Home Brew How To's - Hambleton Bard

How to calibrate a hydrometer - the easy way

A hydrometer used in homebrew and in professional wine making needs either very precise factory calibration (expensive!) or you need to calibrate it every time you use it. Fortunately, this is very simple so you can turn your standard homebrew hydrometer into a precision instrument with minimum hassle.

You can easily calibrate for most errors and use a low cost hydrometer to get a high quality reading using the following procedure:

Keep a trial jar with water near your brew. The idea is that this water have the same temperature as your brew so leave it there all the time.

Before using your hydrometer (every time!): Check the reading of the water. It should be around 1000 s.g. (0 on hydrometers showing oechsle degrees) but it will vary depending on temperature and scale errors.

If your hydrometer shows, say 1002 (+2) you obviously need to take your reading minus 2 to get it right. Then simply take all your wine/beer readings minus 2 and thats it!

Similarly of course, if your hydrometer shows 997 (-3) on the water - just add 3 to all your readings.

Remember that you have to check the water reading every time because temperature might have changed and/or your hydrometer scale might have moved inside the hydrometer. The beauty with this procedure is that it compensates for both temperature and scale moving errors at the same time so no need any more to make sure that the wine has a certain temperature before measuring the specific gravity.

Why do you need to calibrate the hydrometer?

There are many sources of errors when you use a hydrometer, either it is a cheap one or an expensive lab quality one.

Temperature: The same liquid will show different readings at different temperatures. High temperature means lower reading. Your hydrometer usually states what temperature to use it at (normally 15-20 C). If you have a liquid with another temperature there are formulas to calculate the correct value. But then of course you need an accurate thermometer as well.
Scale movement: The scale inside your hydrometer is usually glued inside to the glass and quite often the glue is not good enough which means it slides up or down.
Scale misplacement: In production, especially on cheap hydrometers, the scale might have been fitted in the wrong position. A more expensive hydrometer would probably have been tested in water before approval so you are less likely to have this problem the more money you spend on your hydrometer.

You can easily calibrate your hydrometer for all the above problems which means that you can use a low cost hydrometer and still get a high quality reading. Calibrating your hydrometer - here is how you do it.

Scale distortion: Scales are often photocopied from previous production run and eventually this might distort the scale. This is a less common problem and if it is there, it us usually very small, which is fortunate because we can not calibrate this error away (since it is non-linear).

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