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Help reading my Mangrove Jacks Hydrometer

Aussie Home Brewer

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What you have there is a pretty standard 3 scale hydrometer with some bonus information that you can probably ignore. In the order you have taken your pictures: -

Potential Alcohol.
Not really use all that much by brewers, some wine makers use this scale. You can calculate Alcohol Content by Change in Gravity/7.5 (i.e. if the OG was 1.050 and your beer finished at 1.010, the change is 0.040 or what people call 40 points, 40/7.5=5.333% Alcohol by Volume) on that scale the equivalent start would be 5 and a bit% (around 5.25) the FG would be around 1%. You are supposed to take the finish from the start to get the reading which would be ~4.25%ABV. You can see why it's not to popular for brewers.
If you were doing a super dry and it finished lower than water (say 0.990) it would be Start - (-Reading) so the two would add i.e. 5-(-1)=6 not 4.

Specific Gravity or SG.
This is the one most home brewers use almost exclusively. SG is just times the weight of water. If we assume 1 Litre of water weighs 1kg, 1L of a 10% sugar solution would weigh 1.040kg or 1040g, as sugars are eaten by yeast and turned into Alcohol that is less dense than water and CO2 that escapes, the solution becomes less dense, you can use this info as above to calculate alcohol or to tell you when fermentation is finished as the SG stops changing.

Start Finish.
I don't know why they bother, this information might have made some sense 25-30 years ago when kits weighed 1.5kg (3lbw - yes that long ago) and people added 2lbw of white sugar. Ignore!

Balling, Brix, Plato
Balling was the first scale of this type, Brix was a more accurate version, mostly used in wine making, Plato is the latest most accurate scale and the one professional brewers use, often in conjunction with SG for doing lots of brewmaths.
Balling and Brix don't use the decimal place we use in Plato so the 104 Brix would be 10.4oP (Plato).
What it means is that a 1.040SG wort (brew) is the same density 10% W/W sugar solution.
W/W means Weight in Weight 100g of Sucrose (white sugar) dissolved in 900g of water, its now 100g of sugar in 1000g or 10%.

Stick to the SG scale for now, if you move to all grain brewing or even kits and bits you will find the oP scale has its uses.

To be accurate and useful you need to use an hydrometer properly: -
Free floating in a big enough jar to move freely and not touch the bottom. Must not be touching the sides either.
At the right temperature, outside the US hydrometers are all calibrated at 20oC, to be accurate you need to measure the temperature and correct for changes. It should have the reading temperature on the hydrometer.
To be accurate you should put the hydrometer in the sample and leave for 10-15 minutes for the temperature to equilibrate, spin the hydrometer to knock off any bubbles that can stick to the hydrometer and lift it up giving a false high reading.
Sight Line read like this, unless it says read top of meniscus on the hydrometer. There are special hydrometers designed to be read from underneath (used in big breweries to take readings on foamy beer) but doing so on yours will give a dud reading.
Be consistent, if you do everything the same any errors will tend to cancel out, better to do it right but be consistent.
SG 2.jpg

Never put the hydrometer sample back into the brew, you will get an infection
Here is a decent read on using an hydrometer if you want more Home Brew Academy.

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