Correlation Flowmeters

E-Zine October 2010

Correlation flowmeters are not ultrasonic flowmeters, however at least one design uses ultrasonic means to obtain its measurements. Correlation flowmeters measure fluid velocity by measuring parameters associated with flowing stream at various locations in the piping. To illustrate the general principle of operation, consider a flow stream that abruptly changes color from red to green. The color of the fluid could be sensed at two points that are one meter apart in the piping. If the second sensor detects the green fluid one second after the first sensor detects the green fluid, the velocity in the pipe could be calculated to be one meter per second. There are various flowmeter designs that utilize different measurements to determine the flow rate. Note that correlation flowmeters do not disturb the fluid flow, but rather passively or actively sense its characteristics.

Correlation flowmeters are typically applied to fluids in the turbulent flow regime and other fluid flows with coherent disturbances, such as slurries.

Pressure Sensor Array Technique
A multiple pressure sensor array attached to the outside of the pipe is used to measure pressure gradients caused by fluid vortices that travel down the pipe as a natural consequence of the fluid flow. These vortices are passively sensed at each pressure sensor. Computational techniques derived from sonar technology are used to determine the velocity of the fluid. This technology can also be used to determine the speed of sound of the fluid in the pipe.

Ultrasonic Technique
An ultrasonic transmitter/receiver pair is located at a fixed distance downstream of another ultrasonic transmitter/receiver pair. Their respective ultrasonic beams are used to actively sense turbulent eddies in the pipe. Cross-correlation computational techniques are used to determine the velocity of the fluid.

Excerpted from The Consumer Guide to Ultrasonic and Correlation Flowmeters

ISSN 1538-5280