As heat treaters, we often focus on the measurement of gases. However, there are a number of different devices used to sense and measure the flow of both gases and liquids, and these fall under the generic term of “flowmeter.” For example, anyone who has seen a nitrogen/methanol system is aware that liquid methanol must be metered into the furnace. Some of the many types include flowscopes, mass flowmeters, orifices, rotameters, positive displacement meters, electromagnetic meters, ultrasonic (Doppler effect) devices, turbine meters, wedge flow devices, impact meters and turbine meters.
General Types of Flowmeters
Flowmeters typically measure either volumetric or mass flow and can be described as follows:
- Volumetric flow measurement looks at the flow of a given volume of the medium over time (for example, cubic feet per hour). This technology uses primarily mechanical flow-rate indication with electronic output normally available as an option.
- Mass flow measurement looks at the flow of a given mass over time (e.g., pounds per hour). Industrial thermal mass flowmeters with electronic output are discussed later.
Conversions between the two measurements can be made if you know the pressure, temperature and specific gravity of the flowing medium.
Flowmeters can be subdivided into several general types.
- Variable area: Fluid-flow rate is measured as the flowing medium passes through a tapered tube. The position of a float, piston or vane placed in the flow path changes as higher flows open a larger area to pass the fluid, providing a direct visual indication of flow rate.
- Differential pressure: Calculating a fluid-flow rate from the pressure loss across a pipe restriction is perhaps the most commonly used flow-measurement technique in industrial applications. The pressure drops through these devices are well understood, and a wide variety of configurations are available, each having specific strengths and weaknesses. Variations on the theme of differential-pressure flow measurement include the use of pitot tubes.
- Mechanical: In these instruments, flow is measured either by passing isolated, known volumes of a fluid (gas or liquid) through a series of gears or chambers (positive-displacement type) or via a spinning turbine or rotor. Measurements using a positive-displacement flowmeter are obtained by counting the number of passed isolated volumes.
- Electronic: Magnetic, vortex and ultrasonic devices are available, all of which have either no moving parts or vibrating elements and are relatively nonintrusive.
- Thermal mass: In contrast to volumetric flow devices, thermal-mass flowmeters are essentially immune to changes in gas temperature and pressure. Because measurements can be very accurate and repeatable, these devices are used in critical-flow measurement applications.
Of these types, variable-area and thermal-mass flowmeters are most often used in heat-treating applications.