Understanding flow meter types: laminar differential pressure, thermal, coriolis, ultrasonic and variable area
There are many different ways to measure flow. Here, we present a brief overview of some of these technologies to understand how flow meters work.
Laminar DP 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.
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.
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 Doppler flow meter principle of operation
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.
- What is the difference between turbulent and laminar flow?