Nortek Vector User manual

Comprehensive Manual2
© 2018 Nortek AS
Table of Contents
Chapter 1 Principles of Operation 7
.................................................................................................................. 8
1.1 Working Principle
.................................................................................................................. 10
1.2 Doppler Beams
.................................................................................................................. 11
1.3 Measuring Currents
.................................................................................................................. 12
1.4 Auxiliary Sensors
.................................................................................................................. 13
1.5 Measurement Volume
.......................................................................................................................................................... 13
Vector1.5.1
.......................................................................................................................................................... 14
Vectrino1.5.2
.......................................................................................................................................................... 14
Vectrino Profiler1.5.3
.................................................................................................................. 15
1.6 Coordinate System
.................................................................................................................. 16
1.7 Signal Strength
.................................................................................................................. 17
1.8 Correlation
.................................................................................................................. 18
1.9 Distance Measurement / Bottom Check
.................................................................................................................. 19
1.10 Velocimeter Accuracy
Chapter 2 Using a Velocimeter 22
.................................................................................................................. 23
2.1 Vector
.......................................................................................................................................................... 23
Technical description2.1.1
.......................................................................................................................................................... 25
Instrument software2.1.2
.......................................................................................................................................................... 26
Initial preparation2.1.3
.......................................................................................................................................................... 27
Configuring a Vector2.1.4
.......................................................................................................................................................... 28
Data Collection2.1.5
.......................................................................................................................................................... 29
Mounting and Handling the Probe2.1.6
.......................................................................................................................................................... 30
Orientation of Vector probes2.1.7
.......................................................................................................................................................... 31
Measuring Waves2.1.8
Processing Method
Wave Parameters
Deployment Considerations and Cautionary Notes
Corrections, Measurement Errors and Uncertainties
.......................................................................................................................................................... 35
Optional IMU2.1.9
.......................................................................................................................................................... 36
Data handling2.1.10
.................................................................................................................. 37
2.2 Vectrino
.......................................................................................................................................................... 37
Technical description2.2.1
.......................................................................................................................................................... 38
Instrument software2.2.2

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.......................................................................................................................................................... 38
Initial preparation2.2.3
.......................................................................................................................................................... 39
Configuring a Vectrino2.2.4
.......................................................................................................................................................... 41
Mounting and Handling the Probe2.2.5
.......................................................................................................................................................... 41
Data Handling2.2.6
.................................................................................................................. 43
2.3 Vectrino Profiler
.......................................................................................................................................................... 43
Technical description2.3.1
.......................................................................................................................................................... 44
Instrument software2.3.2
.......................................................................................................................................................... 45
Initial preparation2.3.3
.......................................................................................................................................................... 45
Configuring a Vectrino Profiler2.3.4
.......................................................................................................................................................... 46
Mounting and Handling the Probe2.3.5
.......................................................................................................................................................... 46
Data Handling2.3.6
.................................................................................................................. 47
2.4 Things to Consider
.......................................................................................................................................................... 47
Probe Check2.4.1
.......................................................................................................................................................... 49
Distance Check2.4.2
.......................................................................................................................................................... 49
Velocity Ambiguity and Phase Wrap2.4.3
.......................................................................................................................................................... 50
Extended Velocity Range2.4.4
.......................................................................................................................................................... 50
Weak Spots2.4.5
.......................................................................................................................................................... 52
Interference from Highly Reflective Bottoms2.4.6
.......................................................................................................................................................... 52
Mooring Vibrations2.4.7
.......................................................................................................................................................... 52
Acoustic Streaming (Vectrino and Vectrino Profiler)2.4.8
.......................................................................................................................................................... 53
Incorrect Speed of Sound2.4.9
.................................................................................................................. 53
2.5 Applications
.......................................................................................................................................................... 53
Mean Currents2.5.1
.......................................................................................................................................................... 54
Low Flow Measurements2.5.2
.......................................................................................................................................................... 54
Turbulence Measurements2.5.3
.......................................................................................................................................................... 54
Laboratory environments2.5.4
.......................................................................................................................................................... 55
Wave Measurements2.5.5
.......................................................................................................................................................... 57
Near Boundary Measurements2.5.6
.................................................................................................................. 57
2.6 Common Configuration Scenarios
.................................................................................................................. 59
2.7 Quality Assurance of Velocimeter Data
.................................................................................................................. 60
2.8 Common Data Analysis Scenarios
Chapter 3 Service Manual 62
.................................................................................................................. 63
3.1 Function Testing
.......................................................................................................................................................... 63
Initial Test Setup3.1.1

