The strength of spirits is calculated in accordance with the OIML (Organisation Internationale de Metrologie Legale) system of measurement introduced on 1 January 1980. In this system the strength of spirits is expressed as the percentage by volume of pure ethyl alcohol contained in it at 20°C, and is written as “% alc by vol”, “% ABV” or “% vol”.
Measurement of strength
Traditionally, the strength has been measured by the use of a hydrometer and a thermometer in conjunction with the Practical Alcohol Tables Vol 2. The tables are also required to calculate the amount of pure ethyl alcohol at 20°C in a quantity of spirits of known volume and strength at a temperature other than 20°C. However, as reflected in regulation 18 of the Spirits Regulations 1991, a wider range of methods is now permitted.
For control purposes, the Department has traditionally used a range of 11 glass hydrometers which between them are capable of measuring densities between 780 and 1000 kg per cubic metre. This is sufficient for all strengths of spirit at a temperature of 20°C.
The Departmental hydrometers are “e” marked and conform to the British Standard Institute’s “Specification for Density Hydrometers”, BS 718:1991. Although these hydrometers tend to retain their calibration for a long period it is advisable to have the calibration tested approximately every 10 years. Distillers may possess “e” marked hydrometers, or hydrometers claiming greater or lesser accuracy.
This method should only be used on fresh or otherwise unobscured spirits.
The thermometers used by the Department are graduated between -5°C and 40°C at intervals of 0.5°C.
Practical Alcohol Tables
The Practical Alcohol Tables, Volume 2, form a “deposited table” as required by the Spirits Regulations 1991, for the calculation of the strength of spirits. The Volume contains 2 tables. The first of these gives the strength corresponding to a given hydrometer indication for each temperature between -20°C and 40°C. The second table shows the volume at 20°C of 1000 litres of spirits at temperatures ranging between -20°C and 40°C and at strengths ranging from 0 to 100% alcohol by volume.
A trader may legitimately use a computerised version of the Practical Alcohol Tables. One of these versions uses the formulae the tables are based on to calculate the strength and Thermal Correction Factor of the spirit.
Other tables recognised by law are the Contents by Weight Table and the Laboratory Alcohol Table. The first of these allows the volume of spirits to be calculated from the weight and hydrometer indication. The second shows the percentage of alcohol by volume and percentage by mass as it relates to the density of spirits in air at 20°C.
Any industry-recognised method, used in accordance with the
manufacturer’s protocol, may be used to measure the quantity
Ascertainment of the strength and quantity of alcohol by hydrometer
To calculate the strength and alcohol content of any spirits, the liquid quantity and hydrometer indication must be taken, together with the temperatures at which each measurement was obtained. (These temperatures may be different.)
The strength and alcohol content are then calculated as in the following example:
|Hydrometer indication at 12.5°C||897.2|
|Strength at 20°C from first table||67.7% alc by vol|
|Liquid quantity as measured at 9°C||8734 litres|
|Bulk correction factor from second table||1.010|
|Liquid quantity at 20°C||8821.34 litres|
|taken to be||8821 litres|
|Quantity of alcohol (strength x liquid quantity)||8821 x 67.7%|
|taken to be||5971.81 litres of alcohol|