New CCM-guideline for improvement of determination of uncertainty in fluid flow measurement

During the 8th session of the WGFF in June 2012 in Colorado Springs/USA, specific proposals for implementation of actual documents of CIPM and ILAC regarding the measurement uncertainty given in CMC entries and in calibration certificates have been discussed and summarized in a guideline for the field of fluid flow measurement.

Beside the fact that in fluid flow measurement derived units are applied, it is specific that the measurements are always done under dynamic as well as time variant conditions and that they cannot be repeated under exactly identical conditions. Therefore, a larger number of partial correlated input values being measured and other impact factors are influencing the uncertainty of the measurement result (what can be volume flow or mass flow as well as volume and mass of flowing fluid).

The determination of measurement uncertainty is in all cases of fluid flow measurement a complex task and requires often additional specifications of general rules. Actual there is a need for such specification due to the common definition of the term CMC by CIPM [1] and ILAC [2]:

„A CMC is a calibration and measurement capability available to customers under normal conditions:

   (a) as published in the BIPM key comparison database (KCDB) of the CIPM MRA; or

   (b) as described in the laboratory’s scope of accreditation granted by a signatory to the ILAC Arrangement.“

in conjunction with the requirement that in both cases the contribution of the device under test to the uncertainty budget shall be included.

At its 8th session (June 2012 in Colorado Springs/USA), the CCM Working Group for Fluid Flow (WGFF) has therefore elaborated and discussed the „WGFF Guidelines for CMC Uncertainty and Calibration Report Uncertainty“. The document is based on draft proposals of the EURAMET working group “Fluid Flow”.

The essential points can be summarized as follows:

The CMC uncertainty consists of a base uncertainty u_base for flow standard (determined according the GUM [3]) and the repeatability u_repeat,BED of measurements using the “best existing device”:

.

In a calibration certificate of a customer’s device, the documented uncertainty u_PI for the measurement value (named here with “performance indicator PI”, i.e. e.g. calibration factor, meter deviation, discharge coefficient etc.) contains furthermore contributions of:

.

•    instrumentation uAI associated to the device under test („Associated Instrumentation AI“,  e.g. pressure or temperature sensors)
•    characteristics of the test medium uprop and
•    the short term repeatability ureprd,DUT of the device under test (DuT)

The guide line first time defines an uniform base for realistic determination and evaluation of measurement uncertainties for the dissemination of units in fluid flow. The discussion of this guide line is to be continued.

Fig. 1: Participants of 8th WGFF session from 17 countries, June 18th/19th 2012, Colorado Springs/USA

Literature:

[1] CIPM MRA-D-4: Calibration and Measurment Capabilities in the context of the CIPM MRA, Version 2, October 20120

[2] ILAC-P14:12/2010: ILAC Policy for Uncertainty in Calibration

[3] JCGM 100:2008: Guide to the Expression of Uncertainty in Measurement (GUM)

Contact person:

Bodo Mickan, FB 1.4, AG 1.42, E-Mail: bodo.mickan(at)ptb.de