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Old 01-04-2008, 05:09 PM   #1
Zul'jin
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What's the formula to determine specific gravity at any temperature? I keep reading about 60F being the temperature to take specific gravity readings. It ain't gonna happen that I can always do that at 60F. Some sources say there is a formula to use to figure the correct specific gravity at temperatures not 60F. Great! But none of them tell what the formula is. I'm not looking to buy a hydrometer that I can calibrate for any temperature. Mines gonna be the typical floating glass type with lines on it. Even if I were to get crazy and draw different lines for different temperatures, I still need a formula to do that, which I ain't gonna do.

Thanks!
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Old 01-04-2008, 05:10 PM   #2
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http://www.myhomebrew.com/hc0.html

As for the specific formula, I am not sure what it is, but I suppose you could look at that pages source code.
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Old 01-04-2008, 05:20 PM   #3
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For tracking the ABV%, adjusting to a 60 degree reading is not that important if your final gravity reading will be taken at the same temperature as your original gravity.

Now, if you want to check your efficiency, then yes, most recipes are based around a 60-degree calibration.

Most my readings are taken at 68 degrees. That's my pitch temp and the ambient temp in my brewshop.

 
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Old 01-04-2008, 05:27 PM   #4
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The formula is rather complicated... but here it is, as it is coded into StrangeBrew Java. This is for Celsius temps.

Code:
	//	 Hydrometer correction formula
	static double deltaSG(double TempC, double SG)
	{  // hydrometer correction formula based on post by AJ DeLange
	   // in HBD 3701
	   double coeffic[][] = {{56.084,   -0.17885,   -0.13063},    // 0-4.99
	                        {69.685,   -1.367,     -0.10621},    // 5 - 9.99
	                        {77.782,   -1.7288,    -0.10822},    // 10 - 14.99
	                        {87.895,   -2.3601,    -0.10285},    // 15 - 19.99
	                        {97.052,   -2.7729,    -0.10596}};   // 20 - 24.99

	   double plato = SGToPlato(SG);
	   int coefficIndex=4;
	   if (plato < 20)
	     coefficIndex = 3;
	   else
	   if (plato < 15)
	     coefficIndex = 2;
	   if (plato < 10)
	     coefficIndex = 1;
	   if (plato < 5)
	     coefficIndex = 0;

	   double deltaSG = (coeffic[coefficIndex][0])
	                   + (coeffic[coefficIndex][1] * TempC)
	                   + (coeffic[coefficIndex][2] * TempC * TempC);

	   // changed + to - from original
	   double CorrectedSG = platoToSG(plato - (deltaSG/100));
	   return CorrectedSG;

	}

	public static double SGToPlato(double SG)
	{ // function to convert a value in specific gravity as plato
	  // equation based on HBD#3204 post by AJ DeLange
	  double plato;
	  plato = -616.989 + 1111.488*SG - 630.606*SG*SG + 136.10305*SG*SG*SG;
	  return plato;
	}

	public static double platoToSG(double plato)
	{  // function to convert a value in Plato to SG
	   // equation based on HBD#3723 post by AJ DeLange
	   double SG;
	   SG = 1.0000131 + 0.00386777*plato + 1.27447E-5*plato*plato + 6.34964E-8*plato*plato*plato;
	   return SG;
	}

 
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Old 01-04-2008, 05:30 PM   #5
Zul'jin
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Wow! I think I heard a sonic boom from how fast those answers came in

With that conversion site I should be able to figure a formula, at least an approximation. I'll work on that later.

Quote:
BierMuncher: For tracking the ABV%, adjusting to a 60 degree reading is not that important if your final gravity reading will be taken at the same temperature as your original gravity.
Good to know as ABV% is what I want to figure out. I have the formula for that and ABW% too.

And in comes JimC to crit my brain
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Old 01-04-2008, 05:35 PM   #6
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From the above it looks to be a simple (second order fit iirc).

SG = A + B*T + C*T^2

The lookup table has the dependency as to what your input gravity is, they fill in A, B, and C respectively based on degrees Plato, the comments to the right of the table show which ones go with which (or use the index ). The rest is convering to Specific Gravity it looks like or visa-versa.

yeah it takes the SG, puts in Plato, does the correction then back to SG...I think.
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Old 01-04-2008, 06:56 PM   #7
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Be careful using this conversion. It is accurate as long as you are close to 60F - like up to about 75. It gets increasingly less accurate the farther off you are. so don't go measuring your gravity at 150F...

GT

 
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Old 04-13-2009, 03:43 PM   #8
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What about at colder temperatures? 34?
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Old 04-13-2009, 07:17 PM   #9
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If you are specifically looking for a formula I don't know, but if you want to know what the adjustment is for any given tempurature i suggest http://www.brewcalcs.com/
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