2001/998 - Precious Metal

This Technical Report specifies an acceptance sampling system of single sampling plans both for inspection by attributes and for inspection by variables. The attributes plans are of the accept-zero form, i.e. no lot is accepted if the sample from it contains one or more nonconforming articles. (For the purposes of this Technical Report, a nonconforming article is a precious metal article containing less than the nominal proportion by weight of the precious metal). The variables plans generally require smaller sample sizes than the attributes plans, but require the precious metal content of all the sampled articles to exceed the nominal content by at least a specified amount.
The objective of this Technical Report is to provide accept-zero schemes and procedures for assuring an upper limit to the long-term percentage of nonconforming precious metal articles in the market place (i.e. the percentage of articles reaching the market place that contain less than the nominal proportion of precious metal) by means of the lowest practicable sample sizes. The upper limit provided by this Technical Report is 1,47% nonconforming. The objective is achieved in three ways:
a)   the sample size reduces as the total number of articles accepted in all the lots since the last non-acceptance increases;
b)   if quality is consistently high, then subject to certain conditions it will be possible to switch from sampling by attributes to sampling by variables;
c)   under sampling by variables, further reductions in sample size may be achieved by switching from the unknown standard deviation method (the "s" method) to the known standard deviation method (the "s" method) if there is sufficient evidence that the process standard deviation is constant....

This standard applies to gully tops and manhole tops with a clear opening up to and including 1000 mm. for installation within areas subjected to pedestrian and/or vehicular traffic. This standard does not apply to surface boxes nor to floor and roof gullies in buildings which are specified in EN XXX. The purpose of this standard is to establish definitions, classes, materials, design and testing requirements, marking and quality control of gully tops and manhole tops.