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FLOW MEASUREMENT + INSTRUMENTATION
M
easurement of flowing liquid in closed conduits is present
in many different applications in our daily life, e.g. water or
heat metering, gas metering, filling stations, in the chemical
industry, the food industry, for batching or for process regulation.
Usually liquid flowmeters are used to fulfil most of these
requirements. The demands of the application on flow
meters are high. We have a general expectation that
meters ‘show the correct value’, which is necessary
for correct billing. Without external help it is nor-
mally impossible to check whether a flowmeter
shows the correct values or not. Behind the
accuracy of flow meters is a relatively complex
system of calculations, standards, normative
documents and special calibration facilities with
special knowledge of the operators.
The transfer of measurable goods is regulated
metrologically in the area of custody transfer measure-
ment by the European Measuring Instruments Directive
2004/22/EG [1] the German Mess- und Eichgesetz [2] and the Eichord-
nung [3]. The requirements for suitability, usability and application
area are covered under general definitions. Special requirements
for flow meters are defined depending on the different applications.
Maximum Permissible Errors (MPE) depending on the measurand
(m³, kg or kWh) define the ‘accuracy requirements’ for the meters,
which have to be fulfilled by every single meter.
Many technical official standards and documents are available
which the basis for comparable and repeatable measurements is. In
industrial metrology, many referenced guidelines for process appli-
cations exist to ensure correct installation of flow meters, where the
application can influence the meter measurement.
To achieve reproducible results andmeters which can be replaced
one for another, the flow meters have to be calibrated with the
same reference condition, which leads to traceable and comparable
measurements.
Traceability of measurements
The transfer of measurable goods generally requires a consistent
measurement of the units which are used like kilogram or metre. The
question about a standard definition of the measurands was
discussed by the ancient Egyptians and Romans. The goal
of the discussions was to achieve equal, comparable
and reproducible values of a measurand so that a
fair trade of the goods could be realised.
Definitions of reference lengths and refer-
ence weights can be found in a lot of different
areas of measurement. For a long time in Europe
these definitions were established by the local
kings.
The result was a huge number of different
standards for the kilogram, for example, and dif-
ficulties in conversion of these standards. In 1875 with
the Meter convention 17 different European countries
signed a contract to use the same reference kilogram and the
same reference meter and to establish an institution to keep these
standards.
For the first time this allowed traceability on the same standards.
Up to the present 51 states have signed this contract [4] and there are
two associated states. In addition to metre and kilogram, five new
measurands have been added. Since the 13
th
general conference of
mass and metre (CGPM) in 1960 the system is called SI-System, the
international unit system.
The SI-units today are kilogram, metre, second, Kelvin, ampere,
mole and candela. The transfer and traceability of the SI-units
has to be done by the national metrology institute of the different
countries. In Germany this is done by the Physikalisch-Technische
Bundesanstalt (PTB).
The technical definitions of the SI-units can be found at the home-
page of the PTB [5]. Next to the SI-units several additional derived
units exist, for example typical flow units as l/h, l/s, kg/s or kg/h.
Traceability
and
validation
of liquid
flow measurands
Dr. Nicolaus Mathies, Krohne Messtechnik GmbH, Duisburg
This article describes the requirements on liquid flow meters with regard to the
measurement accuracy.
Figure 1: Liquid flow calibration rig.
Electricity+Control
July ‘16
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