It is required that liquid edible oils are sold by volume. By law, the declared, or nominal, volume is the volume measured at 20°C. If the oil is warmed to a temperature higher than 20°C, the oil will expand, so a larger volume will have to be filled into the container to achieve the required volume at 20°C. Conversely, if the oil is cooled below 20°C it will contract. Hence a smaller volume will be required to achieve the declared volume at 20°C.

In view of the above, it is often more convenient to control filling by weight and because of this, the guidelines to the relationship between the volumes of edible oils at different temperatures and their weights are given below.

All containers must, on average, be not less than the declared nominal value. In addition, not more than 2½% (1 in 40) of all containers may be more than "The Tolerable Negative Error" (T.N.E.) below the nominal value. For 20 litre containers the T.N.E. is 1% or 200ml (0.2 litre).

In addition no container may be more than twice the T.N.E. below the nominal volume. For 20 litre containers, this means that no container may be more than 400 ml (0.4 litre) below the nominal volume.

To summarise, in the case of 20 litre declared volume containers:-

1. All containers must, on average, be not less than 20 litres, measured at 20°C.
2. Not more than 1 in 40 may be less than 19.80 litres, measured at 20°C.
3. No container may contain less than 19.60 litres, measured at 20°C.

Note:

For full details covering your responsibilities, you are referred to the "Weights and Measures Act 1979, S.I, 1979 - No. 1613; Weights and Measures (Packaged Goods) Regulations 1979", as amended by S.I's 1980 - 1064, 1980 - 1070 and 1982 - 144.

The major implications of these are summarised in the D.O.T. "Code of Practice for Importers and Packers", available from HMSO.

Members will be familiar with the expression "Specific Gravity", or S.G for short.

If the specific gravity is determined on a batch of an oil, such as rapeseed oil, at a given temperature, t°C, the relationship between the volume of the rapeseed oil at that temperature, V_t, and the corresponding weight is given by the equation:-

For example, if we wish to determine the weight of 20 litres of groundnut oil at 20°C, i.e. the nominal weight, and the specific gravity at 20°C is determined as 0.9146 g/ml the nominal weight is calculated as:-

Hence, the average net weight of all the 20 litre cans of groundnut oil packed from this particular batch should equal or exceed this weight.

Given below is the table of typical specific gravities at 20°C for a range of oils, published by NEODA, together with calculated nominal weights for 20 litre cans.

* Note:

The figures are typical only. For precise weight control, the specific gravity should be determined on each individual batch of oil.

Similarly, the nominal weight for other pack sizes can be calculated and the other requirements can also be determined, applying the appropriate T.N.E. given for that pack size.

Weigh about 1 pack in 40.
Keep records of the net weights achieved to show that filling is being carried out to maintain the required average and that the requirements for not more than 1 in 40 to be below 1 × T.N.E. and no pack to be below 2 × T.N.E. are being met.

Take corrective action where indicated by the recorded weights to ensure that the required average is achieved. If any sample is more than 2% T.N.E. (see end column in the table in Section 4) below the nominal weight, every pack must be checked and refilled where necessary, to achieve the correct weight. Record the action that has been taken.
All the weights given in Section 4 are net weights. When checking the gross weight of the containers, a similar number of empty containers (including caps, seals, labels, etc.) must also be weighed prior to filling, to ensure that a proper target weight has been set and is being achieved.

These are the volumes which have to be packed at the given temperature, to achieve the nominal weight.

Note:

To be more precise, the term "Litre Weight in Air" rather than "Specific Gravity" should be used. This expression will be used in future revisions of these guidelines.