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Pow er
Converter
Connect cables
Start the appropriate softw are
.......................................................................................................................................................... 64
Sensor Checks3.1.2
Temperature
Tilt or IMU (Vector)
Compass (Vector)
Pressure (Vector)
.......................................................................................................................................................... 67
Noise floor3.1.3
.......................................................................................................................................................... 68
Beam Amplitude and Velocity Checks3.1.4
Amplitude
Velocity
Probe Check
.......................................................................................................................................................... 70
Test Recorder Function (Vector)3.1.5
.......................................................................................................................................................... 70
Battery degaussing3.1.6
.................................................................................................................. 70
3.2 Customer Calibration
.......................................................................................................................................................... 70
Compass Calibration (Vector)3.2.1
.......................................................................................................................................................... 71
Set Pressure Offset (Vector)3.2.2
.................................................................................................................. 73
3.3 Working Inside an Instrument
.......................................................................................................................................................... 73
Changing a Battery Pack (Vector)3.3.1
.......................................................................................................................................................... 73
Accessing the Circuit Board (Vector)3.3.2
.......................................................................................................................................................... 74
Installing a Memory Board (Vector)3.3.3
Installing a Memory Board - Pre-midlife (Vector)
Installing a Memory Board With Cable - Really Old Paradopps
.......................................................................................................................................................... 76
Installing a DC/DC (Vector)3.3.4
.......................................................................................................................................................... 76
Installing IMU sensor (Vector)3.3.5
.......................................................................................................................................................... 77
Changing the Wiring Harness (Vector)3.3.6
.......................................................................................................................................................... 77
Changing the Output Power for Auxiliary Sensors (Vector)3.3.7
Midlife Electronics
Pre-midlife Electronics
.................................................................................................................. 79
3.4 Changing a Probe
.................................................................................................................. 79
3.5 Upgrade Firmware
.................................................................................................................. 80
3.6 Erase Recorder (Vector)
.................................................................................................................. 80
3.7 Update Headfile (Vector)
.................................................................................................................. 81
3.8 Mechanical Aspects and Maintenance
.......................................................................................................................................................... 81
Mechanical Tolerances3.8.1

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.......................................................................................................................................................... 81
Instrument Care3.8.2
.......................................................................................................................................................... 82
Recommended Torque for Securing Screws3.8.3
.......................................................................................................................................................... 82
Connector Care3.8.4
.......................................................................................................................................................... 82
Cable Care3.8.5
.......................................................................................................................................................... 83
O-ring Care3.8.6
.......................................................................................................................................................... 83
Replace Desiccant Bag3.8.7
.......................................................................................................................................................... 83
Biofouling3.8.8
.................................................................................................................. 84
3.9 Cables and Wires
.......................................................................................................................................................... 84
Vector3.9.1
.......................................................................................................................................................... 87
Vectrino3.9.2
.......................................................................................................................................................... 88
Vectrino Profiler3.9.3
.......................................................................................................................................................... 88
Using Long Cables (Vector)3.9.4
.......................................................................................................................................................... 88
Harness (Internal wiring)3.9.5
Chapter 4 Integrators Guide - Light 90
.................................................................................................................. 91
4.1 Analog Inputs (Vector)
.................................................................................................................. 92
4.2 Analog Outputs
.......................................................................................................................................................... 93
Vector4.2.1
.......................................................................................................................................................... 93
Vectrino4.2.2
.................................................................................................................. 94
4.3 Synchronizing with Other Instruments
.......................................................................................................................................................... 94
Vector4.3.1
.......................................................................................................................................................... 97
Vectrino4.3.2
.......................................................................................................................................................... 98
Vectrino Profiler4.3.3
Chapter 5 Troubleshooting Guide 100
.................................................................................................................. 101
5.1 Communication Problems
.......................................................................................................................................................... 101
Test with battery and power supply5.1.1
.......................................................................................................................................................... 101
Cable Test5.1.2
.......................................................................................................................................................... 102
Serial-to-USB converter5.1.3
.......................................................................................................................................................... 102
Baud rate5.1.4
.......................................................................................................................................................... 103
Serial COM port5.1.5
.......................................................................................................................................................... 103
Break Response5.1.6
.......................................................................................................................................................... 104
Loop Back Test5.1.7
.......................................................................................................................................................... 105
Test with both RS422 and RS232 communication (Vector)5.1.8
.......................................................................................................................................................... 105
Software5.1.9
.......................................................................................................................................................... 105
Check Current Drain5.1.10

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.......................................................................................................................................................... 105
Check fuse5.1.11
.......................................................................................................................................................... 106
Check LED Indications5.1.12
.......................................................................................................................................................... 106
Checksum Error5.1.13
.......................................................................................................................................................... 106
Flash corrupt5.1.14
.......................................................................................................................................................... 107
"Calibration matrix not stored"5.1.15
.................................................................................................................. 107
5.2 Instrument Stopped During Data Collection
.......................................................................................................................................................... 107
Power5.2.1
.......................................................................................................................................................... 107
Full Recorder (Vector)5.2.2
.................................................................................................................. 108
5.3 Suspicious Data
.......................................................................................................................................................... 108
Signal Strength5.3.1
.......................................................................................................................................................... 108
Diagnostic Testing Using the Probe Check Function5.3.2
.......................................................................................................................................................... 112
Boundaries5.3.3
.......................................................................................................................................................... 112
Incorrect speed of sound5.3.4
.......................................................................................................................................................... 113
Noisy Velocimeter Data5.3.5
.......................................................................................................................................................... 113
Grounding Problems5.3.6
.......................................................................................................................................................... 113
Phase Wrapping5.3.7
.......................................................................................................................................................... 114
Mooring/mounting Vibrations5.3.8
.......................................................................................................................................................... 115
Error and Status Codes (Vector)5.3.9
.................................................................................................................. 117
5.4 Returning Instruments for Repair
.................................................................................................................. 118
5.5 Appendix A - Final Test Checklist
.................................................................................................................. 119
5.6 Appendix B - Example of Proforma Invoice

