Electricity + Control July 2016

FLOW MEASUREMENT + INSTRUMENTATION

as it is done for mass units is not possible because of the complex- ity of the liquid flow calibration systems. For a description of the measurement accuracy for the calibration systems, measurement uncertainty calculations are used [7] and afterwards validated with key comparisons between the calibration rigs. This is usually done with reference flow meters, which have been tested and characterised for a long time so that the measurement history of the instrument is well known. These liquid flow meters are called comparison standards, because their flow scale is used for a comparison of the flow scale of different liquid flow calibration rigs. Key comparisons can be done in different ways, see Figure 4 .

described in a measurement uncertainty calculation of the system. The validation of these calculations can be done with key compari- sons, which considers the expected measurement uncertainty of the comparison, the rigs, and different reference flowmeters. The result of the sum of requirements for the comparison provides a general way to compare liquid flowmeasurements at different locations with a traceable accuracy and acceptance of the meter measurement true- ness at the end customer.

References [1] Measuring instruments directive (MID), 2004/22/EG.

[2] Gesetz über das Mess- und Eichwesen (Eichgesetz) vom 23. März 1992 (BGBl. I S. 711), zuletzt geändert durch: Art. 2 G vom 3. Juli 2008 [3] Eichordnung vom 12. August 1988 (BGBl. I S. 1657), Zuletzt geändert durch Art. 3 § 14 G v. 3.12.2007 I 2930. [4] BIPM, Meterkonvention, http://www.bipm.org/en/convention/ [5] Physikalisch-Technische Bundesanstalt, www.ptb.de [6] Mathies N, Adunka F, Schupp, R.Volumenstrommessung mit gravimetrischen Normalmessanlagen’ tm, Technisches Messen, Ausgabe 11-2008, Oldenbourg Wissenschaftsverlag, München Oldenbourg Wissenschaftsverlag, München. [7] Mathies N, Engel R, Baade, H-J. Modellbasierte und messtech- nische Validierung von Umschalteinrichtungen in Durchflusska- libriereinrichtungen für Flüssigkeiten, tm Technisches Messen, Ausgabe 5-2010. [8] Mathies N. Messunsicherheit einer gravimetrischen Kalt- und Warmwasser Normalmessanlage für große Volumenströme’, Dissertation, Mensch- und Buch Verlag, Berlin, 2005. [9] Leitgen G, Espig F. AGFW-Ringversuch für Prüfstellen für Wärme- Messgeräte’ EuroHeat & Power, Heft 1-2/2010, VWEW-Verlag, Frankfurt. [10]Mathies N. Präzisions-Durchflussmessung mit Normalmessanla- gen, ATP, Automatisierungstechnische Praxis, Ausgabe 7-8/2010, S. 42-49, Oldenbourg Industrieverlag, München.

Figure 4: Different structures of key comparisons.

The structure of a key comparison can be realised as a circle or a star, as shown in Figure 4 . Comparisons which are organised in a circle rotate the reference flowmeter after it has been measured in the pilot laboratory. At the end of the circle it is measured in the pilot laboratory again. Measurement errors which occur when the reference meter is measured at the different laboratories can’t be detected during the comparison so that establishing the time of the error is difficult. A comparison which is organised in a star structure improves detection of measurement errors. Here the reference meter is measured at the beginning in the pilot laboratory and afterwards in another laboratory which sends the meter back to the pilot laboratory. In this case meas- urement errors can be detected quickly. In practice combinations of both structures are used. The District HeatingWorking Group (AGFW) key comparison uses a combination of the circle and star structure for a comparison of calibration rig for the flow sensor of heat meters [9]. Before a comparison takes place, the goal of the comparison, the measurement uncertainties of the calibration rigs, flow rates, pres- sure, temperature of the medium and the measured quantity have to be precisely defined. The investigation of a standard calibration including calibration procedure and operators has been done in [10]. Conclusion Normally we accept the quantity of the measured medium which is displayed by the flowmeter without being able to check technical de- tails of the meter. For this, a wide range of technical descriptions, ap- provals, standards etc are available. Liquid flowmeters are calibrated depending on the requirements for the measurement accuracy with different reference systems like reference flowmeters, gravimetric or volumetric calibration systems. Therefore a lot of different influences on the reference systems have to be taken into account. These are

• Liquid flowmeasurement is generally used for the measure- ment of cold, potablewater, heat metering, gas measurement or fuel dispensers, truck, rail or ship loading. • Industrial processes are often coupled to flowmeasurement. • For the calibration of the flow meters, reference methods depend on the required measurement uncertainty of the meter under test and the application.

take note

Dr. Nicolaus Mathies. KROHNE Messtechnik GmbH. Enquiries: Email n.mathies@krohne.com or J.Alexander@KROHNE.com

Electricity+Control July ‘16

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