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Flow measurement is arguably the most complex, most varied and most interesting of process measurements that an Instrument Engineer is likely to encounter.
For example fluid flow can be measured in volumetric or mass terms e.g. m3/hr or kg/sec. These measurements being related by the material's density. And since gases are compressible and change volume when placed under pressure, are heated or are cooled then the volumetric flow rate may change under differing process conditions, where as the mass flow rate may not. This introduces the need for terms like "actual flow rate" through a meter and "standard flow rate". So the flow rate that you display on the operator's control panel may in fact not be the flow measured. See our page on Flow Conditions for a guide showing how to convert between standard, actual and normal flow.
Types of Flow Meter
Then there is the task of how to measure flow. And here the choice can be huge:
- mechanical flowmeters like positive displacement meters or oval gear meters,
- differential pressure based meters like the ubiquitous orifice plate or equally popular venturi meter,
- thermal mass meters,
- vortex meters, electromagnetic, ultrasonic and coriolis flow meters
- and not forgetting laser and acoustic doppler methods.
Flow Meter Selection
All of these types of flow meter have their place; some being more suited to a particular application than others. Many factors may influence your choice when selecting a flow meter, including:
- Required accuracy (or uncertainty),
- Whether mass or volume flow rate is required,
- Properties of the fluid to be measured e.g. pressure, temperature, viscosity, conductivity etc,
- Installation restrictions e.g. straight length requirements, vertical or horizontal pipeline, diameter of pipe etc,
- Familiarity of operators with the flow meter technology
The links below lead to a wide variety of flow measurement related reference data, tools, resources and look up tables as used by instrument engineers when specifying flow instruments, sizing orifice plates and purchasing inline meters.
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For those who want to read further about the theory of flow measurement and the differing types of flow instrumentation, then the following books from Amazon will be of interest: