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Brewing (pomegranate) wine with a Gay-Lussac hydrometer at my disposal. Reading prior to fermentation was way below the deeper (estimate -30 GL) and floats at 10 GL after fermentation.

The vinometer, on the other hand, indicated 20 but that’s very unlikely using GV3 which is supposed to max at 15.

I can’t find any clear info as to how I should measure alcohol content using the Gay-Lussac scale.

Gay Lussac hydrometer

Please help.

  • Is the Gay-Lussac scale between 0 and 100 ? – Philippe Nov 20 '18 at 20:25
  • Yes. I just added a picture for reference. – Benjamin Nov 23 '18 at 5:29
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To the best of my knowledge, a Guy-Lussac hydrometer is intended for use with distilled alcohol. It will indicate 0-100% concentration of ethanol in water. For beer, wine, cider etc. you need a hydrometer that will give you the specific gravity (in kilograms per liter) before and after fermentation.

A vinometer works on the basis of the viscosity of the wine and gives you an indication of the alcohol content of most wines but can be off if the wine is particularly syrupy due to back-sweetening.

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I got this information from the web, basically you should take a measure at 15°C. The scale should indicate the alcohol content directly from 0 to 100.

A hydrometer scale calibrated so that readings are the percentage of ethanol (ethyl alcohol) by volume. Zero on the scale is the level to which the instrument sinks in pure water at 15°C, and 100 is the level to which it sinks in pure ethanol at 15°C. It differs from Tralles' scale only in the temperature at which the set points are defined. It was used in France. The Gay Lussac scale replaced the Cartier scale in 1824.

A vinometer, can only be used in a dry wine (fermented dry), if any residual sugar is left it won't work.

  • Thanks. I've read that if the measurements are not done at the required temperature (15°C) there are some tables to convert the value proportionally. – Benjamin Nov 23 '18 at 5:26
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Unfortunately you can't easily do it, because your first measurement was so far off. If the hydrometer is way off the scale, you've gotta ignore the measurement, unless you have reason to believe the scale is linear and applicable outside the marked range. The problem is that the OG measurement matters more than the FG, assuming your yeast and technique are both good. (That may be a bad assumption, though I mean no offense, as I've made more bad mead than good. I'm still catching up.)

The good news is that you can still do it. First, you need a table of ethanol/water ABV/density measurements, so you can convert your Gay-Lussac hydrometer readings to specific gravity. Next, you need to measure it and convert to SG. Then pour a sample of known volume (250 mL is the common convention), and boil it until the volume is reduced by half. Replace the lost volume with water, to the 250 mL you started with. Measure the SG again. The difference is due to ethanol having been replaced by water. Then you need to apply some calculations. The method is described more here:

https://www.meadmakr.com/how-to-determine-alcohol-by-volume/

They link to a table and a formula. The formula is:

Spirit indication (SI) = (SG2 – SG1)*1000

%ABV = (0.008032927443 * SI^2) + (0.6398537044 * SI) – 0.001184667159

musther.net has a handy calculator to assist with this computation:

http://www.musther.net/vinocalc.html#spiritindication

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