Temperature correction for Plato hydrometers

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Sep 18, 2012
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Port Angeles
Since SG & Plato are often used in tandem for calculations (e.g. dH2O(20C)*SG(20/20)*Plato = w extract), it is understandable that most homebrewers use either a SG hydrometer and calculate Plato (my current method) or use a dual scale SG/Brix=Plato hydrometer - although I don't particularly trust the dual or triple scales. When taking SG readings, rather than do the fiddling required to cool samples all the way to 20C I just use a "standard" formula to correct the reading.

Now, suppose I could get a good deal on a precision Plato or Brix hydrometer and want to calculate SG. That I can do but first I need Plato at 20C.

Is there a "standard" temperature correction for the Plato scale measured with a hydrometer? If the sample was cooled to less than 30C (for example), would Plato linearly track the density of water down to 20C?

Thanks for any and all suggestions.
Yes, the ASBC has published a table of values to be added to °P as measured at temperature other than 20 °C. A good fit to that table is 0.000383644+0.0496253*T-6.35525e-05*P+0.00110774*T*T+0.00091996*T*P+1.36646e-05*P*P where P is the °P read at whatever temperature and T is the difference between that temperature and 20 °C. Thus, if you read 10 °P at 40 °C T in the formula would be 40 - 20 = 20 and the correction to be added to the 10 ° reading is 1.62 °P i.e the corrected estimate of the extract is 10 + 1.62 =11.62 °P.

And yes, you can derive something very close to what the ASBC table gives by assuming that the density of wort changes at the same rate as the density of pure water with respect to temperature.

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