Home Brew Forums > Specific gravit before boil

01-13-2012, 02:25 PM   #1
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 Specific gravit before boil

Does anyone know if you can calculate OG from a gravity taken before boil? I took a gravity before boil but forgot to take a gravity after boil before I pitched the yeast. Also does anyone take gravity as they are collecting their wort, say at the 4 gallon mark to ensure their gravity is going to be correct? Is there a formula for this?

01-13-2012, 02:28 PM   #2
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Yes, if you know your pre & post boil volumes.

Essentially, volume x gravity reading = gravity units. This remains unchanged unless you add or remove sugars.

Preboil volume x preboil gravity = post boil volume x post boil gravity, in a perfect world. So, if you know your volumes, you can find for OG.

And yes, I take readings as I collect my wort. I batch sparge, so I take readings of each runoff. Just a good way to check my math-
1st run volume x 1st run gravity + 2nd run volume x 2nd run gravity ~ preboil volume x preboil gravity, assuming no losses.

01-13-2012, 02:35 PM   #3
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I would think the gravity will be unreliable before you boil because unless you are taking gravity of just water, then you would most likely be taking gravity of water at around 150 degrees or higher. You also would be taking gravity before you boil off some of the water you started with, so it again would be inaccurate.

Maybe I am way off base, but I just don't see why you would take a reading before you boil. If you have a reliable recipe or use something like beersmith, your OG should be pretty close to what it predicts it will be, or if you are doing extract recipes, its pretty hard not to hit your OG unless you didn't follow the recipe.

01-13-2012, 02:36 PM   #4
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Quote:
 Originally Posted by TyTanium Yes, if you know your pre & post boil volumes. Essentially, volume x gravity reading = gravity units. This remains unchanged unless you add or remove sugars. Preboil volume x preboil gravity = post boil volume x post boil gravity, in a perfect world. So, if you know your volumes, you can find for OG. And yes, I take readings as I collect my wort. I batch sparge, so I take readings of each runoff. Just a good way to check my math- 1st run volume x 1st run gravity + 2nd run volume x 2nd run gravity ~ preboil volume x preboil gravity, again, assuming no losses.
Anyway you could give me an example that you have done in the past?

01-13-2012, 02:38 PM   #5
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It's not hard. Gravity should be taken at a temperature of near 60F. Some amount near there can be adjusted with good accuracy.

If you know how much water has been taken out of the boil (comparing volumes), then math can figure out the gravity that results.

Beersmith and many places online can tell you what the OG is from the two volumes and the preboil gravity.

01-13-2012, 02:38 PM   #6
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Quote:
 Originally Posted by hagelini I would think the gravity will be unreliable before you boil because unless you are taking gravity of just water, then you would most likely be taking gravity of water at around 150 degrees or higher. You also would be taking gravity before you boil off some of the water you started with, so it again would be inaccurate. Maybe I am way off base, but I just don't see why you would take a reading before you boil. If you have a reliable recipe or use something like beersmith, your OG should be pretty close to what it predicts it will be, or if you are doing extract recipes, its pretty hard not to hit your OG unless you didn't follow the recipe.
I use all grain, so won't taking gravity as you collect wort tell you whether you are pulling enough sugars off or not? I don't know I just want to know if I'm wasting my time or not

01-13-2012, 02:41 PM   #7
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Quote:
 Originally Posted by RUNningonbrew Anyway you could give me an example that you have done in the past?
Sure.

I collected 6.5 gallons of wort with 1.044 gravity. 6.5 x 44 = 286 total gravity.
After the boil, I had 5.5 gallons of wort. What's my OG? 286 / 5.5 = 52, or 1.052

01-13-2012, 02:44 PM   #8
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Quote:
 Originally Posted by hagelini I would think the gravity will be unreliable before you boil because unless you are taking gravity of just water, then you would most likely be taking gravity of water at around 150 degrees or higher. You also would be taking gravity before you boil off some of the water you started with, so it again would be inaccurate. Maybe I am way off base, but I just don't see why you would take a reading before you boil. If you have a reliable recipe or use something like beersmith, your OG should be pretty close to what it predicts it will be, or if you are doing extract recipes, its pretty hard not to hit your OG unless you didn't follow the recipe.
Dissolved sugars don't evaporate. Unless you add or remove sugars, your dissolved sugars remain constant. So taking a preboil reading gives you your total sugar. If you have a target gravity, just divide total sugars by target gravity to know what volume you need to boil down to.

And yes, temp does affect gravity readings, so you correct for that.

01-13-2012, 02:46 PM   #9
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Quote:
 Originally Posted by hagelini I would think the gravity will be unreliable before you boil because unless you are taking gravity of just water, then you would most likely be taking gravity of water at around 150 degrees or higher. You also would be taking gravity before you boil off some of the water you started with, so it again would be inaccurate. Maybe I am way off base, but I just don't see why you would take a reading before you boil. If you have a reliable recipe or use something like beersmith, your OG should be pretty close to what it predicts it will be, or if you are doing extract recipes, its pretty hard not to hit your OG unless you didn't follow the recipe.
Taking a pre-boil gravity reading isn't necessary for an extract recipe, but it's invaluable for all-grain brewing. Every mash is a little different, and if you know you missed your numbers up front, you can correct for it by boiling more / less or, in severe cases, adding some DME.

As for the temperature thing, using a refractometer solves that problem nicely. You only need 2 or 3 drops to take a reading, so it cools to a reasonable temperature almost instantly.

01-13-2012, 02:48 PM   #10
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I take gravity readings from dough-in to flame out to ensure I am right on target with the gravity. PH, brix, and temp are all things that get measured 100% of the time throughout the entire brewing process all the way up to kegging.

I'm using a refractometer, but it's easy to keep track of all that info with a hydrometer. On my system, my post boil will be 15 points higher than my pre-boil, so I know if I am within range during my run off, or if I need to adjust by slowing down the sparge or recirculating the wort through the mashtun to pickup some more sugars.