This article focuses on how High Resolution mode is used by Gen1 Aquadopps and Vectors. These systems are designed to produce detailed vertical profiles of water velocity and turbulence.
General information on high resolution measurements can be found here: HR mode.
For detailed guidelines and best practices for using HR mode on our Gen1 instrument you can also consult this paper: Practical Primer for Pulse-Coherent Instruments
How to avoid phase wrapping?
To increase the maximum unambiguous velocity, a few things can be done. For a velocimeter, increasing the nominal velocity range in the Nortek Vector Deployment planning software will change the horizontal and vertical velocity ranges. These values should be above the expected flow velocities. For the AquaPro HR, keep an eye on the Profile Range parameter on the right-hand side of the Deployment Planning configuration window. To change this parameter, the user must play with the pulse distance. As this is not always an option, it can be easier to select the Extended Velocity Range (EVR) checkbox on the Current Profile section of the software.
When the EVR is enabled, it sends a pulse with a pulse distance of 1/3 of the lag shown in the Pulse Distance field. Using the data measured at 1/3 of the distance into the full profile the instrument makes an initial estimate of the phase shift, which is then used to refine the standard pulse distance phase shift estimates by accounting for any phase wrapping. If the phase shifts from the two estimates don’t agree, the standard pulse distance estimate is increased to match the EVR estimate. This adjustment is continued for all bins in the profile. The velocity range product for EVR operation is 0.9 m^2/sec, compared to 0.3 m^2/sec for standard pulse coherent operation.
How to calculate time lag to calculate the maximum velocity range?
Time lag (\( \Delta t \)) is specified in two fields for the Vector, Vectrino and AquaPro HR. In an exported ASCII header file (*.hdr) these are labelled System 38 and 39 (under “User Setup”). Note, however, that these are listed as “not used” in the Integrator’s Manual, and in most instruments, they are not. For pulse-coherent instruments, they contain the lag information. The smallest non-zero value of these two lags is used to calculate the velocity range. This information is given in number of counts.
To convert these lags from digital counts to time, we use a simple linear scaling that varies according to the instrument:
Vector: timeLag = lagCounts / 480e3
Vectrino: timeLag = lagCounts/1e6 Hz
AquaPro HR (2 MHz): timeLag = lagCounts/111,111 Hz
AquaPro HR (1 MHz): timeLag = lagCounts/55,555 Hz
where the scaling factor is the internal sample clock timing.
Let's use an example: in a Vector (6000 kHz head frequency) *.hdr file, System 38 is 50 and System 39 is 130 counts, sound speed was measured as 1470.7 m/s.
For the vector, timeLag = 50/480e3 = 1.042e-4
Velocity range will be:
| \begin{equation} VR= \frac {1470.7 [ms^{-1}]}{4 \cdot 6000 \cdot 10^3 [Hz] \cdot 1.042 \cdot 10^{-4} [s]} \end{equation} |
(1) |
which leads to:
| \begin{equation} VR= \frac {1470.7 }{2500.8}=0.59 [ms^{-1}] \end{equation} |
(2) |
As the Velocity Range refers to the beam velocity, the final vertical velocity range will be:
| \begin{equation} \text{Horizontal Velocity Range} = \frac{0.59}{sin(15 [^{\circ}])} \sim 2.3 ms^2 \end{equation} |
(3) |
| \begin{equation} \text{Vertical Velocity Range} = \frac{0.59}{cos(15 [^{\circ}])} \sim 0.6 ms^2 \end{equation} |
(4) |
Details are discussed in this FAQ: Pulse-coherent measuring methodology
What is the *.hr2 file from my AquaPro HR?
The *.hr2 file is generated when you enable the option for extended velocity range (EVR) in the instrument configuration. The velocity range is extended by placing a single measurement cell 1/3 into the profile. The *.hr2 file shows the measured data in this cell. Since then a shorter lag is also used, this cell gives us the velocity at this point with a factor 3 higher velocity range than the regular profile. We then use this velocity to resolve the velocities of the whole profile. You should normally not need to look at the data in the *.hr2 file since we have used that information already in the profile output. But if you have a data set that looks suspicious, you may want to check that the correlation in the *.hr2 file to see if there is a problem with the measured data from the second (higher velocity) lag.
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