14.11.2005
With a high temperature stability and the possibility of adjusting the temperature of the test fluid over a wide range, the hydrodynamic test field, as a high-accuracy water flow calibration facility, offers new qualitative possibilities of more precise calibrations of measuring devices and reduces the measurement uncertainty during traceability and comparison measurements.
As in other metrological applications, in the case of flowrate measurements of flowing media, too, there are other quantities - besides the measurement quantity concerned - which affect the indicated measurement value (influence quantities and/or process parameters). Depending on the respective sensor principle being applied for the realisation of flowrate measurement, these are the flow profile (velocity distribution in the section of the measurement pipe), the temperature and the process pressure of the flowing medium in the measurement pipe section.
In order to determine the influence to the respective flowmeter with regard to the water temperature and the process pressure in the measurement pipe quantitatively, the flow calibration reference facility (see figure 1) is provided with corresponding devices for temperature adjustment and highly precise measurement and control devices which guarantee a high temperature stability during a measurement. The latter is, amongst others, an essential precondition for measurements on a low measurement uncertainty level.
Figure 1: Turbine flowmeter mounted for measurement purpose in one of the two measuring sections of the hydrodynamic test field
During traceability measurements for industrial testing and calibration devices (traceability to national PTB standards), it is thus possible to carry out the respective measurements under application-relevant temperatures of the measurement medium. The systematic influence of the transfer measuring devices used for this purpose is thus reduced; this means an improvement in the measurement uncertainty both for the measuring facility concerned and for the calibrations carried out there.
In addition, the hydrodynamic test field of PTB offers the almost unique capability of adjusting the temperature to target values over a comparatively wide range so that not only a selective adjustment with regard to a certain measuring temperature is possible, but also an analysis of the influence of the medium temperature on the metrological properties of flowmeters (e.g. such as the error curve).
Figure 2 shows the behaviour of a DN 150 turbine flowmeter within the temperature range between 10°C and 35°C. These unique properties of the hydrodynamic test field open up a very broad field of application for measurements and investigations aimed at improving the temperature characteristic of flowmeters for liquids.
Measurements for which these capabilities are applied also contribute, amongst others, to the evaluation of international comparison measurements.
Figure 2: Measured temperature characteristic of a DN 150 turbine flow meter
Contact person:
Rainer Engel, FB 1.5, AG 1.52, E-mail: rainer.engel@ptb.de