It's prompted by a few recent posts over at Gotmead. A newer mead maker was asking about final gravities and the possibilities of sweetening a brew, yet the persons post was a little confusing with the numbers that were quoted.
A brewing hydrometer measures original gravity for solutions that contain sugars, in the range commonly found in the making of alcoholic beverages.
There's a number of different scales that they can be calibrated too, though it seems generally the best/easiest one to use, is that of original gravity. They sometimes have a %ABV scale and colour bands with numbers and other bits like "bottle at this point" etc.
IMO, there's only one necessary scale that you need to think about using. Yes, I've mentioned it already, the original gravity scale.
You can see that the top, is numbered 0.990, with the numbers increasing as the scale goes down the shaft of the hydrometer. That is because the higher the sugar level present in the liquid, the higher the hydrometer floats in the liquid.
So a new ferment, or ideally, a must mixed for fermentation, irrespective of whether it's grape juice or honey/water, will make the hydrometer act in the same way. The lower the device floats, the lower the amount of fermentable sugars present.
Which is what makes it suitable to be able to measure the levels of fermentable sugars and once the yeast is in the brew, where it's got to on the fermentation i.e. as the alcohol is produced by the yeast munching the sugars, the device sinks further into the sample, giving a different reading.
Water on its own would normally measure 1.000 - and there will usually be an indicator of temperature, whether it's shown in Celsius or Fahrenheit, just depends on where it was made to be marketed or even the age of the device (ones for the UK market are now calibrated in Celsius, but older ones may show Fahrenheit, as would (most likely) one made for the US market). The temperature is shown for accurate readings i.e. if it says 20C, then measuring water to get an accurate reading of 1.000, the sample water should be at 20C etc.
Before I forget, I'm gonna link the Grapestompers guide to using a hydrometer. I haven't read it all, I just found it when I was looking for images to illustrate this point.
Now one thing that I've noticed, is that some new mead (and wine) makers get a hydrometer that's been calibrated and then has 3 different scales printed on (well "in" really) it. Here's a link to a very good set of pictures of one of those, the third picture at the bottom can be enlarged to show the printed scale laid out flat.
These are fine, but they can cause confusion, and while the image shows one that has gravity, percentage alcohol and brix scales, the only scale necessary is the gravity one. There are uses for the %ABV scale and brix, but I've found the gravity scale the most useful because you can see the numbers and those numbers or at least the amount that they drop during a ferment, correspond directly to the % alcohol, even if you "step feed" a ferment, where you start it at one point e.g. 1.100, then you let it ferment to say 1.020, then you add more fermentable sugars (honey generally with meads) and the reading goes back up to 1.040, then you repeat that process, say 2 times before you just let the ferment finish, that is a drop of 80 points, but with 2 step feeds that allowed for an increase of 20 points each time, so that would give a total drop of 120 points, then the third step feed back to 1.040 and then allowing it to finish, lets use 1.000 as the final gravity, that adds a further 40 points to the total drop, so a total of 160 points (which is rather unlikely given the ability of wine yeasts - but this is just a hypothetical example anyway), by using the Alcohol Calculation Chart posted by Lockwood over at wines at home (link to a number of charts he's posted, but the one we're interested in is the alcohol calculation one - and you'll need something that can display a .xls file to view it), we can work out that after the initial 80 point drop, the batch has a %ABV of about 10.8%, then after the first step feed to 1.040 that drops back down to 1.020, it would have a strength of 13.5% ABV a total drop of 100 points, this being repeated increases the total drop to 120 points or 16.3% ABV and after the final feed back up to 1.040 which is then allowed to drop to the final gravity of 1.000, that makes the total drop 160 points or 21.7% ABV.
As far as I can work out, those figures are pretty accurate as to how the drop in gravity points equates to %ABV. Plus it's worth pointing out, that modern wine yeasts generally can be obtained that will ferment or "tolerate" 18% ABV, there are a few yeasts on the market that are advertised as being capable or tolerant to 21 or even 23% ABV. I can't vouch for those, I haven't used them, but I frequently use yeasts that will ferment to 18%.
I'm hoping that that explains why I prefer the single, gravity scale, hydrometers are the easiest to follow.
Of course, it's never all "going to be plain sailing". If you've used something in the batch that has fermentable sugars in it, that can't be easily measured, like fruit for example, then there's always likely to be some discrepancy to the numbers. It can only be a reasonably accurate measurement when all the fermentable sugars can be dissolved in solution, and that solution measured. There are various charts around the net that list the approximate gravity of juices or fruit, I can't say how reliable these are or might be - a few juices can be tested, like apple juice, orange juice and grape juice, as long as you remember that any pulp or other solid matter in the juice can skew the reading one way or another. A bit like dropping a hydrometer into your sample and finding that bubbles attach themselves to the side of the glass. You can usually just spin the glass shaft with your fingers and that removes the bubbles or if there is a lot of effervescence in the test sample, then you need to let it stand for a while for the bubbles to rise/escape.
Damn I hope this makes as much sense to any reader, as it does to me.......
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