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Old 01-03-2016, 11:42 AM   #1
terrym
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Default How to measure Specific Gravity using a Homebrew Hydrometer

A hydrometer is a simple device that measures the Specific Gravity (SG) of a liquid. SG is a number which compares the densities of a liquid being tested to a reference liquid. The type normally used by home brewers and wine makers is graduated using water as the reference liquid, usually at a temperature of 20*C.
As sugars are added to water the density increases and therefore so does the SG. As these sugars are fermented they form ethyl alcohol which has a density less than water, and so the effect is to reduce the density of the fermenting liquid, and with it the SG. A typical range of a brewing/wine hydrometer is SG 1.120 to 0.990 (note position of the decimal point).
To find out the SG of the liquid simply immerse a clean hydrometer in the liquid with or without the help of a sample jar. Try to ensure that there are no gas bubbles attached to the hydrometer surface since they could cause the reading to be inaccurate; gently spinning the hydrometer sometimes helps. When the hydrometer has settled compare the liquid level to the scale on the hydrometer.
Note - You should check from your hydrometer instructions where your SG scale reading should be taken, relative to either the liquid level or the top meniscus level. Conventionally hydrometers will require a reading to be taken at the liquid level (as shown in the diagram below). However some hydrometers are available where the instructions state the SG reading is to be taken at the meniscus level (i.e. the opposite of what the diagram shows). If in doubt, test (calibrate) your hydrometer using water at the calibration temperature to confirm how the scale compares to the 'reality'.


Diagram shows hydrometer with a scale calibrated against liquid level (not meniscus top)

Basically that's it.

On a Stevenson Reeves hydrometer commonly used by homebrewers, examples of readings are given in the picture below. However note that the example readings '1010', '1014', and '1040', are incorrectly stated and should be 1.010, 1.014, and 1.040 (again note the decimal point).



For complete accuracy a small temperature correction should be applied to a measured SG, since the densities of liquids do vary with temperature. Online calculators are available to do this, typically
http://www.brewersfriend.com/hydrometer-temp/.
If you intend to apply a temperature correction you should check the calibration temperature of your hydrometer, and take this into account.

The SG measurement taken at the beginning of the fermentation is usually called the Original Gravity (OG) and that at the end Final Gravity (FG).

Tables and calculators are also available to convert SG values to potential and actual alcohol content using the OG and FG values you have taken.

Finally if you suspect that your hydrometer is not reading correctly simply immerse it in clean water at 20*C (or the calibration temperature of your hydrometer) and it should read 1.000.
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Last edited by terrym; 01-03-2016 at 10:42 PM. Reason: 1. Some hydrometers may not be calibrated at 20*C 2. Edit for scale calibration type
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Old 01-03-2016, 01:04 PM   #2
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One more abbreviation you may see is PG which stands for present gravity, that is what it is now assuming it's not complete, a test maybe.
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Old 01-03-2016, 02:44 PM   #3
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So if I use one of these 'hydrometer' you talk of, will I stop falling over and going temporarily blind after a bottle of my homebrew?

Brilliant write up mate. Everyone can learn something here.
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Old 01-03-2016, 02:55 PM   #4
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I've got 2 of them, I think I'm going to try them both tonight in 200C water tonight to see if they both read the same...
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Old 01-03-2016, 03:59 PM   #5
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It pays to read the instructions on your own hydrometer as mine states to measure at the top of the meniscus
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Old 01-03-2016, 04:59 PM   #6
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Quote:
Originally Posted by CelticNate View Post
It pays to read the instructions on your own hydrometer as mine states to measure at the top of the meniscus
Conventionally hydrometers are read as I indicated above, so what your instructions are saying is unusual, possibly incorrect. It would be useful if you could immerse your hydrometer in water at 20*C to confirm where it measures 1.000, and so confirm what is correct one way or the other for your hydrometer, and then share the outcome with us. If your hydrometer is read differently from 'normal' I will edit the above to take this into account.
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Old 01-03-2016, 05:53 PM   #7
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Quote:
Originally Posted by terrym View Post
Conventionally hydrometers are read as I indicated above, so what your instructions are saying is unusual, possibly incorrect. It would be useful if you could immerse your hydrometer in water at 20*C to confirm where it measures 1.000, and so confirm what is correct one way or the other for your hydrometer, and then share the outcome with us. If your hydrometer is read differently from 'normal' I will edit the above to take this into account.
Well I'd hate you to think I'm incorrect Attachment 4790
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Old 01-03-2016, 06:08 PM   #8
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Quote:
Originally Posted by CelticNate View Post
Well I'd hate you to think I'm incorrect Attachment 4790
To be clear I said your instructions could possibly be incorrect not yourself.
Nonetheless if you could please check and advise whether the method in the instructions matches the reality of the actual reading in water at 20*C, and if it does, I'll do as I said which is to amend the words in the OP to take this into account.
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Old 01-03-2016, 06:40 PM   #9
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I would tend to believe that the instructions Terrym has given are correct. The flat portion is the correct reading and the meniscus is caused by stonger attraction and adhesion of the water to the glass rather than other water molecules and it creeps upward. This attraction to the glass distorts the reading at this point so the correct reading it taken at the level portion.

It would seem that the supplier/manufacturer has put the wrong instructions on the pack, This should not be instrument specific as it is a phenomenon due to relative attraction of water and glass.
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Old 01-03-2016, 06:41 PM   #10
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Assuming, of course, your hydrometer is calibrated at 20°c. Some aren't, but the instructions should say what water temperature to use.
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