Principles of Operation 7
© 2018 Nortek AS
1 Principles of Operation
This section covers
Basic Doppler principles
Differences between the Velocimeters
Current measurement and auxiliary sensors
Measurement volumes
Measurement method and measurement accuracy
Velocimetry is the measurement of the velocity of fluids. The name "Velocimeters" belong to a
special class of high-resolution 3D instruments, that comprises the following instruments:
The Vector is a field instrument designed for measurements of rapid small scale changes in 3D
velocity, used for turbulence, boundary layer measurements, surf zone measurements, and
measurements in very low flow areas.
The Vectrino is used in hydraulic laboratories to measure turbulence and velocities in flumes and
physical models. It also comes in a "field" version that has a more rugged design.
The Vectrino Profiler is a profiling version of the Vectrino. Its strength lies in its ability to collect
data comprising of up to 30 cells that are only 1 mm in height.
Common for them is that they are typically used in difficult measurement situations such as
turbulence measurements, very slow, low energy flows, and rapidly varying flows in the laboratory or
the ocean. They can work well in areas with high shear, near boundaries, under breaking waves, and
many other situations. Pulse coherent data from single or small ensemble average measurements
are useful because of the individual measurement accuracy and low noise. In the following part, the
concept of current measurements are divided into relevant topics.

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1.1 Working Principle
The Nortek instruments measure the velocity of water by utilizing a physical principle called the
Doppler Effect. The Doppler Effect is the change in frequency of a sound wave when a wave source
moves with respect to an observer, or when the observer itself moves relative to the wave source.
You can hear the Doppler effect whenever a train passes by – the change in pitch you hear tells you
how fast the train is moving.
There are many ways to measure the Doppler Effect, each with its own advantages and drawbacks.
The Velocimeters measure velocity by transmitting a pair of short sound pulses of a specific
frequency into the water column. Part of the sound wave reflects back to the instrument, the
detected return signal will be further processed by the instrument. The reflected signal will be shifted
("Doppler Shifted") in accordance with the water velocity, which is what the processing is based on.
More details about that will follow.
The sound pulses do not reflect from the water itself, but rather from passive tracers suspended in
the water. These tracers or "scatterers" are typically zooplankton or suspended sediment that move
with the same average speed as the water, hence measured velocities corresponds to the water
flow. It is thus the moving particles that generates the change in pitch or frequency (the Doppler shift)
in the return signal that the instrument measure at.
The Velocimeters utilize a pair of short acoustic pulses with a known time lag, or separation, to
determine a Doppler induced phase shift. The use of two transmit pulses is commonly referred to as
a pulse-to-pulse coherent method. By utilizing this method the instrument is able to differentiate
relatively small differences in the velocities (corresponding to small differences in phase) and
therefore achieving great accuracy (often able to achieve velocity variance orders of magnitude lower
than standard incoherent Doppler systems) and low noise measurements. A limitation is the
maximum profiling range.
The figure below visualize how a sound wave transmitted through the central transducer (shown in
blue), hits a particle (red), which in next turn reflects parts of the sound (Doppler shifted - shown in
black) and the slanted beam which is the receiver arm which will detect the reflected sound wave.
Only one beam is shown for the sake of overview.
Figure: Transmit pulse
pair (blue), a m oving
particle (red), and the
reflected, Doppler
shifted pulse (black)
Water flows can be measured in many different ways, depending on how accurate the
measurements need to be and how often the measurements need to be taken to detect the variation
of interest. As described in the section above, the Velocimeters use sound waves and the Doppler

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© 2018 Nortek AS
effect to measure velocity fluctuations underwater, and can do so several times per second. But
exactly how does this transmission and receive of sound waves turn into velocity estimates?
Processing Technique
The phase difference between two transmit pulses is the basis for estimating current velocities using
the pulse coherent processing technique.
Figure: Tw o pulses are transmitted by the central transducer, w ith a time lag. The difference in phase
betw een the tw o transmit pulses is used to calculate velocity. The reflecting particle can be seen in
green, w ith a vector indicating its speed and direction
The phase difference between the two reflected pulses is a direct measure of the velocity, and can
be expressed as
Here, V is the current velocity, ∆φ is the phase difference, F_source is the transmitted frequency
and ∆t is the time difference between two consecutive pulses ("Lag"). The V is scaled with the speed
of sound in the liquid (C).
There are some aspects with the pulse coherent method that affects what the instrument is able to
measure.
The Doppler phase shift is computed using the covariance method. By computing the complex
covariance of the two return signals, the Doppler phase shift can be calculated by taking the four
quadrant arctangent (atan2 in most programming languages) of the real and imaginary parts of the
covariance function. This restricts solving the phase difference within the range of [-π, π]. The
calculation of φ is beyond the scope of this manual, but the impact of using this method is
important. It introduces the problem of ambiguous determination of the phase shift; if the phase
difference goes outside the range of ±π, the measurement uniqueness is lost, something known
as Velocity Ambiguity. For example, a sine wave with phase -π looks the same as a sine wave
with phase π. Read more about this here; Velocity Ambiguity and Phase Wrap, but the take home
is that there is a maximum velocity the instrument is able to measure.
The size of the lag (∆t in the equation above) determines the maximum unambiguous velocity that
can be measured. Longer lags have lower maximum velocities, while shorter lags have higher
maximum velocities. Conversely, longer lags will typically have higher velocity accuracy (lower
noise levels) than shorter lags, but potentially degraded signal correlation due to particle advection

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and turbulence. It is therefore important to set the time lag properly. Information on what you need
to think about when setting this parameter can be found in the Using a Velocimeter chapter.
Near boundary measurements are prone to interference. This may happen if the first pulse is
reflected from a boundary and reach the sampling volume on its way back at the same time as the
second pulse is in the sampling volume. This is referred to as a Weak Spot.
1.2 Doppler Beams
All Acoustic Doppler Velocimeters send out short acoustic pulse pairs from the transmitter (center)
element. When the pulses travel through the focus point for the receiver beams, the echo is recorded
in each of the acoustic receiver elements and the phase shift between two consecutive is found. The
phase shift is then processed to find the actual velocity of the water, as described in the previous
section. The instrument samples on all three of four receivers simultaneously.
The Doppler shifts measured at the three or four receivers provide estimates of flow velocity along the
different beam directions, which are then combined geometrically to obtain the three orthogonal
components of the water velocity vector V (illustrated by the blue arrow in figure below). The
Velocimeters are in this way able to calculate 3D water velocity in the sampling volume by knowing
the exact orientation of the beams.
Figure: The blue arrow indicates a positive velocity.
The Vector, which is shown in the figure above has three receiver arms, while the Vectrino and the
Vectrino Profiler have four receiver arms. Because of the arrangement of the receiver arms, each pair
(composed of two opposing receivers) can measure the horizontal velocity component and the
vertical velocity component. Z1 and Z2 are the vertical velocity estimates associated with receivers 1
& 3 (X) and 2 & 4 (Y). For the standard downward looking probe, Z1 and Z2 are thus independent
measurements of the vertical velocity. Note that the side-looking probe does not have a secondary
measurement of the vertical velocity. Z1 will be the correct vertical velocity and Z2 should be output
as zero.
All Velocimeters go through a probe calibration procedure during its production. This to know the
exact geometry of the probe, and this information is saved in what we call head matrices. As long as
the sensor heads are not physically deformed, the head matrices will, consequently, remain the
same. This also means that the calibration only needs to be done once, and there is no need to re-
calibrate the probe again - unless it has physically been deformed.
A bistatic system
Bistatic geometries have been used with intersecting narrow beams to produce a measurement
volume in the intersection zone. A bistatic system is one where the transmitter and receivers are
physically separated; the transmitter generates sound along the vertical beam, and the receivers
listens for it. This results in motion along two axes, the component of motion of the object in the
direction of the transmitter and the component of motion in the direction of the receiver. The motion

Principles of Operation 11
© 2018 Nortek AS
can be considered as proportional to the rate of change along the bisector of the transmit and receive
beams (indicated by the blue arrow in the figure below). Knowing the relative orientation of the
bistatic axes and the receivers makes it easy to calculate 3D velocities. The receive beams are
slanted 30°, thus the angular bisector is 15° away from the transmit beam.
Figure: The transm it/receive beam pair is sensitive to
velocity in the direction of the angular bisector betw een
the beam s.
The Z (vertical) and X- and Y (horizontal) velocity limits are tied fundamentally to the geometry and
are not independent. The conversion from beam to XYZ coordinates has Z in the direction that the
central transducer is pointing and XY is always orthogonal to Z. The "Vertical" and "Horizontal"
velocity range is reported in the Deployment planning dialog. The reason that the Vertical range is so
much lower than the Horizontal velocity range is because of the angle used by the passive receivers
(30 degrees). This makes the projection of the passive beams onto the horizontal plane about ~1/4
(sin(15)) (so the maximum horizontal velocity measurable is ~4x the maximum beam velocity) while
the projection of the passive beams onto the vertical plane is 0.96 (cos(15)) (so the maximum vertical
velocity is pretty much the same as the maximum beam velocity). Consequently, the Z-velocity
component yields a lower measurement uncertainty.
In other words, for best measurement, you should always have the largest component of speed in
the horizontal plane from the point of view of the transducers. On another note, even if you do reorient
the probe, when operating at that current, you will likely still have to watch out for wraps in the
velocities if there are any local eddies occur which result in the beam ambiguity velocity being
exceeded.
1.3 Measuring Currents
The processing technique described above is used because the measurement of a phase shift is
more accurate than direct Doppler frequency determinations and provides pulse coherent systems
with their characteristic low noise measurements. The instrument reports the phase shift converted
to an along beam velocity by utilizing the speed of sound in water and the velocity data is output in
m/s.
The user can specify which coordinate system to present the velocity data in. The raw velocity
measurement is a vector in the direction along each of the three or four beams, which is referred to
as beam coordinates. Beam coordinates can be converted to a Cartesian coordinate system (XYZ)
by knowing the beam orientation. Furthermore, the flow can be presented in Earth normal
coordinates (ENU- East, North and Up). In order to get the information referenced to earth
coordinates (ENU - East, North, Up) it is therefore necessary to detect the instrument’s orientation in
space. Attitude sensors, such as magnetometer and tilt are therefore used to aid in the
transformation needed to correct for the instrument’s attitude and motion. The definitions of the
coordinate systems are presented here: Coordinate System

