In conventional dual-tube Coriolis flowmeters, magnets are mounted on one tube, the exciter and sensor coils on the other. However, in very small sizes, for example, where extremely low flow rates demand a tube diameter of 1 mm, the vibrating behavior of the coils' weight can significantly influence measurement results. Therefore, it’s common practice to use one-tube systems for these applications, where the coils are mounted to the chassis.
But eliminating the second tube removes it as a measurement reference, requiring the sensor coils be mounted on the chassis and making the sensor more susceptible to shock and vibration. A costly mechanical decoupling is often required, pricing them out of many applications.
To reduce this sensitivity and deliver accurate measurement at very small flow rates, Heinrichs Messtechnik GmbH, member of the KOBOLD Group, has designed a dual-tube Coriolis meter, in which the sensor coils are mounted between, instead of on, the tubes. The result is “the world’s smallest dual-tube Coriolis mass flowmeter, the High Performance Coriolis (HPC),” says Frank Schramm, managing director, Heinrichs Messtechnik. “With an installation length of less than 6 in., it's now possible to achieve high-accuracy measurements with deviations of just ±0.1%. Furthermore, the sensor shows insensitivity to temperatures of up to 350 °F, to pressures of up to 5,800 psi, as well as to strong vibrations.”
The coils are mounted on a printed circuit board between the tubes. “This method also enables the use of four sensor coils instead of two, as is usually the case with dual-tube Coriolis, providing a higher resolution,” Schramm says. “On the measuring tubes themselves, only very light magnets are mounted, which with a weight of only 0.08 g, have little to no influence on the vibrating behavior of the tubes.”
Instead of conventional brazing, the magnet holders are mounted onto the tubes using a special laser-welding technology. Using this method, Heinrichs Messtechnik aims to hold production costs of the sensor to an absolute minimum. This not only allows for a stress free connection, but also eliminates the time-consuming and elaborate process of brazing in a vacuum oven.
HPC displays extreme insensitivity towards external influences, allowing for precise measurements with a maximum deviation of ±0.1% of the mean value and a zero-point stability between 0.001 and 0.005, making a mechanical decoupling superfluous in most cases. A measuring tube working frequency of more than 200 Hz prevents coupling of installation vibrations or similar oscillations into the measuring system, avoiding fault signals.
Mounting the sensor coils on a motionless PCB also eliminates open wiring in the sensor, as is common in other devices, eliminating a potential weak point as the wire and its connection vibrate with the measuring tubes.
With the exception of the laser-welded measuring tubes, HPC is essentially a solid, drilled-and-tapped, stainless-steel block. Instead of a flow splitter, a reservoir distributes fluid into the measuring tubes, preventing flow disturbances generally caused by splitters. “In principle, the device may also be ordered with Hastelloy tubes and other alloys,” adds Schramm.
HPC is available in four configurations, including conventional straight inline, wall-mount, table-mount with measuring pipes above the supply line (for gas), and table-mount with measuring pipes below the supply line (for liquid). Two measuring ranges cover 2–20 and 5–50 kg/h. Other ranges and adaptations are available on request, such as customer-specific enclosures, connectors or interfaces. Fully-welded, stainless-steel enclosures are also available.
“The whole development phase took just one and a half years,” says Schramm. To achieve this short development time, Heinrichs Messtechnik used simulation. “The required number of prototypes was drastically reduced, reducing development costs significantly,” he says. “Simulation also allows customer-specific requirements to be captured and individual solutions presented quickly.”
For more information, visit www.koboldusa.com.