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Emerald Group Publishing Limited
Copyright © 2005, Emerald Group Publishing Limited
The confusing world of flow sensing
The confusing world of flow sensing
Keywords: Flow, Sensors
Although flow is one of the most widely measured of the physical variables, flow sensing remains a problem area. Unlike, say, temperature or pressure, where most applications can be covered by a small number of techniques, numerous different methods and technologies are routinely employed in flow sensing. Depending on who you consult, somewhere between 60 and 100 different basic flowmeter designs are on the market today; new products are constantly being launched; and the problem is further compounded by the number of vendors: over 200 companies are selling flowmeters and allied products in the UK alone. All of this makes the selection of the right product for the job something of a challenge. Daniel Bernoulli published his work on fluid dynamics in 1738, Reynolds studied laminar and turbulent flows in 1883 and several of the physical principles employed in flow sensing were discovered over a century ago, so one might now expect a greater degree of technological consolidation and stability.
Why should this confusing situation prevail? It largely reflects two factors. Firstly, flow is such a ubiquitous variable that almost every field of human endeavour involves the need to measure it, resulting in a diverse range of flow conditions that must be accommodated (Table I). Secondly, different applications require differing aspects of the flow to be measured, e.g. volume flow, mass flow, flow velocity, maximum flow rate, average rate, total volume and mass transfer etc. Add to this the huge range of temperatures, pressures, viscosities, densities and materials that can be involved, the differing requirements for accuracy, measuring ranges and response times, and that some applications necessitate the use non-intrusive or non-invasive techniques, and the scale of the problem starts to become evident.
Table I Varying flow conditions
|Open channel, closed pipe or free space|
|Liquid, gas or solid|
|Single phase, bi-phase or multi-phase|
|Low or high velocity, supersonic|
|Turbulent or laminar flow|
|Stable or variable/pulsating flow|
|Conducting or non-conducting media|
Nevertheless, one might think that, with all the advances made in sensor technology and microelectronics in recent years, a small number of techniques would have been developed that could cover the majority of these requirements. This is not the case, as despite the best efforts of the flowmeter manufactures and the academic community alike, at least ten very different techniques are still in widespread use (Table II), together with all manner of more specialised options such as tracer methods and ionisation and fluidic sensors etc. However, the growing use of certain newer sensing techniques has led to some technological consolidation by progressively replacing older types such as turbine and positive displacement in the marketplace. The most notable are ultrasonic and Coriolis effect mass flowmeters which, according to recent market research, are presently enjoying the most rapid growth rates. Ultrasonic flowmeters are experiencing the highest revenue growth, due largely to their ability to make measurements in large pipes, of any fluid and without causing a pressure drop, whilst Coriolis devices can determine mass flow directly and are therefore important in custody transfer applications and in the chemicals and other process industries.
Table II Types of flowmeter in widespread use
|Differential pressure (DP transducer with orifice plate or venturi etc.)|
|Ultrasonic (time of flight, Doppler effect etc.)|
|Coriolis mass flow|
|Thermal mass flow|
|Anemometry (optical, thermal, vane etc.)|
Although flow sensing is big business, with global markets presently exceeding $3 billion per annum and forecast to reach $4.2 billion by 2006, no one really benefits from the present situation, bar, perhaps, the flow consultants. Users are confounded by the vast range of products and the major manufacturers have to produce sensors based on a diversity of techniques and technologies – in a highly competitive marketplace where both prices and profit margins are falling. Surely, then, this is a key area that could benefit from some radical thinking and innovation, although whether the “universal flowmeter” is a truly feasible concept remains at best questionable. Today, certainly, this still seems as far away as ever.
Robert W. BogueAssociate Editor of Sensor Review