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10-18-2013, 10:37 PM   #1
goudaphunk
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 Adding DME is Target Gravity is off

Hi,

I have a few questions that I am hoping the wonderful folks on here can help me with. My first question is:

When brewing an AG batch, when should I take my first gravity reading? I just bought a refractometer. When I take my first gravity reading, is there a specific formula I can use? The reason why I am asking is because tomorrow I am brewing an AG recipe from Northern Brewer, "Dead Ringer IPA." The recipe only gives an OG number. I am familiar with how to find the FG, but still slightly confused about the first gravity reading, that's not the OG.

Additionally, if I miss my target OG, how much DME should I add to make up for that mistake? Is there a formula for that?

Thanks in advance for your help. I am learning as I go and this forum has always been helpful.

Cheers,
Jimmy

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10-18-2013, 11:12 PM   #2
feinbera
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The "official" OG is the gravity of the wort right before you pitch your yeast.

However, if you know about how much water you're going to boil off, you can calculate your OG based on your pre-boil gravity. Take the decimal part of the pre-boil reading and multiply it by the pre-boil volume to get your total "points"; then divide the "points" by your anticipated post-boil volume.

For example, if you go into the kettle with 6 gallons at 1.050, and know from previous batches that you boil off a gallon in an hour, for an anticipated post-volume of five gallons....

Quote:
 Decimal part of 1.050 reading = 50 50 * 6 gallons pre-boil = 360 "points" 360 "points" / 5 gallons post-boil = 72 points per gallon 72 decimal part = 1.072 predicted post-boil gravity

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10-18-2013, 11:16 PM   #3
Fishin-Jay
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Personally the first gravity readings I take are of the wort coming out of the mash tun during the fly sparge so i can ensure the gravity never goes below 1.010 during the sparge so that I'm not extracting off-flavors from the grain.

The next reading I take is of my full pre-boil volume, which for my equipment needs to be 7.75 gallons for a 5 gallon batch, assuming I want 5.5 gallons in my kettle at the end of a 90 minute boil (personal preference, I boil for 90 min for nearly every recipe). Here's how you figure out what your gravity should be pre-boil:

beginning gravity*beginning volume=end gravity*end volume
(When inputting gravity use just the numbers after the 1. For example 1.045 is 45)

For the recipe you linked to, the OG is 1.064. So, based on my system I want the pre-boil gravity to be 1.045

x*7.75=5.5*64
7.75x=352
x=45.4

As for using DME to correct your gravity, this is best done pre-boil. A rule of thumb is that 1lb of DME in 1 gallon of water will yield 40 points. Just figure out how many point shy of your target you are, calculate how much DME you would need to add per gallon to add those points, and multiply by the number of gallons in your kettle.

So, for example, let's say I really screwed up my fly sparge when making the recipe you linked and pre-boil my gravity was only 1.035. I need 10 more points to reach the proper pre-boil gravity. Knowing that 1lb of DME per gallon will add 40 points tells me that 1/4lb per gallon adds 10 points per gallon. So: 1/4lb*7.75 gallons=1.9lbs of DME.

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10-18-2013, 11:49 PM   #4
goudaphunk
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Hi @Fishin-Jay,
Ok, so I will do a boil of 60 minutes. I know, based on past brews, that I will burn off about 1 gallon (give or take a little). My pre-boil volume will be 6.5 gallons. What would be my formula for that? Seriously, thanks for your help.

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10-18-2013, 11:53 PM   #5
Fishin-Jay
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Here you go:

x*6.5=64*5.5
6.5x=352
x=54
Preboil volume at 6.5 gallons should be 1.054

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10-19-2013, 12:00 AM   #6
goudaphunk
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Thanks! This is starting to make total sense. Is this the "go-to" formula to use each time, changing of course depending on starting volume, OG of recipe, etc?

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10-19-2013, 02:06 AM   #7
ballsy
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I'm gonna piggy back here. ...so this formula is usable for BIAB doing partial boils as well?? I.e. the volumes will change but otherwise a good indication of whether SG needs adjustment prior to boil?

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10-19-2013, 06:16 PM   #8
Fishin-Jay
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Quote:
 Originally Posted by goudaphunk Thanks! This is starting to make total sense. Is this the "go-to" formula to use each time, changing of course depending on starting volume, OG of recipe, etc?

Yes, if you are having trouble with your targets after sparging this is the go-to formula.

Quote:
 I'm gonna piggy back here. ...so this formula is usable for BIAB doing partial boils as well?? I.e. the volumes will change but otherwise a good indication of whether SG needs adjustment prior to boil?
The method you use shouldn't matter. You can use these formulas to make your adjustments pre-boil no matter what method you use to get your wort.

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