24.10.2012
The current state-of-the-art approach to provide traceability of flow standard facilities – based on the SI units mass, density, temperature and time – relies, in practice, on applying purely static traceability chains of the measurands which are involved in fluid flow measurement. Even a mere model-based theoretical consideration clearly shows that dynamic influences or disturbing effects occur. The essential and dominant erroneous impacts are primarily caused by temporal variations of the reference flowrate generated in the facility in question.
Since for comparison and traceability measurements of the measurands used in liquid flow measurement, no measuring instrument principles with a sufficiently low measurement uncertainty for "direct" traceability measurement purposes are available, traceability is usually ensured element by element. This means that instead of tracing back the flowrate as the elementary measurand, it is broken down into its elementary measurands, namely mass, density, temperature and time, in a purely mathematical manner.
This element-by-element traceability chain is, however, oblivious to the fact that fluid flow measurement – although a static measurand is considered – is a dynamic process where the required temporal stability during a measurement (for calibration or traceability) can only be realized within the technically feasible variation limits of the control devices involved.
As shown in Figure 1, the temporal variation of the flowrate’s magnitude, due to a non-ideal flowrate control operations, represents the main influence quantity for all essential elements of the measurement uncertainty model of a gravimetric liquid flow standard facility. The energy input into the measuring system varies together with the flowrate fluctuations; in turn, this energy variation causes a change in the medium temperature. Temperature variations are responsible for temporal and spatial changes in the medium properties density and viscosity, but also for device and facility parameters such as the temporal change of the connecting pipework’s volume between the measuring device and the gravimetric standard [1][3] (Figure 1: items 2 to 5)
Fig. 1: Gravimetric liquid flow calibration facility – Uncertainty impacting items and impact of flow rate fluctuations [1]
(1) Impact upon diverter timing error
(1a) Systematic effect: Diverter timing error ΔTerror
(2) Material and fluid effects within the interconnecting pipework
(3) Conversion of mass mREF into VREF
(4) Effects due to nonlinearities of the meter characteristics
(5) Time delay caused by the flow sensor or transmitter electronics.
Another functional component that is relevant for the measurement uncertainty of such a standard facility is the flow diverter [2]. Besides its timing error, which is caused by systematic influences (Figure 1: item 1a), this diverter timing error is also influenced by the flowrate fluctuations occurring inside the facility (item 1).
As illustrated in Figure 2, in addition to the generally accepted practice of "purely" static approach of traceability for each measurand, also the generally occurring dynamic effects have to be taken into account when modeling the measurement uncertainty as realistically as possible for the traceability chains of the individual components of fluid flow measurement; this is shown by the additional signal inputs (1) to (5).
Fig. 2: "Comprehensive" traceability chain in fluid flow calibration
- comprising dynamic process effects on the measurement uncertainty of liquid-flow standard facilities:
(1) Dynamic impacts on weigh scale originating from mechanical vibration excitations [4];
(2) Variation of the temperature of the circulating water resulting from flow rate fluctuations and imperfect temperature control operation, respectively;
(3) Variation of the process pressure due flow rate fluctuations;
(4) Fluctuations of the determined water density due to spatial and temporal changes of the water temperature within the water stream;
(5) Impact of flow rate variations on the diverter’s timing error
These additionally effective dynamic components in the traceability chain of flow standard facilities have to be determined by means of adequate model-based experimental investigations (see [4] among other things).
References:
[1] R. Engel, H.-J. Baade: Impacts upon the measurement uncertainty of liquid-flow facilities originating from random-like variations of the flow parameters, Proceedings of the 8th ISFFM, Colorado Springs, CO, USA 2012
[2] R. Engel, H.-J. Baade: Model-based flow diverter analysis for an improved uncertainty determination in liquid flow calibration facilities, Measurement Science and Technology 21(2010) pp. 11
[3] R. Engel, H.-J. Baade: Water density determination in high accuracy flowmeter calibration- Measurement uncertainties and practical aspects, Flow Measurement and Instrumentation 25(2012) pp. 40-53
[4] R. Engel, K. Beyer, H.-J. Baade: Design and realization of the high-precision weighing systems as the gravimetric references in PTB’s national water flow standard, Measurement Science and Technology 23(2012) pp. 12
Contact person:
Rainer Engel, Dept. 1.5, WG 1.53, e-mail: rainer.engel@ptb.de