Greyline ultrasonic sensors are mounted at the top of a tank or in a position above the liquid being measured. The sensor continuously transmits pulses of high frequency sound (typically 42 kHz) which travel away from the sensor, hit the surface of the liquid and return to the sensor.

Since the speed of sound is affected by air temperature, Greyline ultrasonic level sensors include a built-in temperature sensor. Level/distance measurements are automatically temperature compensated throughout the operating temperature range of the sensor.

The sensor should be positioned so that it has an clear "view" of the liquid surface and away from ladders, pipes or other obstructions. Greyline recommends 1 ft. from the sidewall for every 10 ft. depth (300 mm for every 3 m depth). False echoes from agitators (sweeping under the sensor), turbulence and waves can be filtered and disregarded by the instrument.

Greyline models range from simple 4-20mA level indicating transmitters, to sophisticated monitoring, controlling and logging models.

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, Greyline 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.

Greyline 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.

Greyline 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.

Area-Velocity Flow

Measure open channel flow without a flume or weir.

Doppler flow meters measure flow from outside a pipe with a strap-on sensor. Greyline Doppler meters continuously transmit high frequency sound (640 kHz) that travels through the pipe wall and into the flowing liquid. Sound is reflected back to the sensor from solids or bubbles in the fluid. If the fluid is in motion, the echoes return at an altered frequency proportionate to flow velocity. Doppler flow meters continuously measure this frequency shift to calculate flow.

The Doppler technique only works on liquids which contain solids or gas bubbles to reflect its signal. These are "difficult" liquids that may damage regular flow meters: slurries, sludge, wastewater, abrasives, viscous and corrosive chemicals. Because the sensor mounts on the outside of the pipe, there is no pressure drop and no obstruction to flow.

For best performance Doppler sensors should be mounted away from turbulence creating devices like pipe elbows and tees, and away from velocity increasing devices like controlling valves and pumps. Typical accuracy is ±2% of full scale.

Doppler instruments include a strap-on 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.