Accuracy of Home Hydrometer Testing

The adulteration of spirits, especially rum, with sugar and other unlabeled additives is not new. Thanks to ALKO, the Swedes a few rums were finally revealed. Then came the amazing Johnny Drejer whose pioneering method changed the whole ball game. Distillers can no longer hide their illegal and deceptive use of sugar. Read all about it and see if your rum has been altered...
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Capn Jimbo
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Accuracy of Home Hydrometer Testing

Post by Capn Jimbo »

Accuracy of Home Hydrometer Testing


There are some that believe than home hydrometer testing may not be accurate enough. Not true. Let's discuss...

Johnny Drejer's protocol – on which all home testing is based – is actually quite accurate. To test his method he first tested a number of known rums already tested by the governments of Finland and Sweden, and found a close correlation. It is well to remember that we believe that different batches of rum from the same distiller do change from year to year, or even from country to country. There is some evidence that distillers may alter with sugar to taste, and/or for different markets, eg the sweet-toothed Americans.

So whether Drejer's tests matched or not is not definitive; still they matched quite well. More to the point and to control this factor, Drejer then created his own known alcohol and sugar solutions – when tested the protocol agreed quite closely, ergo yes, the tests are quite accurate enough. None of care whether a rum tests for 3 or 3.5 grams; but we DO care if it tests for 30 or even more grams.

But recently and as always, to examine a point, I decided to analyze the method again based on hydrometer error, thermometer error and/or reading errors. I compare both the Cheapo 4-piece kit with the semi-precision 30-40 and 40-50% hydros, along with a low cost digital thermo. It is assumed the user takes reasonable care to read all of them. It is also assumed that readers tend to err in a consistent fashion, ie either consitently a bit high, or a bit low.



An Analysis of Home Testing

Cheapo hydrometer: a cheapo can be calibrated to “0” by testing Bacardi Superior (no wood extractives, no sugar); thus all error will be from reading. Reading error 0.5% = 2 gram error.

Semi-precision hydrometer: an also be calibrated (above); thus all error again, will be from reading, in this case 0.1% = 0.4 gram error.

Low end digital thermometer: can also be calibrated using icewater to 0.1 deg C. Because these thermos are digital, there are no reading error. Thus all error is that of the thermo itself. By design these low priced digitals typically have an error of +/- 1 degree which means +/- 4 grams! Fortunately it's EASY to calibrate thermometer using the icewater method to easily withing 0.1 degree, which for all practical purposes returns NO error.

Of course this demonstrates the importance of calibrating both the thermometer and the hydrometer being used (in that order).

To summarize:
  • 1. If calibrated the max error will be 2 grams or less for the cheap hydro, and 0.4 (less than one half gram) for the semi-precision hydro.

    2. All instruments must be calibrated. Although one could assume the semi-precision hydrometer is relatively accurate out of the box, the same cannot be said of the Cheapo 4-piece kit. At the least the Cheapo error will be around 2 grams.

    3. The big kahuna is the thermo, which has a minimum built-in error of plus or minus 4 grams, and therefore calibration is especially important. Fortunately, this is easy.

    4. A quick and dirty calibration would be to test Bacardi Superior. If your test is say 5 grams high, then a rougher adjustment would be to subtract 5 grams from all your tests. But it's very easy to calibrate the thermo first (crushed ice water), and then to check the Bacardi with the hydrometer to get your adjustment value. Simple, really. You only have to calibrate once.



Flat Ass Bottom Line


Calibration is pretty simple and very effective. The thermo is calibrated first by using the icewater method, to within 0.1 degree, close to a zero error insofar as grams of sugar. The now calibrated hydro is now used to calibrate the hydrometer by testing Bacardi Superior (the anal retentive could make up a 40% mixture of grain neutral alcohol with distilled water).

Doing so limits errors to the reading of the hydro only, thus a maximum error of 2 grams for the Cheapo hydros, and close to zero error for the semi-precision models. Thus, the use of a calibrated thermo and semi-precision hydro means there will be no appreciable error.

To be fair there is yet another source of error, namely that for smaller distillers the labelled percent of alcohol may be off (but not much). The EU allows an error of 0.3%, Canada allow 0.2%. This translates into a possible error of just 1 gram of sugar.

All factors considered, tests with the recommended sem-precision hydrometer should be within 1 gram, with the super-cheepo kit, to 3 grams. Drejer uses precision equipment so his results should be VERY close to the laboratory.

Carry on.
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