(referred to in SPIR3140 - SPIR3150)
General
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.
Hydrometer
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.
Thermometers
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
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
of spirits.
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% |
|
| = 5971.817 |
| taken to be | 5971.81 litres of alcohol |
