Flowmeter Miniaturization: Advantages and Disadvantages

Titan Enterprises has consistently strived to develop compact, high quality flowmeters. These have ranged from oval gear meters and miniature turbines to current ultrasonic devices. This article outlines a range of issues that must be considered when wanting to miniaturize flowmeters.

Flowmeter Miniaturization: Advantages and Disadvantages

Image Credit: Titan Enterprises Ltd.

Turbine Flowmeters

Designing a turbine meter that offers a very low flow alongside high performance is especially challenging, and a whole range of physical issues can hinder the production of such small devices. Radial flow turbines are generally a better choice than axial flow turbines for low flow applications.

Key problems are listed below, in no particular order.

Friction

Stiction can be defined as friction on two surfaces, which may prevent parts being in motion. This is the first hurdle, and stiction’s effect on any bearing will dictate the point at which a turbine will begin to rotate.

Stiction tends not to be an issue for large turbines are these will likely have considerable driving torque relative to the stiction. However, miniature devices will likely have much less available driving torque when compared to the stiction. In these cases, even getting the bearing to spin freely may pose a problem.

The use of sapphire bearings can help overcome this barrier, but the contact area of these highly polished surfaces may still be high in comparison. A point contact can offer the lowest friction, and this can be put in place using cone or ball bearings. However, this may shift the problem onto the bearing load - a single point contact can lead to an enormously high bearing load, ultimately reducing bearing life. Stiction itself will also be dependent on the turbine’s mass.

Weight

Not only does the turbine’s mass in the fluid affect the stiction, but this can also impact response time. Greater mass will result in s slower response time, while a turbine that has minimal mass could be comparable in density to the fluid being metered. In this instance, it would possess neutral buoyancy.

A balance must also be found between blade width and thickness, turbine diameter, materials used in construction and the detection method of choice. The greater the movement on the turbine from the incoming jet’s offset, the more potent the driving force will be. If the mass and rotational resistance is higher, however, then drag on other non-driven blades will also be increased.

Because fine blades can be difficult to optically detect, the blade width will usually be restricted to the incoming jet’s diameter plus some incoming jet “spreading” distance within the fluid itself. Blades that are too narrow will result in too much fluid spilling around the side, while blades that are too wide will result in unnecessary drag.

The Detection System

A no-drag system is vital, and optical detection is an ideal option presuming the fluid being investigated can transmit the light effectively. The system can be reflective, or this may involve some form of beam cutting.

The addition of magnetic material to the turbine (to accommodate a zero drag inductive, Hall Effect or magnetic detector) will increase mass, but this will also enable the flowmeter to accommodate opaque fluids like emulsions. There is a trade-off in between magnetic elements, their mass and size, when working with these sorts of applications.

Fluidics

As flow rates reduce, liquids will tend to appear and act like treacle. Turbines are viscosity sensitive because these will generally be Reynolds number devices, better suited to turbulent flow. Some of these effects can be negated using different fluidic ‘tricks’; for example, using induced secondary vortices that behave like “roller” bearings, thus reducing viscous drag and helping to extend linear flow range.

General Mechanics

General mechanics are difficult to quantify, but there is a clear relationship between turbine diameter, jet offset and size, clearances, and turbine and chamber thickness. A careful balance must be struck between these elements in order to maintain efficient operation.

Oval Gear Flowmeters

Titan Enterprises incorporates the oval gear design into all its gear meter products. This means that there is far greater driving torque available for a specific differential pressure, derived from the gears’ fluidic asymmetry. Oval gear flowmeters offer fewer subtleties in their design options than radial turbines, due to the fundamental principles of their operation.

Oval gear flowmeters’ operation is based on the principle of positive displacement, whereby a parcel of fluid is taken, then transferred from the inlet side to the outlet side, with no leakage. While this may sound straightforward, there is a careful trade-off between gear friction, gear clearance and fluid viscosity that must be considered.

Clearance and Leakage

Small clearances allow low viscosity fluids to be accurately measured over a large flow range. However, if clearances are too large then the flowmeter will not function efficiently until the liquid is suitably viscous. As oval gear flowmeters become smaller, the leak path will therefore become proportionately larger.

