During the last Welmec committee meeting in May 2011, the new Guide 8.8 (modular evaluation of instruments under MID) was accepted and subsequently published online. The first issue of this guide dates back several years and its scope was rather limited. The new extended version now includes NAWIs (non-automatic weighing instruments) and Conformity Assessment Modules D, F and G. Some changes have also been made to the modular evaluation structure.

Consequences for Oil & Gas industry

These changes could have consequences for the Oil & Gas industry. Extensions to Modules D, F and G enable measuring instruments/systems to be verified in steps. It also enables, under certain conditions, components of a measuring instrument/system to be verified by different entities (e.g., companies, producers, Notified Bodies). That effectively could mean, for instance, that a meter is “verified” by its producer in the factory, and the complete instrument/system is verified by a Notified Body, based on the producer’s wet tests.

Module B

Another change only affects Module B (Type Evaluation) Conformity Assessment. Evaluation or part certificates can only be granted if the component has been fully assessed in accordance with either a harmonised standard, normative document or Welmec guide. The underlying idea is to facilitate and thereby increase the acceptance of evaluation or part certificates by other Notified Bodies, but there is a potential downside to this new agreement.

One possible consequence occurs when a harmonised standard, normative document or Welmec guide is less flexible or more rigid than the MID. When submitted to modular evaluation the component needs to meet the potentially less flexible/more rigid requirements whereas when the instrument/system is evaluated in its entirety only the MID essential requirements are applicable. For example, OIML recommendations tend to be quite specific in which tests need to be done, in what manner and with which maximum uncertainty.

Alternative methods

MID also allows alternative methods in order to ascertain conformity to their essential requirements. In other words, when a test is prescribed by an applicable normative document, it must be followed to the letter before evaluation or part certificates can be issued. If this is not possible (think for example of the maximum measurement uncertainty) the component cannot be granted an evaluation or part certificate.