It is a popular misconception that measurement is an exact science. In fact, all measurements are merely estimates of the true value being measured which implies some degree of doubt about the accuracy of that measurement itself. For example, the repeated measurement of a fixed quantity will never yield the same result every time. The degree of doubt about the measurement becomes increasingly important with the requirement for increased accuracy. For example, with regards to fluids, the relative cost of the measured fluid would need to be considered: i.e. the measurement of the flow of petroleum has historically been much more accurate than the measurement of water flow for either industrial or domestic supply. Uncertainty of measurement gives an indication of the quality or reliability of a measurement result.

However, due to issues related to climate change, an increasing population, and the need to reduce leakage levels, obtaining lower uncertainty flow measurements from water networks has never been more important. With current plans being made to transfer water between water companies, taking water from water rich areas and transporting this to areas of water scarcity, accurate flow measurement will become increasingly important for custody transfer monitoring of the volumes of water transferred between water companies. Improving knowledge of measurement uncertainty, and how this impacts flow measurements, is vital to the efficient operation of the water industry.
The purpose of this document is to give the reader an understanding of the factors affecting the accuracy of a measurement, and of the methods used to assess the way in which the various factors contribute to the overall accuracy. This document is by no means a comprehensive review of measurement uncertainty. If more information is required, the reader is asked to consult the ISO/IEC “Guide to the expression of uncertainty in measurement (GUM)”.

Authors:

• Dr Carl Wordsworth – Head of Water Sector, TÜV SÜD