Flowmeters Are Not All Alike
- Monday, 01 December 2008
When it comes to choosing a flowmetering device, some users assume “one size fits all.” In reality, there are significant differences between flowmeter types, and each design has its unique “pros and cons.” The basis of proper meter selection is a general awareness of flow measurement science — and a clear understanding of your specific application requirements.
Flowmeter users should also consider intangible factors such as familiarity of instrumentation personnel, their experience with calibration and maintenance, spare parts availability, mean time between failures, at the particular installation site.
With most liquid flow measurement instruments, the flow rate is determined inferentially by measuring the liquid’s velocity or the change in kinetic energy. Velocity depends on the pressure differential forcing the liquid through a pipe or conduit. Because the pipe’s cross-sectional area is known and remains constant, the average velocity is an indication of the flow rate. The basic relationship for determining the liquid’s flow rate in such cases is:
Q = V x A
Q = liquid flow through the pipe
V = average velocity of the flow
A = cross-sectional area of the pipe
Other possible factors affecting liquid flow rate include the liquid’s viscosity and density, and the friction of the liquid in contact with the pipe.
Volumetric flow rate is the volume of fluid passing through a given volume per unit time. Mass flow rate is the movement of mass per time. Its unit is mass divided by time - kilogram per second in SI units. Mass flow rate can be calculated from the density of the liquid (or gas), its velocity, and the cross sectional area of flow.
End users contemplating a flowmeter purchase should take the time to study the characteristics of respective measurement technologies, and analyze their advantages/disadvantages for different environments. Common industrial flowmeter designs include:
Differential Pressure – Operates by measuring the pressure differential across the meter and extracting the square root. These meters have a primary element which causes a change in kinetic energy, creating differential pressure in the pipe; and a secondary element measuring the differential pressure and providing a signal or read-out converted to the actual flow value.
Electromagnetic – Employs Faraday’s law of electromagnetic induction, which states voltage will be induced when a conductor moves through a magnetic field. The liquid serves as the conductor, and energized coils outside the flow tube create the magnetic field. The amount of voltage produced is directly proportional to the flow rate.
Coriolis – Provides mass flow data by measuring fluid running through a bent tube, which is induced to vibrate in an angular harmonic oscillation. Due to the Coriolis forces, the tube will deform and an additional vibration component will be added to the oscillation. This causes a phase shift over areas of the tube, which can be measured with sensors.
Thermal Mass – Utilizes a heated sensing element isolated from the fluid flow path. The flow stream conducts heat from the sensing element, which is directly proportional to the mass flow rate. The meter’s electronics package includes the flow analyzer, temperature compensator, and a signal conditioner providing a linear output directly proportional to mass flow.