Sorry if this is in the wrong section.
Can someone explain how to use these properly.
For best accuracy, obtain a set of narrow range hydrometers. They are often sold in sets of 3 (low, medium and high density) and are made for vintners. If possible, get ones calibrated for beer as the surface tension has an effect on the reading. That not withstanding I get excellent (0.1 °P) agreement with much more elaborate equipment.
The hydrometer should be scrupulously clean and dry. The test jar should be too but if it is wet you can always rinse with sample a few times.
The sample should be at the reference temperature specified by the hydrometer's manufacturer.
Fill the jar with sample to near the top. Then lower the hydrometer gently into the sample to a bit above where you think it will come to rest. The jar should be full enough so that some sample overflows (think about where you do this - i.e. not over mum's Persian rug). This is a great way to deal with foamy samples as the foam goes off with the overflow. Now let the hydrometer settle for a minute or 2.
Read the meniscus either at the top or bottom according to the manufacturers directions.
I want to know how to relate the figures to alcohol content.
Balling determined years ago that 2.0665 grams of extract produced 1 gram of alcohol, 0.9565 g of CO2 and 0.11 g yeast biomass -
on average. From this basic piece of information various formulae for estimating the alcohol content have been devised. Balling's own is
ABW = f*(p-m) where f is a multiplicative factor, p is the "original extract" i.e. the extract before fermentation began and m is the "apparent extract" at the end of fermentation. p is simple the original hydrometer reading (in °P) and m is the hydrometer reading (also in °P) at the end. Most good hydrometers read in °P and even the cheap ones tend to have a Plato scale on them. This is sometimes labeled "Brix" but the Brix and Plato scales are so close that this doesn't matter. If your hydrometers are only labeled in specific gravity you can convert to Plato using p = -616.868 + 111.14*s - 630.272*s*s + 135.997*s*s*s where s is the apparent specific gravity measured at 20 °C and referenced to 20 °C.
The factor, f, varies with OE and is well approximated by f = 0.39661 + 0.0017091*p + 1.0788E-5*p*p. For nominal strength beer (12 °P) f = 0.4187.
No now you have ABW as a percent. If ABW is sufficient you are finished but if ABV is wanted to have to convert ABW to ABV using the formula
ABV = ABW*s/0.791 with s being the specific gravity of the beer and 0.791 that of pure ethanol.
If you have only °P markings on your hydrometer you will need to convert °P to s. You can get that from s = 1 + 0.00386777*p + 1.27447*p*p + 6.34964E-8*p*p*p.
I put in all this detail because you mentioned doing this at the uni and I supposed, therefore, that some degree of precision was wanted.
One last thing that might be helpful is that °P represent the number of grams of sugar in 100 grams of solution. I mention this because this makes it very easy to make up a solution of given strength. Tare a beaker, add 10 grams of sugar, add water to 100 grams. Presto, a 10 °P sugar solution.
Thanks
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