20.07.2011
In 2003, after successful acceptance tests, the hydrodynamic test field (see Figure 1) of PTB was commissioned as national standard facility for the quantities of flowrate and totalised water flow. With first commercial calibrations and a project carried out for industry, high-accuracy measurement capabilities could already be testified to the facility.
But the standard flow facility still had to pass its first crucial test within the scope of an international inter-comparison measurement campaign with water flow facilities of other national metrological institutes. Between mid-December 2003 and mid-January 2004, the Department 1.5 of PTB took part in the key comparison CCM FF-K1 of CIPM with the hydrodynamic test field.
Figure 1: View of the measuring section of the hydrodynamic test field
Figure 2: Repeatability of the K-factor determination at 72 m³/h
Figure 3: Repeatability of the determination of the K-factor at 171 m³/h
The following NMIs took part in this key comparison (which is not totally completed yet since the official final report will only be available at the end of this year):
- KRISS, Korea (pilot laboratory),
- SP, Sweden,
- PTB, Germany,
- CENAM, Mexico,
- NMIJ, Japan,
- NEL, UK.
Within the scope of the required comparison measurements, the repeatability and reproducibility conditions as well as the flow conditions of the individual flow facilities had to be determined.
To this end, two flowrate measuring devices were selected as transfer measuring devices for the comparison measurements whose measuring principles are based on different physical action principles:
1) Turbine flowmeter (special design with two counter-rotating turbine wheels),
2) Coriolis flowmeter.
The flow velocity profile in the inlet of the measuring section was determined by means of an additional 5-beam ultrasonic flowmeter and visualized by means of an appropriate analysis software.
Each participating laboratory carried out 16 test series with the dual-meter transfer standard.
In the course of these test series it was determined among other things (e.g. the repeatability) to what extent the two measuring devices influence each other as a result of the flow conditions. This was achieved by changing repeatedly the order in which the two devices were arranged.
By mounting and re-disassembling the two measuring devices repeatedly, we did not only gather information on the repeatability but also quantitative information, allowing us to draw conclusions on the reproducibility of the measurements and on the operators' abilities to control the operations of the standard flow facility.
The results obtained within the scope of this key comparison measurements with the above-mentioned flowmeters were used at the same time to verify the test field's measurement uncertainty which had been considered to be a target quantity.
Figures 2 and 3 show the repeatability obtained during the key comparison for the determination of the K-factors of the two measuring devices. With a standard deviation of max. 3.5·10-5 for this characteristic value serving as comparison quantity, a value was evidenced which is necessary to achieve the total uncertainty of 0.02 % for the test facility.
The fact that these extremely good values could be reached for the repeatability is, to a high degree, also due to the stability of the process quantities flowrate, pressure and temperature. During the whole measurements series (from mid-December until mid-January), temperature changes of max. ± 25 mK occurred. This corresponds to a ± one quantisation increment of the temperature measuring devices used.
As we can see from various requests, the excellent metrological characteristic values of the hydrodynamic test field encourage firms and various national metrological institutes to carry out direct traceability and/or bilateral comparison measurements.
From this point of view, direct comparison measurements were carried out in November 2004 between CENAM, the national metrological institute of Mexico and PTB.
Contact person:
R. Engel, FB 1.5, AG 1.52, E-mail: rainer.engel@ptb.de