Electricity + Control July 2016

FLOW MEASUREMENT + INSTRUMENTATION

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.

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.

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