Hach Flo-Dar, User Manual

Page: 11 / 44

9
General information
Depth measurement
The water depth is measured using an ultrasonic pulse echo sensor. An electronic 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 is used to convert the distance to water depth.
The depth sensor on the Flo-Dar unit can measure distances up to 1.5 m (5 ft). For larger
channels, an extended range sensor is available to measure up to 6.1 m (20 ft).
During surcharge conditions, a pressure transducer in the Flo-Dar unit is used to
measure depth.
Flow calculations
The velocity and depth measurements are used with the pipe diameter to determine the
flow rate. The flow rate is calculated from the continuity equation
(1) :
(1)
where
Flow rate = volume of liquid that passes the sensor per unit time (e.g. 200 gallons per minute)
Average velocity = average velocity of the liquid, calculated using surface velocity
measurements and algorithms
Area = cross-sectional area of the liquid in the channel, calculated using the channel
dimensions and depth measurement.
Flow rate Average velocity Area
×
=