The official unit for measuring alcohol content is the Alcoholic Strength by Volume, expressed as a volume percentage of ethanol at 20°C, i.e. A.S.V. % vol. at 20°C. The volume of pure alcohol is calculated from the result of the A.S.V. at 20°C and the volume measurement brought back to 20°C.
However, the measures of the A.S.V. on the alcoholmeter are carried out at a given temperature (T°C).
To convert A.S.V. and volume at 20°C, there are various tables including the “Practical Guide to Alcoholometry” by M. OUDIN (1) or the “Practical Alcoholometric Tables” published by the European Union (2).
Use and Comparison of the White Pages and the Yellow Pages of the Practical Guide to Alcoholometry
White pages of the Practical Guide of Alcoholometry
They make it possible to convert at 20°C, the measurement of A.S.V. carried out at a given temperature with an alcoholmeter calibrated at 20°C. The step in these tables is 0.1% vol. and 0.5°C and the precision of the conversion result of the A.S.V. at 20°C is +/- 0.1% vol.
These tables also give the volume correction factor to convert at 20°C the volume measured at a given temperature, with an accuracy of 1°/oo.
Yellow pages, although they have never been described in the regulations, they are used in the field of spirits to directly calculate the volume of pure alcohol. Thes tables give the volume at 20°C in liters of pure ethanol contained in 100 liters of the hydro-alcoholic mixture.
They can only be used on the only condition that the A.S.V. and volume take place at the same temperature.
Results are less accurate than with white pages.
Example of calculations with white pages and yellow pages:
For a measure of A.S.V. of 68.5% vol. at 15°C and a volume of 100HL of alcohol at 15°C
With the white pages
68.5% vol. at 15°C is converted to 70.1 % vol. at 20°C
Volume correction factor is 1.004
100HL at 15°C = 100 *1.004 HL = 100.4 HL at 20°C
Pure alcohol calculation:
100.4 * 70.1 / 100 HL = 70.38 HL AP
With the yellow pages
The table indicated 70.40 HL AP
So: 2 L AP difference with the white pages
To use the white or yellow pages the A.S.V. measurement and product have to be at the same temperature.
But the A.S.V. measurement could be at a different temperature than the volume measurement. It’s the case, with an electronic densimeter which gives the A.S.V. directly at 20°C.
So when the temperature measurement fo the A.S.V. is different than the temperature of the product, how to get volume correction factor ?
With the White pages, you have to look at the line of the volume measurement temperature, the T.A.V. found at 20°C and thus read the volume correction factor on this same line.
Example: if the electronic densimeter measures 70.1% vol. at 20°C and the product is at 15°C, look in the white pages for the 15°C line, 70.1 and read the volume correction factor given in the first column, i.e. 1004.
Remarks:
The extrapolation method between 2 temperature steps (1°C), as indicated in the Practical Guide to Alcoholometry, is incorrect: refer to the article “Volume Contraction-Dilatation”.
To make calculations easier and more reliable, Boxettes from Labox Applications offer automated conversions.
The conversion of A.S.V at 20°C is done with 2 decimals and the volume correction factor with an additional decimal compared to the Alcoholometry Guide. These calculations are made in accordance with Table II of O.I.M.L Recommendation R22 which is the official density conversion table.
Evelyne CHANSON –Consultant in quality control of Wines and Spirits of EC Consulting
1. Alcohometric Practical Guide – P. OUDIN
2. Practical Alcohol Tables – Volume 1 – Commission of Europen communauties
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Evelyne CHANSON