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> Hitting Target Gravity, contributed by Danno6102
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post Mar 1 2005, 09:42 PM
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Hitting your Target Gravity
contributed by Danno6102

By taking a simple hydrometer reading & doing a couple simple calculations, you can come very close to your target gravity every time you brew. I will quickly outline the concept of Gravity Units, or GUs, as described in Ray Daniel’s book “Designing Great Beers”. I will then go into further detail on how to hit your target gravity in both Extract & All Grain brewing.

Q: What is a GU?
A: GU is an abbreviation for total Gravity Units. This represents the total amount of extract that is added to your brew from either mashing grain, adding extract or adding any other form of fermentable.

Q: How do I know what my GU is for my recipe?
A: Calculating GUs is simple. You take your target original gravity (OG) & multiply it by your post boil volume of wort. Example, say my recipe has a target OG of 1.054 & I’ll have 5.5 gallons of wort post boil. I multiply my desired OG (54) times my desired final volume (5.5) which equals 297 GU. In other words:
GU = Target OG x Target Final Volume

Extract Brewing
Q: How do I hit my target gravity in an extract brew?
A: The first step to hitting your target gravity in both extract & all grain brewing is to calibrate the volumes on all of your equipment. 5 pounds of extract will yield a much lower OG in 6 gallons of wort than it would in 5 gallons. I like to use your typical graduated kitchen measuring cup & a small pitcher. Also, find an object you can mark on to use as a dipstick for measuring the volume in your kettle. Add water to your brew pot in 1-quart increments & indicate the level on your dipstick with a permanent marker. For your fermenters you can just mark the sides of your bucket or carboy with the permanent marker.

Extract is one of the easiest ways of brewing to hit your target gravity. All extract, either liquid or dry, add a somewhat constant Potential Gravity to your brew. For liquid extract the average Potential Gravity is 1.036 & for dry extract, the Potential Gravity is approximately 1.044. This makes hitting your target gravity extremely simple as long as your fermenter is calibrated properly & you know all the variables, such as boil off rate, if doing a full boil.

Q: For partial boils, what should I do?
A: The recipe you have says that you will have 5.5 gallons in the fermenter at an OG of 1.045. Using the calculation from above, we can see that your GU for this brew will be 248. This recipe calls for the use of Dry Malt Extract. We know that DME yields a potential gravity of 1.044. So to determine how much DME we need to add, we just divide our GU (248) by the PG of the extract (44) & we can see we need to add 5.6 pounds of DME to hit our gravity of 1.045:
248 / 44 = 5.6 pounds

You simply add the 5.6 pounds of extract to your partial boil, cool, add to the fermenter & top up to the 5.5 gallon mark you previously calibrated. An important part of measuring your final gravity with partial boils is to insure you have the final volume of wort & top up water mixed well when you take the gravity reading. If it is not mixed well, the water & wort will stratify, leaving lighter wort at the top of the fermenter & thicker wort at the bottom, throwing your gravity reading way off. Seeing as how you need to aerate well before adding the yeast, you can simply shake the bejeesus out of the fermenter or stir it well with a sanitized spoon to mix, then take your gravity reading & add your yeast. You should easily be within .002 of your desired OG.

Q: Is it different for full boils?
A: Full boils throw another variable into the mix when trying to hit your target OG: boil off. This is the amount of wort that evaporates as you boil. For most 60 minute extract boils, a good number to start with is 1 gallon. So you need to add an extra gallon to your desired final volume. You will need to adjust this boil off amount for your system over the course of a couple brew sessions to accurately hit your desired final volume. Other than figuring out your boil off rate, the procedure is the same as for partial boil.

