![]() ![]() other than at the sample temperature) we can simply use the conversion routine built into AlcoDens. If we want to know the strength at any other temperature (i.e. This tells us the strength is 17.20 Vol% at 25✬. This makes SG(true) = 0.97613 x ( 1.0 - 0.000025(25-15) )įor the alcohol solution in Concept 4 - we know the SG is 0.97589 and we know the temperature is 25✬, so we can certainly get the strength from AlcoDens or alcohol tables. T(calib) is the temperature at which the hydrometer was calibrated in ✬ T(act) is the actual temperature of the alcohol solution and the hydrometer in ✬ SG(indic) is the SG indicated by the hydrometer The formula that describes the expansion of the glass is : But we can say that the SG of this alcohol solution is 0.97613 (just as we did in Concept 3), corrected for the very slight expansion of the hydrometer glass. ![]() Again, we cannot say that the strength of this solution is 20.0 Vol%, even though the hydrometer floats at exactly the 20.0% mark, because the hydrometer was calibrated for 15✬ solutions. We put our hydrometer into the solution and it also floats with its single calibration mark exactly at the liquid surface. But the temperature of this solution is 25✬. ![]() Let us imagine a similar situation to Concept 3, but instead of an unknown solution (or a methanol solution) we know that we have a solution of ethanol and water, although we do not know its strength. Concept 4 - Ethanol-Water at Other Temperatures But we can say that the SG of this methanol-water solution is 0.97613 because if we had calibrated it for SG rather than strength, this is what we would have written on the hydrometer scale. What does this tell us? We cannot say that its strength is 20.0 Vol% because the density of methanol is different from that of ethanol. Now imagine that we put this hydrometer with just the single calibration point into some other solution, say methanol and water at 15✬, and it floats with the calibration mark exactly at the liquid surface. If a manufacturer was making an alcometer rather than a general hydrometer he would mark it "Vol% = 20.0". What value would we allocate to this mark? We could write SG = 0.97613, or we could write Vol% = 20.0. We put the hydrometer into the solution (everything is at 15✬) and we make a mark on the hydrometer at the liquid level. Also, we have an uncalibrated hydrometer. From AlcoDens or alcohol tables we know the SG of this solution is 0.97613. Let us imagine that we have a solution of ethanol-water which we know has a strength of 20.0 Vol% and is at 15✬. This is a simple concept and needs no further explanation. If we have the SG and temperature we can use alcohol tables, or graphs or AlcoDens to get the strength in Vol%. ![]() If we have a solution of ethanol and water we must know the SG and the temperature to be able to determine the strength. Once each concept has been understood by itself, they can be combined into an understanding of the total process. The best way to clarify the theory behind this, is to break the process down into separate concepts, and to understand each one individually. The information for the first two factors is built into AlcoDens and the expansion of the hydrometer follows a simple formula, so once we understand the theory it is a simple calculation. The expansion coefficient of the hydrometer itself.The relationship between the density of the ethanol solution and its strength.The expansion coefficient of the liquid solution.In order to correct a reading taken at a particular temperature with an hydrometer that is calibrated for a different temperature there are three factors that have to be taken into account. This calculator allows you to make rapid and accurate measurements of alcohol strength in the plant or laboratory without having to heat or cool the sample to the exact calibration temperature. This theory is used in the hydrometer temperature correction calculator built into the computer program AlcoDens. This article describes the theory that makes it possible to use an ethanol hydrometer at temperatures other than the calibration temperature. Theory for Hydrometer Temperature Correction ![]()
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