Rue J.H. Cool 19a | B-4840 Welkenraedt | BELGIUM
Tel. : +32 (0)87 899 799 | Fax : +32 (0)87 899 790
E-mail : [email protected] | www.flow-tronic.com
-
8 -
2.2.1
Theory of operation
The RAVEN-EYE
®
sensor is mounted above an open channel of water and measures the surface velocity. The
depth from water is measured by an optional associated level sensor. The two measurements are used to calculate
the flow rate using the continuity equation. The RAVEN-EYE
®
converts the surface velocity to average velocity by
analyzing surface velocity distribution using a self-learning technology that doesn’t require theoretical modules nor
site calibration. Then the water level and pipe size is converted to the fluid area. Multiplication of fluid area by
average velocity to obtain the flow rate.
Surface velocity measurement
The surface velocity of the water is measured using radar technology. A radar beam is transmitted from the sensor
to the water surface at the center of the channel. A portion of the signal is reflected back at a slightly different
frequency. The difference in frequency, known as the Doppler frequency, is directly proportional to the speed of the
flow. Proprietary Patent Pending velocity measurement algorithms are then used to calculate the average speed of
the flow stream.
Note: The radar velocity sensor does not operate under surcharge conditions.
Level measurement
The water level is measured using an ultrasonic pulse echo sensor. Two models are available, a standard range
1,75 m and a long range 5,75 m sensor. The sensors can be attached to the RAVEN-EYE
®
sensor body using
specific mounting hardware. For both sensors a sound pulse is sent to the water surface and a portion of the signal
is returned to the sensor. The transit time to the surface and back is used to calculate the distance from the water
surface to the sensor. The pipe diameter and sensor offset are used to convert the distance to water depth.
External 4-20 mA level sensors from other manufacturers can be used to be connected to the UNI-TRANS or any
other PLC or logger.
Flow calculations
The velocity and depth measurements are used with the channel shape and its dimensions to determine the flow
rate. The flow rate is calculated from the continuity equation (1):
(1) Flow rate = Average velocity × Area
where
Flow rate
= volume of liquid that passes the sensor per unit time (e.g. 300 litres per second)
Average velocity = average velocity of the liquid, calculated using surface velocity measurements
and patent pending measurements algorithms
Area
= cross-sectional area of the liquid in the channel, calculated using the channel
dimensions and depth measurement.