20. Every measure of length submitted for testing shall be in a clean condition.
21. Every measure of length shall be tested by comparison with an appropriate Northern Ireland local or working standard linear measure using equipment and under conditions which are appropriate to the degree of accuracy required, taking the following factors into account—
(a)the ambient temperature, when the standard and the measure of length being compared are of different materials;
(b)the tensioning and support of flexible measures of length; and
(c)the method of magnification for viewing.
22. An inspector shall test the nominal length and, in the case of a graduated measure, such other graduations as he considers necessary.