Ultrasonic Level 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.

The instrument electronics measure the time it takes from transmitted sound to return of the echo. With reference to the speed of sound in air, the exact distance of the liquid surface from the sensor can be calculated with high accuracy (±0.25% of maximum range). 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.

Open Channel Flow 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.

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. 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: How it works