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The decision on which coordinate system to use should be based on what the aim with the data
collection is. When handling data, XYZ or ENU coordinates are the most practical for a user. Beam
coordinates are used when dealing with ambiguity (phase wrapping) and higher order turbulence
calculations, as these are the "rawest" format.
Along with determining a phase shift (and then the velocity vector), the instrument will also typically
compute the speed of sound, measure the return signal strength, and measure the attitude sensors
such as pitch, roll, and compass heading (if equipped). For more about this; check the auxiliary
sensors section.
To assure high quality velocity data from a Velocimeter, it is important to configure it properly. The
specifics for each instrument version is described in Using a Velocimeter.
1.4 Auxiliary Sensors
Along with determining a phase shift, the instrument will also typically compute the speed of sound
(needed to change the phase shift to an actual velocity) and measure the return signal strength
(reported in arbitrary units as counts and output as the variable Amplitude). Vectors also measure
the attitude sensors such as pitch, roll, and compass heading (if equipped) to aid in the last step in
processing; coordinate system transformation.
Pressure (Vector)
The pressure sensor measures the hydrostatic pressure and reports in units of dBar. Vectors
sample pressure at the same rate as velocity.
The instrument do not set the pressure offset automatically, so this must be done using the software
right before deployment to corrects for local atmospheric pressure. For more about this, check out
the Service Manual. Note that a Nortek pressure sensor cannot measure negative values.
Temperature
The instruments use the speed of sound to convert time to distance and Doppler shift to velocity, and
since speed of sound depends on temperature, the data from the temperature sensor is of vital
importance. The temperature is measured by a thermistor embedded in the probe and reported in
degrees Celsius. The instruments sample temperature every second. The time response is about 10
minutes for the Vector, 5 minutes for the Vectrino and the Vectrino Profiler.
Magnetometer and Tilt (Vector)
The Vector compass is a system including a 3-axis magnetoresistive magnetometer, a liquid tilt
sensor, algorithms to convert data from these sensors into heading and instrument tilt, and a
calibration algorithm. The magnetometer measures the magnitude of three components of earth's
magnetic field, and the tilt sensor measures two components of tilt and detects up/down orientation.
The Vector (with compass) combines this information to compute the instrument's tilt and heading,
then uses the tilt and heading to correct measured velocities to ENU coordinates.
Tilt sensor:
The tilt sensor measures the orientation of the instrument and comes in two versions; vertical
orientation and horizontal orientation. The tilt sensor is calibrated within +/- 30º, but the accuracy is
significantly better for smaller tilt angles because of the technology used to measure tilts.
If an instrument with vertical orientated tilt sensor is to be mounted horizontally, the tilt sensor data
will be un-useable and the data would need to be collected in XYZ or BEAM coordinates. It is
possible to change the orientation of the sensor, but the instrument would need to be returned to
Nortek. In addition to the physical replacement of the instrument would need to go through
calibration.
The tilt sensors measure tilt relative to earth gravity. If the instrument is moving (e.g. when mounted
on a buoy), the tilt sensor cannot differentiate the acceleration from the buoy from the acceleration of

Principles of Operation 13
© 2018 Nortek AS
gravity. We therefore offer the Vector with an integrated IMU (inertial measurement unit) which
operates over a larger tilt range and can be used to correct measurements for motion contamination.
This sensor is intended to help understand how the Vector's own motion influences the
measurements. The simple configuration allows one to basically record motion data simultaneously
with the Vector velocity data. Physically, the IMU is incorporated in the Vector by simply using the
magnetometer contact interface. This means the standard tilt and magnetometer are removed and
replaced with the IMU, so there is no standard Nortek compass system with the IMU integrated.
More about this sensor can be found here: Optional IMU.
Magnetometer:
Magnetoresistive (MR) magnetometers sense magnetic fields with thin strips of a metal alloy whose
electrical resistance varies with a change in applied magnetic field. These sensors have a well-
defined axis of sensitivity. The Vector's compass includes three MR sensors, one each for X, Y and
Z components.
Each compass system is calibrated at the factory to quantify the characteristic response of the
individual components and of the system as a whole. When it leaves the factory, each system can
measure its tilt and the direction of its magnetic field vector accurately, anywhere in the world.
Note: Users disturb the magnetic field near the instrument when they deploy. Adding a battery pack
and mounting the instrument with deployment hardware adds magnetic materials that change the
magnetic field measured by the instrument. The compass calibration procedure quantifies this
magnetic disturbance, and the instrument's compass algorithm then corrects for it to obtain accurate
heading.
1.5 Measurement Volume
The Velocimeters use separate transmit and receive beams and are therefore referred to as bistatic
sensors. The measurement volume is defined by the intersection zone of the three or four slanted
receivers and the central transmitter.
1.5.1 Vector
The Vector measurement volume is defined as the area where the beams intersect each other, that
is, 157 mm from the transmitter. The transmit transducer sends a short pulse that covers only about
4 mm vertically (user selectable), and the receivers listen to an echo that corresponds to about 14
mm vertically (also user selectable). Since the Vector use three receivers, all focused on the same
volume, it obtains three simultaneous velocity measurements from that very volume.
Figure: The Vector beams intersect each other at a know n
distance from the center transducer, defining the
measurement volume.

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1.5.2 Vectrino
The Vectrino measurement volume is defined as the area where the beams intersect each other. For
the Vectrino that is 5 cm from the transmitter, for the Vectrino field probe, it is 10 cm. The transmit
transducer sends a short pulse that covers 3-15 mm vertically (user adjustable), and the receivers
listen to an echo that corresponds to this volume (see figure below). The diameter of the volume is 6
mm.
The Vectrino side-looking probe (figure below, middle) has two horizontal beams and two slanted 65°
from the vertical. The two horizontal beams measure the U and V velocity. This means that the data
from the two slanted beams are not required when measuring horizontal velocity, and the side-
looking probe then act as a 2D system. This means that this probe can be used in really shallow
water applications, as only the two horizontal beams need to be submerged to get 2D velocities.
Side-looking probes do not have the problem of weak spots, so they are a little more robust to
operate, especially in high flow environments. Due to the design, the side-looking probe has higher
instrument noise in the vertical than in the horizontal.
The Vectrino beams intersect each other at a know n distance form the center transducer, defining the
measurement volume. The Vectrino to the left is the dow n-looking version, the one in the m iddle is a so-
called side-looking proble, and the field probe is presented to the right.
1.5.3 Vectrino Profiler
The mechanical and acoustic characteristics of the Vectrino and the Vectrino Profiler are identical;
both employ the same probe assembly and transducers. However, the Vectrino Profiler provides for
profiling over a 30 mm range interval thereby allowing measurement of the spatial structure of the flow
(see figure below).
The figure above shows how the beams intersect each other 50 mm from the transmitter. The
measurement profile is defined by this intersection and by range gating in time. Four passive
transducers, angled at 30° towards the center surround the central active transducer, allowing 3D
velocity measurements. With a profiling range up to 30 mm, a usable profiling region approximately
40-70 mm in height from the central transducer is provided. The Vectrino Profiler pulse intervals are
longer than the Vectrino because of the need to sample data from farther away.

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© 2018 Nortek AS
1.6 Coordinate System
You may specify the preferred coordinate system in the Deployment Planning menu of the
instrument specific software. The velocity measurement is a vector in the direction of the transducer
beam, which we refer to as beam coordinates. Beam coordinates can be converted to a Cartesian
coordinate system (XYZ) by knowing the beam orientation. Furthermore, the flow can be presented
in Earth normal coordinates (ENU - East, North and Up) if the instrument is equipped with a
compass and tilt sensor.
The coordinate systems are defined as follows:
In Beam coordinates, a positive velocity is directed in the same direction as the beam points.
Beam 1 is defined as the arm with the black / red marking. If the marking is missing, beam 1 can
be identified as the arm opposite of the engraved serial number on the probe. Beam velocities are
not corrected for probe geometry.
In XYZ coordinates, a positive velocity in the X-direction goes in the direction of the X-axis arrow.
The X-axis points in the same direction as beam 1, and newer models have the direction engraved
in the probe endbell itself. Use the right-hand-rule to remember the notation conventions for
vectors. Use the first (index) finger to point in the direction of positive X-axis and the second
(middle) finger to point in the direction of positive Y. The positive Z-axis will then be in the direction
that the thumb points. Remember that XYZ coordinates are relative to the probe head and
independent of whether the instrument points up or down. For instruments with 4 receiving
transducers, an extra Z velocity is outputted (X, Y, Z1 and Z2).
In ENU coordinates, a positive east velocity goes toward east. This is also a right-handed
orthogonal system.
ENU and XYZ coordinates are the most practical when handling data. Beam coordinates are
primarily useful for higher-level turbulence calculations and for dealing with phase wrapping issues.
Figure: Coordinate systems. From left: Beam coordinates, XYZ coordinates and ENU coordinates
The figure below shows an assortment of Velocimeters probes and the definitions of beam and XYZ
coordinates.

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Figure: The Vectrino/Vectrino Profiler and Vector XYZ coordinate systems and beam num bering as
defined relative to the probes. Beam 1 of the Vectrino has a red m arking, a black marking defines beam 1
for the Vector (not show n here). For the Vector, beam 1 is also the arm opposite of the engraved head ID.
Beam 1 point in the direction of positive X-axis. The Y direction can be found in accordance w ith the
right-hand rule.
For the interested reader: For the Vector probe (see illustration third to the left, lower part), one may
notice that the X component is predominantly measured by beam 1 with nearly equal contributions
from beam 2 and beam 3. This makes sense since the XYZ coordinate system is aligned with X
pointing along one receiver arm. Beams 2 and 3 are at some angle α to the X-axis and measure a
component of X proportional to cosα. For the Y-component, beam 1 contributes to zero (or very near
zero) because the Y-component is perpendicular to beam 1. Finally, the Z component is an equal
combination of all three beams since the Z-axis is aligned with the central transducer and each
beam is at the same angle to the Z-axis. Note that this concept applies to the other probes as well.
1.7 Signal Strength
To be confident about the quality of the data and allow proper calculation of the velocities, the
received echo (reflected by particles) need to be over a certain level. Signal strength (amplitude) is a
measure of the magnitude of the acoustic reflection from the water, and is a function of type and
amount of particles in the water. If the echo is weak, the calculation will be statistically "noisy" and
the velocity data will show significant short-term variability. Signal strength is outputted by the
instrument and can be used to assess data quality.
The signal strength is accessed as raw signal amplitude (using dimensionless unit called Counts) or
as a "Signal-to-Noise ratio", SNR (in dB).
Counts
For signal strength, Nortek frequently make use of the word counts, which obviously seem to be
connected to the decibel term, but how and why?
Inside the velocimeters there are circuitries for the amplification of input signals. The number of
counts is an indicator of how much gain we must apply to a signal – the less gain needed, the
higher the input signal level is. The number of counts is inversely proportional to the logarithm of the
gain setting, which means that a higher number of counts reflects a need for less amplification than
a lower number of counts will do.
Strong signals have a higher noise immunity and they correlate much better than weak signals do.
For the Vector, 1 count ~ 0.4– 0.45 dB.