For example, taking a smaller oval gear flowmeter and straightening the leak path would show it to be a strip that is around 60 mm of clearance between the gear and chamber. This would generally be run at a clearance of 0.03 mm, providing an overall potential leak area of around 1.8 mm2.

This leak area is almost equivalent to 1.5 mm diameter round hole. Halving gear size in all directions would essentially halve the leak path, but this would still leave a 1 mm hole for a square-law reduction in gear efficiency and size. It is easier to manufacture larger oval gears because these are relatively more efficient when working at lower flows.

Friction

Stiction will prevent an oval gear flowmeter from starting, while running friction will result in poor linearity. Friction must be kept to a minimum (as with mini-turbine flowmeters), but oval gear flowmeters offer fewer options to achieve this due to their high bearing loads. The load induced by the meter’s pressure drop – necessary for the meter to work - becomes challenging at higher flows.

Fluid Viscosity

Fluid viscosity remains essential in oval gear flowmeters’ performance, and most will struggle with non-lubricating, low viscosity fluids like water. However, problems can be minimized by carefully selecting appropriate high-quality materials and employing precision manufacturing techniques.

For instance, the viscosity of hot water is around 0.6 Centipoise, meaning that hot water can easily slip through the theoretical clearance equivalent of a 1.5 mm diameter hole discussed earlier. It can do this without rotating gears, unless the gear’s mass is low, friction is minimal, and clearances are as tight as possible.

Even the smallest increase in viscosity – and therefore lubrication - can have a huge impact on an oval gear flowmeter’s performance and efficiency.

Ultrasonic Flowmeters

Titan Enterprises are exploring this area of technology as a means of providing the best solution for its low flow developments. While pure mechanics remains the limiting factor in the technologies outlined above, ultrasonic flowmeters require an amount of energy to be injected into the system before lower flow rates can be achieved. The production of low flow ultrasonic devices is also problematic due to the sheer physics of the technology.

Basic Physics

Time of flight ultrasonic meters function due to the fact that sound speeds up when it travels with the flow and slows down when it travels against it. Phase shift between these two signal transit times is effectively double the velocity of fluid in the pipe. As the flow rate decreases, so does this phase shift or time of flight in each direction.

Reduced Bore Sizes

A reduction in pipe diameter will accelerate velocity, resulting in an increase in phase shift between the upstream and downstream signals. Ensuring that the ultrasonic signal reaches the appropriate location within smaller pipes is challenging in itself.

Titan Enterprises’ lowest flow Atrato ultrasonic flowmeter allows the ultrasound signal to be injected and received into a 6 mm diameter bore. The pipe can then be reduced to a 1 mm bore in the central section located between the crystals, accelerating the liquid and increasing velocity. This approach can comfortably meter flows as low as 2 ml per minute.

Future Developments

The company is currently developing even lower flow devices, thanks to a key feature of its patented technology – the ability to send ultrasound around corners.

This has allowed Titan Enterprises to prototype a new device; effectively a 300 mm long 1 mm diameter tube, produced in a 30 mm diameter coil. This unique design’s fundamental physics facilitate an increase in pathlength, significantly increasing the difference between up and down signals.

This design also involves a number of trade-offs; for example, the signals are significantly attenuated in such a long, tight bore, meaning that the signal to background noise ratio is also increased. This ratio essentially determines the flowmeter’s flow range, as well as its low end flow capability.

As such, efficiency suffers at a certain point with greater increased length, and there is no further decrease in low flow measurement. This limit has been determined to 0.25 ml per minute in a laboratory setting, but it is anticipated that improved algorithms, electronics, and ultrasound coupling will lead to this threshold being lowered further.

Thermal Technology Flowmeters

Inherently mass flow devices such as these are becoming smaller while offering more rapid response times. Unlike the flowmeter technologies outlined above, thermal technology flowmeters are only suitable for use with specific fluids, because each fluid’s thermal transfer characteristics will alter the calibration of the device.

In practice, this means that when a flowmeter is set up for one liquid, the instrument could not accurately meter a second liquid without first being re-calibrated. Equivalent thermal technology devices do not offer the same level of accuracy as ultrasonic technology, but these are able to meter far lower flows.

This information has been sourced, reviewed and adapted from materials provided by Titan Enterprises Ltd.

For more information on this source, please visit Titan Enterprises Ltd.

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