Manual – Starflow Ultrasonic Doppler Instrument
Model 6526
Unidata Manual - 6526 Starflow User Manual Issue 17 01 2020.docx
Page 3
The micrologger switches on the Starflow instrument once per scan. The scan rate is specified by
the user. Each time the Starflow switches on, it performs the following tasks:
Measurement
– Each scan interval (user adjustable from 15 seconds to 10 minutes) the
micrologger switches the Starflow on. It then measures velocity, depth and temperature these
signals are then stored in the micrologger memory.
Analysis
– The micrologger then performs data processing and analysis defined in the scheme
you have loaded. This includes averaging, maximum/minimum and totalizing calculations. Flow
computation is also performed at this time.
Communications
– If a computer is connected to the Starflow instrument, the micrologger
establishes communications via the RS-232 channel. An SDI-12 interrogation sequence will also
be initialised at this time. Note: RS-232 and SDI-12 communications will occur simultaneously with
other operations.
Datalogging
– When the scan occurs at a log interval (from 15 seconds to 1 week) the
micrologger records user defined measurement values (velocity, depth, flow, etc) into its memory
(log buffer). Here, the data remains until the Starflow is unloaded into a computer.
2.1
How Starflow Measures Velocity
When sound is reflected from a moving target the frequency of the sound is varied by the velocity
of the target. This variation is known as a Doppler shift. To measure water velocity in open
channels Starflow exploits the particles moving with the water as acoustic targets (or scatterers)
from an instrument fixed to the bed or bank.
There are two distinct types of Doppler instruments that can be used to measure water velocity:
Coherent
(or profiling) Dopplers transmit encoded pulses with the carrier frequency at target
locations, and only measure signals reflected from these targets. This allows the velocity in a
stream to be profiled. These instruments are complex and expensive.
Incoherent
(or continuous) Dopplers like the Starflow, emit a continuous signal and measure any
signals returning from scatterers anywhere and everywhere along the beam. These are resolved
to a mean velocity that can be related to a channel velocity at suitable sites.
During a measurement cycle, ultrasonic sound is transmitted continuously at a fixed frequency,
called a carrier. A receiver listens for reflected signals from any targets. A measuring circuit
detects any frequency changes. A processing system accumulates and analyses these frequency
changes and calculates a representative Doppler shift from the range received.
Each Doppler shift is directly related to the water velocity component along the beam. This is a
physical relationship and if you know the speed of sound in water you can calculate the velocity of
the reflector and thereby the velocity of the surrounding water. Starflow instruments do not need
calibration for velocity measurement.
The velocity measured is the component along the beam. As the beam is at an angle to the flow,
the Velocity is adjusted by the angle cosine.
