16-MHz MicroADV Principles of Operation
The SonTek/YSI 16-MHz MicroADV (Figure 1) is a
single-point, high-resolution, 3D Doppler current meter. MicroADV Doppler processing
techniques provide several important advantages: 3D velocity measurements in a remote
sampling volume; invariant factory calibration (no periodic re-calibration required);
simple operation; direct calculation of turbulent parameters such as Reynolds stress; and
excellent low-flow performance. This document presents the basic operating principles of
the MicroADV. To learn more about specific MicroADV configurations and
applications, contact SonTek/YSI.
The MicroADV measures the velocity of water using a physical principle called the
Doppler effect. If a source of sound is moving relative to the receiver, the frequency of
the sound at the receiver is shifted from the transmit frequency.
Fdoppler = -2Fsource (V / C)
In this equation, V is the relative velocity between source and
receiver, C is the speed of sound, Fdoppler
is the change in frequency at the receiver, and Fsource
is the transmitted frequency.
Figure 2. Bistatic Doppler Current Meter
Figure 2 illustrates the operation of a bistatic Doppler
current meter such as the MicroADV (bistatic systems use separate acoustic transducers for
transmitter and receiver). Both transmitter and receiver are constructed to generate very
narrow beam patterns. The transmitter generates sound with the majority of the energy
concentrated in a narrow cone, and the receiver is sensitive to sound coming from a narrow
angular range. The transducers are mounted such that their beams intersect at a volume of
water located some distance away. The beam intersection determines the location of the
sampling volume (the volume of water in which measurements are made).
The transmitter generates a short pulse of sound at a known frequency, which propagates
through the water along the axis of its beam. As the pulse passes through the sampling
volume, the acoustic energy is reflected in all directions by particulate matter
(e.g., sediment, small organisms, bubbles). Some portion of the reflected energy travels
back along the receiver axis, where it is sampled by the MicroADV and processed by the
electronics to measure the change in frequency. The Doppler shift measured by one receiver
is proportional to the velocity of the particles along the bistatic axis of the receiver
and transmitter. The bistatic axis is located halfway between the center axes of the
transmit and receive beams.
Each transmitter/receiver pair measures the projection of the water velocity onto its
bistatic axis. The MicroADV uses one transmitter and two or three acoustic receivers (for
2D or 3D probes). The receivers are aligned to intersect with the transmit beam pattern at
a common sampling volume. The velocity measured by each receiver is referred to as the
bistatic velocity, and is the projection of the 3D velocity vector onto the bistatic axis
of the acoustic receiver. Bistatic velocities are converted by the MicroADV to XYZ
(Cartesian) velocities using the probe geometry. XYZ velocities give the 3D velocity field
relative to the orientation of the MicroADV probe. As it is not always possible to control
instrument orientation, the MicroADV can be equipped with an internal compass and tilt
sensor. The compass/tilt sensor allows the MicroADV to report velocity data in an Earth
(East-North-Up or ENU) coordinate system, independent of probe orientation.
The location of the sampling volume is determined by the physical construction of the
MicroADV probe, and is 5 cm from the tip of the probe. The size of the MicroADV sampling
volume is determined by the sampling configuration used. The standard sampling volume is a
cylinder of water with a diameter of 4.5 mm and a height of 5.6 mm. For more information,
see Setting the Record Straight: ADV Data
Acquisition Rates & Sampling Volume Size.
The MicroADV records nine values with each sample: three velocity values (one for each
component), three signal strength values (one for each receiver), and three correlation
values (one for each receiver). Naturally, the velocity data are of foremost interest;
signal strength and correlation are used primarily to determine the quality and accuracy
of MicroADV velocity data.
5.1. Velocity
MicroADV velocity data can be reported in XYZ (Cartesian) coordinates relative to probe
orientation or Earth (East-North-Up or ENU) coordinates for systems using the optional
compass/tilt sensor. The MicroADV's velocity output data can be used directly without post
processing. The calibration will not change unless the probe has been physically damaged.
One of the most important MicroADV operating parameters is the velocity range setting.
This determines the maximum velocity that can be measured by the instrument; standard
settings are ±3, ±10, ±30, ±100, and ±250 cm/s. The user should select the lowest
velocity range setting that will cover the maximum velocity expected in a given
experiment. Instrument noise in velocity data is proportional to the velocity range
setting; higher velocity ranges have higher noise levels. The typical noise level in good
operating conditions is 1% of the velocity range when transmitting data at 50 Hz (i.e.,
each sample is ±1 cm/s when using the ±100 cm/s velocity range and a sampling rate of 50 Hz).
