01.10.2009
PTB's white-spirit-operated mineral oil test rig is used to test and calibrate quantity and flow meters which are dedicated to the measurement of fluids other than water. Its operation modus (now also with direct pumping and "flying" start/stop operation) as well as its controlling are presently being radically extended. Within the scope of this extension, also one of the volumetric standards of the white-spirit-operated mineral oil test rig has been equipped with a magnetostrictive height-measurement system and has been newly calibrated.
PTB's white-spirit-operated mineral oil test rig works according to a volumetric measuring principle. Four graduated cylinders having a capacity of 5000 l, 1000 l, 200 l and 100 l, respectively, are used as standards. The testing fluid used is white spirit which, at 20 °C, has a density of 772.4 kg/m³ and a viscosity of 0.803 mPa·s. The measuring instrument to be tested is connected upstream of the corresponding container, and its calibration data is determined from the volume of liquid which has flowed through the container during the measuring time. For this purpose, each graduated cylinder is equipped with two sight glasses on which the level is visually read out at the beginning and at the end of the measurement. The test volume is thus fixed for each container, which therefore makes it hardly possible to carry out the measurements automatically.
The use of a magnetostrictive level-measuring system, which has for the moment being only been installed on the 1000-litre container, can palliate this problem. This system is located in one of the stand pipes connected to the container (Fig. 1).
Figure 1: Three of the four graduated cylinders of the white-spirit-operated mineral oil test rig with the newly installed stand pipe for the magnetostrictive level-measuring system for the automatic level measurement on the 1000-litre container (centre of the picture)
The main component of the new measurement system is a ring-shaped magnetic floater which is led vertically through a so-called waveguide and marks the corresponding level (Fig. 2). If a short electric pulse is fed into the copper wire, which is integrated into the waveguide, an interaction occurs at the position of the floater between the floater's magnetic magnetic field and the circular magnetic field caused by the electric pulse. The consequence is an elastic deformation (magnetostriction) of the material structure in the waveguide. This deformation propagates as a mechanic torsion wave in both directions at ultrasonic speed. It is thus possible to determine the level of the fluid in the graduated cylinder via ranging and from the speed of sound in the waveguide. The advantages of this measuring system reside in the possibility of automating the measuring operations, of obtaining a significantly higher resolution in the order of 10 µm in comparison to the visual read-out and of avoiding subjective influences. Furthermore, this system makes it possible to determine levels at any random height.
Figure 2: General set-up of a magnetostrictive length-measuring system
In order to be able to determine the actual volume of liquid in the tank from the measured level, it is necessary to establish a so-called level table in which each level position of the floater is attributed a corresponding contents volume. For this purpose, the 1000-litre tank was progressively filled with water by means of a 100-litre volume standard (expanded measurement uncertainty of its liquid volume: 1·10-4) which had been calibrated directly on PTB's pipette testing facility. The filling processes were carried out at different fluid temperatures (15 °C, 20 °C, 25 °C).
Further tasks consist in verifying the volume-measuring system by means of a turbine meter and with the actual test fluid (white spirit) under the true measurement conditions, as well as in comparing the results with those obtained statically. This also includes the creation of a measurement uncertainty budget in order to assess the final calibration results.
Contact person:
Jörg Riedel, Dept 1.5, WG 1.53, E-mail: joerg.riedel@ptb.de