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SNR
The Signal-to-Noise ratio (SNR) is defined as
SNR is displayed for each receiving beam. It is a measure of the level of the signal with reference to
the background noise level. Strictly speaking, we are unable to measure the signal without the noise
present, so Amplitude_signal should read Amplitude_signal+noise. However, for SNR values in the
magnitude applicable to typical Velocimeter situations, the difference is negligible.
The particles that is necessary to generate reflections of the acoustic signal can be naturally
occurring suspended sediments (rivers, ocean, sand-filled or polluted model basins), bubbles
entrained from pump systems (often the case in flumes), or artificial ("seeding").
When measuring in basins or flumes, seeding is often needed. If practical, we recommend that
stagnant, clear basins or flumes are seeded with hollow spheres that have a density close to that of
water and a size around 10 µm. Particles that are close to neutrally buoyant will remain in
suspension without additional stirring and are ideal for low-flow experiments. The choice of size is
motivated by the relatively strong echo that these particles generate per unit of concentration.
Smaller particles (e.g. 1 µm particles used to seed Laser Doppler systems) are not recommended
because the required concentration is quite high.
When collecting raw data (e.g. at 25 Hz), we recommend an SNR that is consistently above 15 dB.
When collecting mean data (for example, 10 second averages), we recommend an SNR of at least 5
dB. If the SNR remains low even after seeding, try to clean the receive and transmit transducers by
gently rubbing the surface with a finger. This will clean off residues or bubbles that may have
collected and are blocking the sound. Take a look at the Troubleshooting Guide for more about how
to increase SNR.
The determination of the quality of the data is the primary use of the signal strength, for the Vectrino
Profiler it also determines the range over which the instrument can measure velocity accurately. It
may also be used to retrieve information about the suspended sediment or the concentration of the
scattering materials. Note that different types of scattering elements produce different echoes, and it
is difficult to distinguish between conditions with few, relatively big scattering elements or many
small elements. For users interested in sediment concentration estimates it is recommended to take
a look at this subject in the Frequently Asked Questions (FAQ) on the Nortek web.
Figure: Signal to Noise Ratio from one Vectrino Profiler beam. A fixed boundary can be seen at about 6cm
from the transm itter
1.8 Correlation
Correlation is a measure of the similarity of the two pulse echoes being measured. Zero correlation
means nothing at all is similar between the two echoes, and a correlation of 1 means the two echoes
are identical. The number reported by Nortek instruments is a normalized correlation value and will
always lay in the interval 0-100%. The aim is high correlation because it gives confidence the system
measured the two pulses it originally sent out and is determining a valid phase shift. In practice we

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will never see correlations of zero because of noise due to electronics, temperature fluctuations, and
other factors that will always correlate above zero.
Discarding measurements with correlations lower than a threshold significantly reduce the variance
of the data set, which is generally the goal of most data screening operations. Removing low
correlation measurements is still a good idea because correlation is a strong indicator of data quality
in the sense of a valid Doppler phase shift determination. However, high correlation is not always
indicative of a valid measurement of the flow. It is also important to note correlation is tied to a
specific beam velocity measurement, not to an orthogonal coordinate direction such as X, and any
screening should take this into account.
In the early days of the acoustic Doppler velocimeter, if a user did some tests and determined
correlations were above ~70% then the instrument was considered to be generating good quality
data. Many users still use this number for screening out bad data, although a generalization to some
universal value is unwarranted and a close examination of the dataset is the best way to set a
correlation threshold (if any) for discarding bad data points.
Figure: Correlation from one Vectrino Profiler beam. This plot corresponds to the SNR figure in the
previous section. Note how the boundary show s up in this plot
1.9 Distance Measurement / Bottom Check
The distance check (also referred to as Bottom Check) is used to measure the distance to the bed
from the central transducer. The return from this pulse is listened to over some time to obtain an
amplitude profile. The amplitude profile is checked for peaks, and an interpolation scheme is used to
locate the peak location based on the largest amplitudes recorded.
The Distance Check function is available in the Vectrino Plus firmware, the Vector and the Vectrino
Profiler, but not in the Vectrino standard firmware.
Vector: The Vector reports the distance to the boundary at the start and end of each burst, or at the
start when running the instrument in Continuous mode. That means that it is possible to do very
short bursts and get almost simultaneous measurements (requires Firmware V1.20 or higher).
Vectrino Plus: The Vectrino+ firmware comes with a separate boundary distance mode (10 Hz),
where the distance to the boundary is measured. The distance measurement is not done at the
same time as the velocity measurements. The Boundary Distance functionality is typically used in
experiments where bottom changes as a function of time are needed, or when the Vectrino+ is
mounted on traverse systems for bottom topography measurements.
Vectrino Profiler: The Vectrino Profiler has an interleaved boundary distance functionality. In other
words, you get both velocity profiles and distance to the bottom at the same time. The reasons for
this are both that it can be done technically, and also that the problem of weak spots is much more
challenging with the Vectrino Profiler than with a single point instrument. Hence, in order to operate
the Vectrino Profiler properly we depended on interleaving velocity and distance measurements, such
that the time lag between the two pulses in every pulse pair can be adjusted adaptively.

