (Typical yield is 1 Billion cells per gram of extract) After a few days add the remainder of the extract.

The first few posts do have the correct way to calculate the OG.