All Grain & Partial-Mash Brewing
Q: How does this work in all-grain brewing?
A: Hitting your target gravity doing an AG batch is just as easy as with extract, it just requires a pre-boil hydrometer reading & a couple more equations. Let say your recipe gives you 1.071 wort with 6 gallons in the fermenter at 75% efficiency. Your total GU for this recipe will be 426 GU. As long as you do not add any more fermentables to your post-mash wort, it should equal this GU number of 426, no matter what the volume. Let say you need 8 gallons pre-boil volume to compensate for boil off to hit 6 gallons post boil volume. If you divide the GU (426) by the 8 gallons, this will give you the desired pre-boil gravity of 53 or 1.053:
Preboil Gravity = GU target / Preboil volume
So: 426 / 8 = 53 or 1.053

Q: My pre-boil gravity is a lot LOWER than what I need. How do I fix it?
A: Let’s say for this batch your efficiency suffered for some reason & your 8 gallons of pre-boil wort only measured 1.048. That’s a GU of only 384, 42 points shy of where we wanted to be. If you were to proceed, you would end up with 6 gallons of only 1.064 wort. Quite a few points below what you were shooting for.

So how do you correct this? Simple! You just add some dry or liquid extract! We know that 1 pound of dry extract will yield 1.044 points of potential gravity & liquid will yield 1.036. So all we need to do is divide the points we are missing by the Potential Gravity of the extract we are going to use:
Amount of extract = (GUtarget – GUmash) / Extract PG
For dry extract: Amount = (426 – 384) / 44 = .95 pounds of dry extract
For liquid extract: Amount = (426 – 384) / 36 = 1.17 pounds of liquid extract

Add this additional extract with 10 to 15 minutes left in the boil & you should easily be within .002 of your desired OG.

Another option is to boil your wort longer than normal to yield a more condensed wort. You can figure the post boil volume needed that will give you the 1.071 OG you desire by dividing the GU mash by the target OG:
Volume Post Boil = GU mash / OG target
So: 384 / 71 = 5.4 gallons

So if you boil off 2 gallons in your normal 90 minute boil, you would need to boil for approximately an additional 30 minutes to reach 5.4 gallons. I show the equation below as to how to determine the total boil time needed.

Calculating boil off from previous batches:
Boil off = (Volume Pre Boil – Volume Post Boil) / Boil length in hours
So: (8 – 6) / 1.5 = 1.33 gallons per hour

Calculating Boil length to hit target gravity for this batch:
Boil length = (Volume Pre Boil – Desired Volume Post Boil) / 1.33 from above
So: (8 – 5.4) / 1.33 = 1.95 hours or 117 minutes

Q: My pre-boil gravity is a lot HIGHER than what I need. How do I fix this?
A: Let’s look at the flip side of this scenario. Let say you have great efficiency with this batch & your 8 gallons of pre-boil wort came in at 1.058! That’s 464 GU’s, 38 points higher than what we were shooting for. You can figure your volume you need to hit for your target OG in the same manner as above when you had bad efficiency. Divide your GU mash by your target OG.
So: 464 / 71 = 6.53 gallons post boil

And you can also calculate your necessary boil time as outlined above
So: (8 – 6.53) / 1.33 = 1.10 hours or 66 minutes

You also have the option of dilution if you want to stay on your 90-minute boil schedule. After your 90 minute boil we have already established that you will have 6 gallons of wort left. The gravity of this wort will now be 1.077 seeing as how you had better efficiency than your recipe stated. We still need to have a volume of 6.53 gallons to hit our gravity of 1.071 as shown in the equation above. This means you would have to top up with .53 gallons of sanitized water.

But let’s say you still only want 5.5 gallons of 1.071 wort in your fermenter. We now need to break these volumes out into percentages to calculate how much wort to add & how much water to add to hit 5.5g & 1.071 OG. You need to divide each addition, wort & water, by the total original volume needed (6.53).
% water = .53 / 6.53 = .08 or 8% water
% wort = 6 / 6.53 = .92 or 92% wort

Now all you have to do is take the volume you want in the fermenter & multiply it by the percentages to figure how much of each to add to get the 5.5g & 1.071 wort you want.
Amount of water = 5.5g x .08 (8%) = .44 gallons of water
Amount of wort = 5.5g x .92 (92%) = 5.06 gallons of wort

With an accurately calibrated fermenter, these additions should be easy & fairly accurate to make to hit your target gravity to within .002.

Q: Okay, what about partial-mash/mini-mash brewing?
A: Partial Mash brewing equations, procedures & adjustments are the same as all grain brewing only they are dealing with different ingredient proportions. All of the above equations for all grain can be used with figuring partial mash target gravities.
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