01.10.2009
Quantity and flow meters are tested and calibrated under reference conditions by means of stationary measurement equipment. This becomes a problem when the measurement conditions in the industrial application differ from these conditions, so that the behaviour of a flow meter changes and the measurement results are, thus, biased. Such problems can be avoided by using a patented method which makes it possible to calibrate quantity and flow meters under operating conditions at the place of installation by means of a static standard (based on the exact determination of the mass or the volume).
The particularity of the method presented in the following is that it is based on a combination of a static standard and continuously operating flow meters. Figure 1 shows a corresponding measurement arrangement in which the static standard is designed, e.g., as a displacer measurement system and enables a quantity of fluid to be extracted or injected between two quantity or flow meters. This fluid volume is determined as a reference quantity in the form of a volume by the displacer measurement system and serves to calibrate the measuring instruments.
Figure 1: Possible designs of the facility for the determination of a reference fluid volume by means of a displacer measurement system
This method consists of two steps. First, the measurement signals of the two flow meters are recorded simultaneously. Then, during the simultaneous signal recording, a reference fluid quantity is, e.g., injected, so that a known partial flow is added to the main flow. When the measurement signals acquired by means of the two steps mentioned for both measuring instruments and from the partial flow are known, the flow meters can be calibrated.
It is thereby preferable to use turbine meters since, in the case of this method, it is essential that the flow meters display a high repeatability. Furthermore, one can thereby assume that the measuring instruments are stable with regard to their zero point.
The advantage of this method is that the manipulation is easy to execute. It can be applied at any time at the place of installation under real measurement conditions in the flying start/stop operation and, therefore makes it possible to generate a history of the measurement stability of flow meters. In addition, calibration can be planned in advance, since the measurement uncertainty depends on the main flow to partial flow ratio. It is possible to select a ratio which is so low that, depending on the reference quantity, the measuring time and the main flow, a low measurement uncertainty can be achieved.
First tests have taken place using a simple measuring arrangement having a pipe nominal width of DN50 which confirm that the use of this newly developed method is relevant in practice. Figure 2 shows the measurement set-up at PTB's white-spirit-operated mineral oil test rig. The figure shows two turbine meters (highlighted by means of white squares) which are mounted into the main pipe, as well as a static standard consisting of a glass container (10 litres) and a balance.
Figure 2: Measurement arrangement at PTB's white-spirit-operated mineral oil test rig to demonstrate the relevance of the newly developed method
The future task will be to develop an extracting system , e.g., in the form of a displacer measurement system, in such a way that it is possible to extract a quantity of fluid – as a reference quantity – automatically and in a regular and reproducible way, and to determine the fluid reference quantity. This also involves the development of a suitable computer control system including the recording of the data and a time measurement device.
[1] J. Riedel: Transfer method for the validation of quantity and flow meters using Newtonian fluids. Thesis, Otto-von-Guericke-Universität Magdeburg, 2009 (in German)
[2] J. Riedel, H. Többen: Method for the calibration of a flow metering system. Patent DE 10 2007 019 601 B3, 2009 (in German)
Contact person:
Jörg Riedel, Dept 1.5, WG 1.53, E-mail: joerg.riedel@ptb.de