The MicroADV is designed to measure velocity as rapidly as possible. A single estimate
of the 3D velocity field is referred to as a ping. The MicroADV pings 200-400 times per
second. The noise in a single ping is too high for practical use, so the MicroADV averages
a number of pings before outputting a velocity sample. The number of pings averaged is set
to meet the user specified sampling rate within the range of 0.1 to 50 Hz. For example,
when sampling at 50 Hz the MicroADV will collect as many pings as possible over a 20-ms
period and output the average as one sample. An important result of the MicroADV sampling
scheme is that reducing the sampling rate decreases the noise in each sample (by
increasing the number of pings averaged per sample).
5.2. Signal Strength
Signal strength, recorded for each MicroADV receiver, is a measure of the intensity of
the reflected acoustic signal. The primary function of signal strength data is to verify
that there is sufficient particulate matter in the water. If the water is too clear, the
return signal may not be stronger than the ambient noise level of the electronics. Without
sufficient signal strength, the MicroADV is unable to make accurate velocity measurements.
In general, the MicroADV requires a minimal amount of scattering material (typically 10 mg/L) for excellent operation.
Since the return signal is a function of the amount and type of particulate matter in
the water, signal strength values can be used as an indicator of sediment concentration.
While MicroADV signal strength data cannot be directly converted to sediment
concentration, it does provide an excellent qualitative picture of sediment fluctuations.
With proper calibration, signal strength data can be used for reasonably accurate
estimates of sediment concentration.
5.3. Correlation Coefficient
The MicroADV correlation coefficient is a data quality parameter that is a direct
output of the Doppler velocity calculations. The MicroADV computes three correlation
values (one for each acoustic receiver). Correlation is expressed as a percentage: perfect
correlation of 100% indicates reliable, low-noise velocity measurements; 0% correlation
indicates that the output velocity value is dominated by noise (no coherent signal).
Ideally, correlation should be between 70 and 100%. Values below 70% indicate that the
MicroADV is operating in a difficult measurement regime, the probe is out of the water,
the signal-to-noise ratio (SNR) is too low, or that something may be wrong with the
MicroADV. In some environments (highly turbulent flow, highly aerated water), it may not
be possible to achieve high correlation values. Low correlation values will affect the
short-term variability in velocity data (i.e., increase the noise), but will not bias the
mean velocity measurements. For mean velocity measurements, correlation values as low as
30% can be used.
The MicroADV probe is available in several configurations for different measurement
needs. These variations are divided into four areas: location of the sampling volume,
acoustic sensor mounting, sensor orientation, and coordinate resolution. Probes can be
constructed with almost any combination of these configurations.
- Sensor mounting - The sensor can be mounted on a 25 or
40-cm long stainless steel stem, or a 100-cm flexible cable. The 100-cm cable
allows increased flexibility in the sensor orientation, but requires more
complicated mounting arrangements.
- Sensor orientation - The MicroADV acoustic sensor can be oriented looking down,
to the side, or up. Down is most common as it allows for easy mounting and measurements
close to the bottom boundary. Side looking is typically used in wave tanks with the sensor
mounted looking across the direction of propagation, reducing the chance of flow
interference. Up-looking is typically used for measurements near the surface, under a
layer of ice, or near the bottom of a vessel or structure.
- Coordinate resolution - The MicroADV can be built to measure either 2D or 3D
fluid flow, using 2 or 3 acoustic receivers respectively. 3D is the most common
configuration. 2D is most commonly used for very shallow water (minimum 3 cm) with a
side-looking sensor, or very narrow channels using a down-looking sensor.
Because of the remote 3D velocity measurements, the MicroADV is extremely well suited
to flow studies in boundary layers. The MicroADV can be used for detailed boundary layer
studies and direct measurement of turbulent parameters such as Reynolds stress.
Under good operating conditions, the leading edge of the sampling volume can be placed
within about 0.5 mm of a boundary. The vertical extent of the sampling volume is precisely
defined; thus, this leading edge can be placed very close to a boundary without
interference.
One significant advantage of the MicroADV is that there is no minimum measurable
velocity, with no potential for a zero offset or zero drift. The lowest MicroADV velocity
range, ±3 cm/s, will yield good results for flows down to about 0.1 cm/s. If working in
an environment with extremely low flows, the MicroADV software can be modified to use
lower velocity ranges to further improve performance.
More details about the MicroADV can be found at:
