sensors supplement
View of the inside of MARIN’s The Atmosphere with the flume to create sloshing, which is measured by 100 pressure sensors from Kistler. Source: Kistler group
gas to liquid. To prevent noise and bias effects, the Kistler engineers developed a special cable solution that transmits the signals from the test chamber through a vacuum tube to the outside of the autoclave. Signal processing and data acquisition are then handled by 25 automatically synchronised LabAmp 5165A charge amplifiers, which directly digitise the signals of all 100 pressure sensors at a rate of 100 kHz. “We’re very happy with the solution from Kistler. It’s reliable and robust, and it works almost flawlessly,” Ezeta reports. What are the main goals of research at
MARIN’s The Atmosphere, and what results can be expected? “The installation only became fully operational in early 2020, so we’re still at the beginning of our research. One major goal is to improve the scaling models that the industry has used until now,” Ezeta explains. At present, sloshing tests are typically conducted with a small tank on a moving hexapod so as to simulate sloshing. But when the sloshing dynamics are upscaled to real-size dimensions, biases arise in the tests due to the complexity of the underlying physical mechanisms at play. Ezeta continues: “One of
our main objectives is to reduce the uncertainty caused by this scaling. And a good way of doing that is to start by trying to understand the physics behind these
impacting events. To achieve this, we perform tests with waves under varying temperature, pressure and gas conditions so we can see the effects on the impact loads that are measured by the Kistler sensors. One key question is: why do the pressures vary so much?” The answer to this question is a topic of active research, because it is influenced by other important aspects such as the appearance of small perturbations along the wave crests, the oscillation of entrapped gas pockets and the magnitude of the gas-to-
liquid density ratio, which changes with temperature, pressure and different gas mixtures. This turns out to be a key factor for sloshing events, so it also plays an important part in the design of tanks within ships.
results also exPected to benefit hydrogen tanker design
“Research so far has already shown that it is important to take all physical parameters and effects into account,” Ezeta notes. “Our results will give businesses a better understanding of what is happening inside their tanks, and this will help them to keep on improving their testing methodologies. And, last but not least, our research is an enabler for a new type of tanker that will carry hydrogen rather than natural gas.” In theory, hydrogen is even more sustainable than natural gas because its combustion does not produce any greenhouse gases; however, it has a very low liquefaction temperature of 33 Kelvin (corresponding to -253°C under atmospheric conditions). The world’s first liquefied hydrogen tankers are expected to become operational in 2021.
Precisely synchronised fast- samPled charge signals
The LabAmp 5165A from Kistler is a dynamic dual-mode laboratory charge amplifier and data acquisition device for piezoelectric sensors. Key features include:
Low noise 24-bit A/D conversion
High sampling rates (up to 200 kSps per channel)
Universal inputs (charge, IEPE, voltage) Wide frequency and charge range Fully flexible analogue output scaling
Scalable to high channel counts thanks to PTP device synchronisation
A high sampling rate combined with scalability and excellent signal quality make the 5165A the perfect choice for this challenging 100-channel application.
Rodrigo Ezeta sums up his experience with the measurement technology: “We’re very satisfied with our collaboration with Kistler, and we’re looking forward to many successful tests thanks to sensors that are renowned for their long- term durability and stability. We are also constantly looking for new collaborations within academia and different industries, where the ATM with its unique features can provide accurate measurements. Our Atmosphere is open to everyone!”
Kistler
www.kistler.com
The 601C piezoelectric pressure sensor from Kistler is used in a T- shaped array comprising 100 of these transducers in MARIN's test facility for sloshing wave impact measurements. Source: Kistler group
Instrumentation Monthly August 2021 29
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