Types of gas flow meters
There are numerous types of gas flow meters, each utilizing unique measurement technology. Here, we present a brief overview of some of these technologies, including some lesser known flow meter types.
Skip ahead to…
Laminar differential pressure flow meters
Laminar differential pressure-based flow meters use the pressure drop created within a laminar flow element to measure the mass flow rate of a fluid. A laminar flow element converts turbulent flow into laminar flow by separating it into an array of thin, parallel channels. The decrease in pressure, or pressure drop, across the channel is measured using a differential pressure sensor. Because the flow is not turbulent, but laminar, the Poiseuille Equation can then be used to relate the pressure drop to the volumetric flow rate. The volumetric flow rate can also be converted to a mass flow rate using density correction at a given temperature and pressure.
Thermal flow meters
As the name implies, thermal flow meters use heat to measure the flow rate of a fluid. Thermal flow meters traditionally work in one of two ways. The first type measures the current required to maintain a fixed temperature across a heated element. As the fluid flows, particles contact the element and dissipate or carry away heat. As the flow rate increases, more current is required to keep the element at a fixed temperature. This current requirement is proportional to the mass flow rate. The second thermal method involves measuring the temperature at two points on either side of an element or ‘hot wire’. As the fluid flows over the element it carries the heat downstream, increasing the temperature of the downstream temperature sensor and reducing the temperature of the upstream sensor. The change in temperature is related to the fluid’s mass flow.
Alicat BASIS-Series are thermal flow controllers.
Coriolis flow meters
The Coriolis flow meter uses the Coriolis effect to measure the mass flow of a fluid. The fluid travels through single or dual curved tubes. A vibration is applied to the tube(s). The Coriolis force acts on the fluid particles perpendicular to the vibration and the direction of the flow. While the tube is vibrating upward, the fluid flow in forces down on the tube. As the fluid flows out of the tube, it forces upward. This creates torque, twisting the tube. The inverse process occurs when the tube is vibrating downward. These opposing forces cause the tube to twist, the phase difference of which is directly related to mass flow of the fluid through the tube.
Alicat CODA-Series are Coriolis instruments.
Ultrasonic flow meters
Ultrasonic flow meters use sound waves to measure the flow rate of a fluid. Doppler flow meters transmit ultrasonic sound waves into the fluid. These waves are reflected off particles and bubbles in the fluid. The frequency change between the transmitted wave and the received wave can be used to measure the velocity of the fluid flow. Time of Flight flow meters use the frequency change between transmitted and received sound waves to calculate the velocity of a flow.
Ultrasonic flow meters can be an excellent option if you need to measure flow, but can’t install an invasive, new fixture to your flow path; some meters’ transducers can be strapped or clamped directly to the exterior of a pipe! The two methods used by ultrasonic flow meters are Doppler and transit time. Both types of meters emit an ultrasonic beam into the fluid medium. Doppler meters measure the change in frequency of the beam caused by the Doppler effect, and use the known speed of sound of the fluid to determine flow. Transit time flow meters emit two beams which reflect back into receiving transducers in the meter. The transmit times of the two beams can be used to find both the average fluid velocity and the speed of sound of the fluid.
Variable area flow meters
Variable area flow meters, or rotameters, use a tube and float to measure flow. As the fluid flows through the tube, the float rises. Equilibrium will be reached when pressure and the buoyancy of the float counterbalance gravity. The float’s height in the tube is then used to reference a flow rate on a calibrated measurement reference.
Optical flow meters
Lasers have penetrated so many different technologies, so of course they can be applied to flow measurement too! Optical flow meters are used for fluids containing small solid particles—which could cause a clogging problem for other techniques that may rely on capillary bypasses or other restrictions to flow. They are also used to measure gas with liquid droplets, or liquid with bubbles. A laser shines perpendicular to the flow stream and collides with a particle. The light scattered by the particle is picked up by a photodetector, which generates an electric pulse signal. A second laser positioned further downstream of the first laser repeats this process with a second photodetector, and the velocity of the flowing gas is calculated as the distance the particle travelled over time.
Venturi flow meters
Venturi devices have the advantage of being very low cost, but at the expense of flexibility. The Venturi effect is a reduction in pressure caused by a constriction in a fluid’s flow path. Pressure sensors measure the pressure before and inside the length of constriction, and the meter calculates fluid velocity using Bernoulli’s Equation; Bournoulli’s principle states that the a fluid’s speed is inversely proportional to its pressure, so decreasing the pressure of the gas with a known constriction and measuring the differential pressure yields a flow measurement. Without strict control of system pressure and temperature, though, the result is volumetric, not mass flow, so one’s results may vary by environmental conditions.
Different applications will need different gas flow meters, with different outputs, and these various technologies cover a wide range of use cases.