Concept
of a New Primary Standard for
Liquid Flow Measurement
As
a national standard, the new test rig will be the basis to prove and
confarm the claimed total uncertainties of water flow calibration facilities
in accredited laboratories in Germany, say Walter Poschel and Rainer
Engel
Introduction
accredited
test laboratories. Now-adays, for test facilities in these laboratories,
expanded measurement uncertainties of 0.1 % or even 0.05 % are claimed.
Therefore, the aim for the new primary standard was set to achieve a
total expanded uncertainty (2s value) of 0.02 %. That means - in terms
of the ISO-Guide[1] - a total standard uncertainty (1s value) of 0.01
%, including all components type A and B. These Introduction At Physikalisch-Technische
Bundesanstalt Braunschweig (Germany), a new water flow test rig is under
construction, which is intended to establish the national primary standard
for the field of liquid flow measurement. One of its main tasks will
be to calibrate secondary flow standards and to prove by inter-comparison
the accuracy of calibration facilities in uncertainties refer to the
mass or volume of liquid passed through the meter under test or to the
average flow rate during a certain time period. This article describes
the test rig and in particular the measures how to attain the high accuracy
goal and how to prove, that it is realistic with respect to the conditions
of the dynamic measurement process.
General Description of the Test Rig
Fig. 1 shows a simplified diagram and Table 1 gives the specification
of PTB’s high accuracy water flow standard facility that is under
construction now. In principle it is a combined gravimetric-volumetric
test rig, comprising diverter-weighing systems and a pipe prover, which
can be used separately or together for comparison purposes. For generating
and stabilising flow rates from 0.3 m³/h to 2100 m³/ h the
supply system has a 200 m³ storage tank, a set of eight pumps (all
frequency controlled), a constant head tank (30 m³, at a height
of 30 m) and two measuring sections (lane A for nominal diameters from
DN 200 to DN 400 mm and lane B for DN 20 to DN 150 mm). For each diameter
an upstream straight pipe length more than 50D and downstream 20D is
available. All pipe work and valves consist of stainless steel. The
pressure in the measuring section will be between 1.5 and 3 bars when
using the constant head
cont....
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