Flow, level & control c. Fluid Viscosity
The fluid viscosity is fundamental to the performance of oval gear flow meters. Most are not very happy with non-lubricating low viscosity fluids such as water. However, we can minimise problems through selection of high-quality materials and precision manufacture. For example, hot water has a viscosity around 0.6 Centipoise which is very happy slipping through that theoretical clearance equivalent of a 1.5mm diameter hole without rotating the gears unless the mass of the gear is low, the friction minimal, and the clearances as tight as possible. A very small increase in viscosity (and usually lubrication) makes a vast difference to the efficiency of an oval gear flowmeters performance.
C: UltrasoniC Flow meters Titan is targeting this area of technology as providing the best solution for its many of current low flow developments. For the two technologies outlined above the pure mechanics of these systems is the limiting factor. Some energy needs to be injected into the system to achieve lower flow rates. There are also some problems to solve when producing low flow ultrasonic devices due to the sheer physics of the technology.
1. Basic physics
Time of flight ultrasonic meters work because sound speeds up when it travels with the flow and
is retarded when it travels against it. Effectively the phase shift between these two signal transit times is twice the velocity of the fluid in the pipe. The lower the flow rate the lower this phase shift or time of flight in each direction becomes.
2. reduced bore sizes
Decreasing pipe diameter accelerates the velocity so increasing the phase shift between the upstream and downstream signals. Getting the ultrasonic signal into these smaller pipes is itself a problem. On Titan’s lowest flow Atrato ultrasonic flow meter we inject and receive the ultrasound signal into a 6mm diameter bore but then reduce the pipe to a 1mm bore in the central section between the crystals to accelerate the liquid and increase the velocity. This comfortably permits us to meter flows down to 2ml/min.
3. Future developments INS-OCT20-QUANTUM.qxp_Layout 1 19/10/2020 10:32 Page 1
Titan Enteprises is working on even lower flow devices by utilising one of the unique features of our patented technology. The company can send ultrasound around corners. It has successfully prototyped a device which effectively is a 300mm long 1mm diameter tube but produced in a 30mm diameter coil. The basic physics of this unique design increases the pathlength which significantly increases the difference between the up and down signals. However, there are also trade-offs with this design. The signals are seriously attenuated in such a long tight bore so
signal to background noise ratio increases accordingly. This effectively dictates our flow range and low end flow capability. There is currently a point at which we start to lose efficiency with greater increased length and do not see any decrease in low flow measurement. Currently 0.25ml/min is the lowest flow Titan has successfully measured in the laboratory but improved electronics, algorithms and ultrasound coupling is likely to lower this threshold further.
D: thermal teChnology Flow meters These inherently mass flow devices are also becoming smaller with faster response times. By definition (unlike the flow meter technologies outlined above) they can only handle specific fluids as the thermal transfer characteristics of each fluid changes the calibration of the device. Thus, a flow meter set-up for one liquid would not accurately meter a second without a change in calibration factor. Currently for equivalent devices they do not perform with the same accuracy as our ultrasonic technology, but they do meter much lower flows.
ConClUsion Fast paced developments in miniaturising flow sensors is opening the way to measurement of flow in applications such as medical devices that would have previously never been thought possible.
Titan Enterprises
www.flowmeters.co.uk
F71/F41 Teslameters Technician-friendly instruments for manufacturing application
TruZero™ technology eliminates the need to re-zero probes
Touchscreen on TiltView™ display simplifies technician use
FP Series probes have smaller, ultra-thin Hall sensor active areas for improved accuracy
Single- and multi-axis versions Instrumentation Monthly January 2021 Distributed throughout the UK and Ireland by:
1 Mole Business Park, Leatherhead, Surrey KT22 7BA Tel: +44 (0)1372 378822 | Email:
info@qd-uki.co.uk
www.lakeshoref41.co.uk 35
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