Areal flowmeters for laminar flow applications

An Areal-Velocity Flow Meter continuously measures both Level and Velocity to calculate flow volume in an open channel or pipe.

The ultrasonic sensor is installed at the bottom of a pipe or channel. To measure water level the sensor transmits ultrasonic pulses that travel through the water and reflect off the liquid surface. The instrument precisely measures the time it takes for echoes to return to the sensor. Based on the speed of sound in water, the level is measured with accuracy of ±0.25%.

Flow velocity is measured with an ultrasonic Doppler signal continuously injected into the water. This high frequency sound (640 KHz) is reflected back to the sensor from particles or bubbles suspended in the liquid. If the fluid is in motion, the echoes return at an altered frequency proportionate to flow velocity. With this technique the instrument measures flow velocity with accuracy of ±2%.

Areal-Velocity Flow Meters work in partially full and surcharged pipes, rectangular, trapezoid and egg-shaped channels.

Areal flowmeters for turbulent flow applications

A separate down-looking ultrasonic sensor can be used for highly aerated or turbulent flow applications. It measures level by transmitting ultrasonic pulses through the air to the liquid surface with accuracy of ±0.25%. Along with the level sensor, a submerged Doppler velocity sensor is used to measure the water velocity.

Ultrasonic Open Channel/Area Measurement

The common method of measuring flow through an open channel is to measure the height or HEAD of the liquid as it passes over an obstruction (a flume or weir) in the channel. Using ultrasonic level technology, MCD open channel flow meters include a non-contacting sensor mounted above the flume or weir. By measuring the time from transmission of an ultrasonic pulse to receipt of an echo, the water level or "Head" is accurately measured.

Flumes and weirs are specially designed channel shapes that characterize the flow of water. Common types are Rectangular Weirs, V-Notch Weirs, Parshall flumes and Palmer Bowlus flumes. The choice of flume or weir type depends on the application: flow rate, channel shape and solids content of the water. Contact MCD (info(at)messcontrol.de) for advice on selection of a suitable flume or weir for your application.

Open channel flow meters can be calibrated to any flume or weir by menu selection. The open channel flow meter electronics use an internal formula to calculate flow rate (Flow = K Hn, where 'K' and 'n' are constants and 'H' is Head as measured by the instrument). Calibration to uncommon or custom flumes can be done by direct entry of 'K' and 'n' constants. Greyline also offers a PC software program "Find K&n" to develop calibration constants from a flume or weir flow chart.

Open channel flow meters include a non-contacting ultrasonic sensor, connecting cable and an electronics enclosure which can be mounted at a convenient location nearby (within 500 ft / 152 m). Sensors can be rated intrinsically safe for mounting in hazardous-rated locations. The instruments display, totalize, transmit and control, and some models include data logging/flow reporting systems.