This article is the second in a three-part series on flowmeter technologies and selection. February's installment described new technologies and introduced the paradigm-case selection method. In April, users reveal how they choose flowmeters.

A great deal of attention is being paid today to new-technology flowmeters. Coriolis, magnetic, ultrasonic, and vortex are called new technologies mainly in contrast with differential-pressure flowmeters, which have been around for nearly 100 years.

New-technology flowmeters also contrast with traditional-technology flowmeters. Traditional technologies include turbine, positive displacement, thermal, variable area, and open channel. While some traditional technologies are less complex than new technologies, there are still some very interesting developments among these flowmeters. New products are still being introduced, and the annual sales of some of these types of meters exceed those of some new technology meters.

This article describes the operating principles of each of the traditional-technology flowmeters. It also looks at the paradigm-case applications for each type: the case where conditions are optimal for the operation of that type of flowmeter. Paradigm cases are a subset of the broader class of applications where a given technology will work.
Table 1.
Paradigm Cases

Technology	Paradigm case conditions	Comment
Turbine	Clean liquids or gases Enough flow to spin	Low-viscosity liquids are best
Positive-displacement	Liquid and gases Clean, non-corrosive, non-erosive	Medium to high viscosity is best
Thermal	Clean gases Known heat capacity	Low cost is an advantage
Variable-area	Clean liquids Low viscosity High accuracy not required	Spot checks of flow Most have no output signal
Open-channel: Weir and flumes Area-velocity	Free-flowing streams Hydralic structure is feasible Partially filled pipes	Weir: Stream has substantial slope Flume: Stream has little slope Liquid has impurities Flow velocity is high Diameter 6 in. and larger

Which flowmeter is best for which application?

According to this method, users should select the type of flowmeter whose paradigm cases are closest to their application. The method then advises looking at application, performance, cost, and supplier criteria to narrow the choice down to a particular flowmeter.

Turbine Style
Turbine for Clean, Low-Viscosity Fluids
Turbine meters have a spinning rotor with propeller-like blades mounted on bearings in a housing. The rotor spins as water or other fluid passes over it. Flowrate is proportional to the rotational speed of the rotor. A variety of methods are used to detect the rotor speed, including mechanical shafts and electronic sensors.

Turbine meters differ according to the design of the spinning rotor. Several variations include paddlewheel meters and propeller meters. Paddlewheel meters have the axis of rotation perpendicular to the direction of the flow--many paddlewheel meters are insertion devices. Propeller meters have a rotor that is suspended in the flowstream.

Turbine meters can be used on both liquids and gases. Paradigm-case conditions for turbine flowmeters include clean liquids or gases flowing at sufficient speed to operate the meters. Since turbine meters are sensitive to swirl and to flow profile effects, a straight run prior to the meter is recommended. Dirt or impurities in the liquid or gas can damage the meter.

Turbine meters are also sensitive to viscosity: low-viscosity fluids are best. Gas and liquid meters require different designs due to the different densities.