This document provides guidance on estimating the flowrate when using pressure differential devices constructed or operated outside the scope of ISO？5167 series.

Additional tolerances or corrections cannot necessarily compensate for the effects of deviating from ISO？5167 series. The information is given, in the first place, to indicate the degree of care necessary in the manufacture, installation and maintenance of pressure differential devices by describing some of the effects of non-conformity to the requirements; and in the second place, to permit those users who cannot comply fully with the requirements to assess, however roughly, the magnitude and direction of the resulting error in flowrate.

Each variation dealt with is treated as though it were the only one present. Where more than one is known to exist, there might be unpredictable interactions and care has to be taken when combining the assessment of these errors. If there is a significant number of errors, means of eliminating some of them have to be considered. The variations included in this document are by no means complete and relate largely to examples with orifice plates. An example with Venturi tubes has been placed at the end of its section. This document does not apply to cone meters or wedge meters. There are, no doubt, many similar examples of installations not conforming to ISO？5167 series for which no comparable data have been published. Such additional information from users, manufacturers and any others can be taken into account in future revisions of this document.