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Measurement range:
The maximum and minimum distances that can be measured are approximately as listed below, but
note that it will depend a lot on the hardness of the bottom.
Vector: 4 cm - 45 cm
Vectrino+: 1 cm - 35 cm
Vectrino Profiler: 20 mm - 2 m. The Vectrino Profiler can be configured where to start collecting
bottom check data. Refer to SW user guide (available within the software) for more details.
We provide all of the data used to determine the peak location if users need to create their own
bottom detection algorithm. This is available as the probe check data in the Vector and Vectrino, and
in the Bottom Check fields in the Vectrino Profiler data structure.
Quality (relevant for Vectrino Plus)
Quality is a number indicating how valid the distance measurement is. Above 100 should be a good
distance measurement. It is always good to have an independent estimate of position though, so we
usually recommend using the probe check feature of the Vectrino software to get a ballpark number.
More information about the Distance Check can be found in the Using a Velocimeter chapter.
1.10 Velocimeter Accuracy
The word accuracy is often used loosely to cover some combinations of bias and short-term
uncertainty. Bias and short-term uncertainty are different, and they have different ramifications for the
results of your data collection. Here is the difference:
Bias is error that remains after taking long-term averages.
Short-term error is the random error of individual measurements, which can be removed by
averaging.
These errors have different sources and different consequences for the results. The following will
address each of these errors separately.
The velocity measurements are the average of many velocity estimates (called pings). The
uncertainty of each ping is dominated by the short-term error. This error is uncorrelated from ping to
ping, so by averaging together many pings, the measurement uncertainty is reduced. The short-term
error depends partly on internal factors, such as the size of the transmit pulse, the measurement
volume and the beam geometry. Beams parallel to the dominant flow will have smaller short-term
errors than beams at a steep angle relative to the flow.
The instrument’s software predicts errors based on the short-term error of a single ping and the
number of pings averaged together. Averaging multiple pings reduces errors according to the formula:
Where σ represents the standard deviation and N is the number of pings you average together. Note
that the software predicts only the instrumental error.
In many situations, external factors such as the environment itself dominate the short-term error.
This is true near an energetic surface and in turbulent flow such as boundary layers and rivers. In
situations like this, your data collection strategy should take into account the nature and the time
scales of the environmental fluctuations. Here are two scenarios:
Example: Waves. When measuring mean velocities in the presence of waves you should sample
velocity at roughly ¼ the interval of the dominant wave period, and you should sample through 6-10

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wave cycles.
Example: Turbulent flows. In boundary layers, a rough rule of thumb is that the root-mean-square
(RMS) turbulent velocity is 10% of the mean velocity. If, for example, your mean velocity is 1 m/s,
you could estimate turbulent fluctuations to be 10 cm/s. Obtaining 1 cm/s RMS uncertainty would
require at least 100 pings.
When averaging several pings to reduce the error, there is a difference between the resulting “mean
current” and the measured current. This deviation from the actual current measurement is called bias
. Velocimeter bias depend on the geometry of its probes and the stability of its internal oscillators.
Over most of the velocity range, bias can be measured in terms of an offset and a scale-factor error.
Offsets would appear as a non-zero velocity in still water. If probe geometry were entirely fixed and
repeatable, then we could determine the velocity scale factor based entirely on dimensions,
frequencies and our physical model. In practice, small variations in probes require that probes are
calibrated individually in the factory.
A factory-calibrated velocimeter should have a scale-factor bias that is less than 1% of the measured
velocity. A velocimeter will retain this accuracy as long as its probe remains unbent.
Bias increases near the extremes of a velocimeter's velocity range because velocities that would be
measured beyond the extremes wrap around to the opposite end of the range. This can be avoided
by keeping the maximum observed speed toward the middle of the velocimeter's maximum range.
The configuration parameter to adjust is the Velocity Range.
Uncertainty, variance and standard deviation
Uncertainty is often measured in terms of measurement variance or standard deviation. These two
are related:
variance = (standard deviation)^2
Standard deviation quantifies the amount of variation of a set of velocity values, around the true value
of the velocity. For example: Imagine taking measurements of an unvarying velocity. The best
estimate of the true velocity is the mean of the measurements, and the uncertainty of each of the
measurements is the standard deviation of the collection of measurements.
Averaging
When averaging a collection of velocity measurements, the average is a better estimate of the true
velocity than each individual estimate. The uncertainty of the average is reduced according to:
variance(mean) = variance(individual measurements)/N
std dev(mean) = std dev(individual measurements)/N^1/2
In other words, by reducing the variance of individual measurements by a factor of 2, you can collect
data twice as fast.
Measurement Load (Vector)
A 100% measurement load means that the instrument pings as fast as it is able to. Consequently, a
measurement load of 50% means that the instrument pings at half that rate. From signal theory we
know that the more pings there are within an averaging period, the better the estimate of the true
value we are measuring will get. By increasing the measurement load, the precision in the velocity
measurements increase, but alas at the expense of battery consumption. By decreasing the
measurement load it is the other way around.
The measurement load for the Vector is adjusted according to the velocity range selected.
Variance and Noise Level