3.4.2. Transducer installation
A special flow adapter cell made from stainless steel was designed for simultaneous in-line measurements of velocity profiles and acoustic properties for straight pipe flow. Ultrasonic Transducers (TDX) were installed at 70° with respect to the direction of fluid flow and in direct contact with the fluid to avoid attenuation and reflection of the pressure wave. In order to avoid measurements where the ultrasonic pressure wave is irregular, the standard transducers were also pulled back creating a cavity equal to the transducer near-field distance (∼17 mm) between the transducer surface and actual pipe wall interface. This transducer installation has previously been described in Wiklund et al. [4] and the same setup was used in Kotzé et al. [13]. Fig. 4 shows a schematic diagram of the flow adapters with standard and delay line transducers.
An Agilent 100 MHz Digital Oscilloscope (Model 54622A) was used as an integral part of the setup for velocity of sound measurements and the procedure has been described in previous work by Wiklund et al. [4]. The velocity of sound was monitored continuously as it changes with temperature and consequently influences the magnitude of measured velocities.
3.5. Acoustic characterisation of ultrasonic transducers
For acoustic characterisation tests a high performance needle hydrophone (Precision Acoustics Ltd., http://www.acoustics.co.uk, 1 mm piezoelectric crystal) setup was used in combination with a high precision (±0.03 mm) robotic arm (KUKA Robotics GmbH, http://www.kuka-robotics.com). Two ultrasonic transducers (4 MHz, 5 mm active element) were tested: one delay line transducer and one standard transducer. The transducer was moved in a two-dimensional field from the centre of the needled hydrophone using the robotic arm. This enabled measurement of a range of acoustic intensities in increments of 0.1 mm in the Y direction and 1 mm in the X direction (see Fig. 5), which gave a complete acoustic map of the transducer. Tap water was used throughout all the tests and the temperature was continuously monitored using a temperature sensor submerged in the tank. A complete Graphical User Interface (GUI) was written in Matlab® to control the robot arm using a RS232 interface between the robot controller box and PC. It was also possible to automate the measurement procedure as it can take up to 4–6 h for one complete acoustic map, depending if no errors occurred (robot failure or temperature changes). An Agilent 100 MHz digital oscilloscope (Model 54622A) with RS232 connectivity to a PC was used to record data at different points along a two-dimensional grid. Fig. 5 shows the robot arm and needle hydrophone setup together with a delay line transducer submerged in the water tank at SIK.