Mass flow measurement techniques across the spectrum
A few flow measurement principles dominate the flow technology landscape, but there’s a rich variety of ways to measure flow that might not make themselves apparent without a bit of research. Anyone who works with flow is likely to be familiar with thermal flow meters, differential pressure-based flow instruments, and rotameters, so here is a look at a few flow technologies you may hear less about, depending on your specialization.
Laminar Differential Pressure Flow
For high accuracy, rapid response, and a great turndown ratio metering clean dry gases, many experts turn to LDP techniques. Alicat instruments measure differential pressure, similarly to Venturi products, the type of restriction is a bit different. In an Alicat flow instrument, a stack of plates and spacers (called laminar flow elements) is used to laminarize the flow and create a pressure drop. Laminarizing the flow lowers the Reynolds number of the fluid, and allows calculation of the mass flow rate using known temperature vs viscosity data. A sensor measures the differential pressure across a section of the laminar flow stack, another measures the absolute pressure of the fluid, and a temperature sensor measures the temperature of the gas. All of these parameters, combined with the fluid’s physical constants programmed into the instrument, give the Alicat meter everything it needs to calculate volumetric and mass flow.
Because Alicat flow meters don’t just collect but also report pressure, volumetric and mass flow measurements, they are multivariate devices: you can use the data that matters most to you in your application. For many Alicat fans, this versatility is one of the best reasons to use Alicat flow meters and controllers. Measuring both pressure and mass flow means that users can control flow based on either one, using a controlled loop function built into Alicat mass flow controllers. It means being able to measure mass flow while controlling on pressure, or controlling mass flow while knowing pressure. The benefit is saved time, improved precision, and reduced complexity in your system designs.
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.
Magnetic flow meters
Magnetic flow meters can be used for measuring liquid flows so long as they are sufficiently conductive, and can be tolerant of corrosive or aggressive liquids. Magnetic flow meters utilize Faraday’s Law to measure the velocity of conductive fluids. According to Faraday’s Law, any conductor moving at a 90° angle to a magnetic field experiences an induced voltage proportional to the conductor’s velocity. The meter creates a magnetic field in the fluid path and uses electrodes in contact with the conductive fluid to measure the induced voltage. The meter calculates the fluid velocity using the measured voltage, the known strength of the emitted magnetic field, and the distance between the electrodes.
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 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.