ISO 4259 Petroleum products - Determination and application of precision data in relation to methods of test
8 Specifications
8.1 Aim of specifications
The purpose of a specification is to fix a limit or limits to the true value of the property considered. In practice, however, this true value can never be established exactly. The property is measured in the laboratory by applying a standard test method, the results of which can show some scattering as defined by the repeatability and reproducibility. There is, therefore, some uncertainty as to the true value of the tested property.
Petroleum product specifications are controlled in accordance with Clauses 9 and 10. By prior agreement, a supplier and recipient can use the alternative procedures described in Annex I.
It is important that a test method is selected that is sufficiently precise to determine whether or not the product satisfies the specifications.
8.2 Construction of specifications limits in relation to precision
Usually specifications deal with limits for the values of the properties. To avoid uncertainty, such limits are normally expressed as "not less than" or "not greater than". Limits are of two types:
- a double limit, upper and lower, for example viscosity not less than 5 mm2/s and not greater than 16 mm2/s; boiling point 100 °C +/- 0.5 °C;
- a single limit, upper or lower, for example sulfur content not greater than 2 %; lead content not greater than 3.0 g/l; solubility of bitumen not less than 99 %.
The single limit situation becomes relevant when, as in most cases, there is an additional implied limit which effectively converts it into a double limit situation. This is illustrated by the examples above in which the additional implied limits are 0 %, 0 g/l, and 100 %, respectively. In cases of a true single limit situation, for example flash point not less than 60 °C, the following considerations do not apply. In Clauses 8 to 10, A1 denotes the upper limit and A2 denotes the lower limit.
The value chosen for a specification limit shall take into account the reproducibility of the test method adopted, as follows:
- for a double limit (A1 and A2), the specified range (stated or implied) shall be not less than four times the reproducibility R, i.e. (A1 − A2) ≥ 4R;
- for a single limit (A1 or A2), the specified limit shall be a distance not less than twice the reproducibility, R, away from the implied limit, that is, if the upper implied limit is 100 %, then (100 − A1) ≥ 2R, or if the lower implied limit is zero, then A2 ≥ 2R.
The requirements of this International Standard apply to specifications drawn up in accordance with these principles.
In cases where, for practical reasons, the value of (A1 − A2) is less than 4R, the results obtained will be of doubtful significance in determining whether a sample does or does not satisfy the requirements of the specification. According to statistical reasoning, it is desirable for (A1 − A2) to be considerably greater than 4R. If not, one or both of the following courses shall be adopted:
a) the specification limits shall be examined to see whether they can be widened to fit in with the precision of the test method;
b) the test method shall be examined to see whether the precision can be improved, or an alternative test method adopted with an improved precision, to fit in with the desired specification limits.
As a consequence of the above restrictions on specification limits, it is recommended that the lower limit of the scope of petroleum test methods be a value not less than 2R greater than the lowest achievable result, and the upper limit of the scope be a value not greater than 2R less than the highest achievable result.
