This page is dedicated to numerous questions that we have been asked over the years and
hopefully the answers provided by our team of experts and other experts in the field of hydrology,
hydraulics etc., will be helpful to you, the user.
If you have any questions that you would like to ask, please do so and contact us and hopefully our
experienced flow experts will be able to answer them.
hopefully the answers provided by our team of experts and other experts in the field of hydrology,
hydraulics etc., will be helpful to you, the user.
If you have any questions that you would like to ask, please do so and contact us and hopefully our
experienced flow experts will be able to answer them.
Q. Why do I have to monitor flow accurately? Will it not be sufficient to just know approximately how
much the flow is?
A. Flow is an important hydrological measurement, especially for:
Q. There are many methods of flow measurements, especially velocity measurements. Which
method would give us the most accurate data?
A. The most common method of flow measurement is the Continuity Equation, Q = A*Vm where Q is
the flow, A is the wetted cross-sectional area and Vm is the average velocity across the flow. Hence to
measure flow, one must measure the level of water and the mean velocity. Level measurements are
easy, however, mean velocity measurement can be difficult and extreme care must be taken when
measuring mean velocity.
Some of the common methods to measure velocity of flow are:
doppler methods.
Note: For full-pipe applications, the electro-magnetic methods are also very accurate.
Q. What is the difference between the Continuos Wave (CW) and Pulse Wave (PW) Doppler velocity
measurement methods? Which is more accurate?
A.In continuos wave doppler measurement, signals are received from all scatterers within the entire
ultrasonic beam. They have no range discrimination. The returned signals must be evaluated with the
help of analytical methods to determine the characteristic velocity within the measurement distance.
In pulse wave doppler measurement, both, the range and velocity are determined and the signals are
received from a limited sample volume which is controlled by the transmitter. This makes it possible
to assign defined measurement windows for velocity measurements and obtain a velocity spectrum
that provides accurate measurement of the mean velocity.
PW velocity measurement is more accurate than the CW velocity measurement. Also, CW doppler
doesn't give you an accurate average velocity, while using spectrum analysis, PW doppler can obtain
accurate average velocity.
Q. How do you obtain accurate average velocity measurement using Pulse Wave Doppler
measurement?
A. PW doppler measures velocity in a defined cell (window) and reflections from particles in other
areas do not have any influence on the velocity measurement. After obtaining the velocity spectrums,
a spectral correlation can be made to obtain the average velocity. In the Pulse Spectral Correlation
(PSC) Method, 110 power spectrums (representing the distribution of velocities in the flow) are
obtained which are then further analyzed using the 5th order polynomial to obtain the average velocity
of flow.
Q. We often hear about Cross-Correlation (CC) over 16 scan layers. What is the difference between
cross-correlation and Pulse Spectral Correlation (PSC)?
A. In cross-correlation, a first scan of maximum 16 windows is opened and the velocity picture stored
in memory. The 2nd scan of another 16 windows is opened and the velocity picture is superimposed
on the first scan to get the temporal movement of the reflectors. These movements are then
correlated to obtain the time dependent movement of the reflectors, and hence the velocity of the
reflectors. Cross-correlation is made by:
In the Pulse Spectral Correlation, scans of velocity profiles are made and stored in memory as power
spectrums. In total, 110 profiles are stored, which are grouped into 11 defined groups and using a
complex Gaussian process, an Auto-Correlation is performed to obtain the average flow velocity. This
is done by:
Both methods are accurate, but better measurement is obtained using PSC method as opposed to CC
Q. Can we say then that Cross-correlation, Pulse Spectral Correlation and Profilers are the same?
A. Very loosely, cross-correlation has been referred to as Profiler by some. However, a true profiler
would be one where not only the magnitude, but also the vector of the velocity is measured. The
reason you need to know the vector of the velocity is because there are many secondary flows within
the primary flow and they greatly influence the true average velocity of flow.
Therefore, when you talk about Profilers, a profiler should be able to measure both, the magnitude and
vector of the velocity. Also, it should be able to obtain more than just 16 windows for creating a profile
as secondary flows can exist in even a very narrow window of flow.
In our opinion, you can compare the CC to PSC but not to Profiler. Profiler does more than either of
the former.
Q. I have as flow meter that is not intrusive, i.e. I can measure velocity with the sensor outside the flow
and do not have to go and clean the sensor.
A. I think the flow meter you are referring to uses Radar velocity measurement techniques. This
method is very similar to the CW doppler measurement, the only difference being that the velocity is
measured on the surface of the flow.
You have to note that this gives you only a point velocity at the surface of the flow and a correction
factor, k, should be made to account for the average velocity of the flow. This means that you will have
to perform in-situ calibrations to obtain the correction factor, k, which can then be used in:
Q = A * Vs * k
where Vs is the point surface velocity and k is the correction factor.
Note: The k values are not the same for all levels of flow, so you will have to perform lots of
calibrations at different levels and different flow conditions to obtain a series of k values and also
spend lots of time performing data analysis to correct the measured values.
Q. The radar flow meter comes with the so called k factors already integrated into the system and so I
do not have to perform field calibrations. These k values have been obtained by tests and analysis in
a hydraulics laboratory.
A. Yes, that is correct. Also, there are k values that are defined under the ISO 6416 and can be used
for estimation of average velocity. These k values have also been obtained by tests and analysis in
the hydraulics laboratory.
However, to obtain an accurate average velocity reading, a site specific correction factor, k2 is required.
Hence, you are back in the field performing your site specific calibrations.
You should also note that the velocity profiles change with silting conditions and low flows. Although
you have pre-defined k values integrated into your flow meter, these values are values obtained in
laboratory controlled conditions and cannot emulate silting going on in the field, or other roughness
factors that cause a change in the velocity profiles of flow.
If you still insist on using a non-intrusive velocity measurement device and do not want to perform
extensive field calibrations, our suggestion would be to either use SIMK finite element numerical
analysis (download article), or SoftwareQ (download brochure).
Note: With SoftwareQ, you will still have to at least perform one site velocity profile measurement.
much the flow is?
A. Flow is an important hydrological measurement, especially for:
- appropriate water management of available water;
- flood protection;
- proportioning and operation of water supply and distribution installations;
- capacity analysis, extraneous flows into collection systems, etc.;
- water billing networks;
- calibration and validation of hydraulic / hydrological models;
- creation of hydrological records, statistics, expert's certificates etc.;
- permits from regulatory agencies and reporting agencies etc.
Q. There are many methods of flow measurements, especially velocity measurements. Which
method would give us the most accurate data?
A. The most common method of flow measurement is the Continuity Equation, Q = A*Vm where Q is
the flow, A is the wetted cross-sectional area and Vm is the average velocity across the flow. Hence to
measure flow, one must measure the level of water and the mean velocity. Level measurements are
easy, however, mean velocity measurement can be difficult and extreme care must be taken when
measuring mean velocity.
Some of the common methods to measure velocity of flow are:
- electro-magnetic method - measures point velocity in the flow, around the sensor.
- continuos wave doppler methods - measures flow velocities along a line and estimates
average velocity. - pulse doppler methods - measures flow velocities along at known distances and obtains a
better estimation of average velocity than continuos wave doppler. - radar method - measures point velocity at the surface of flow.
- profiler - measures flow velocity profile using pulse doppler techniques and either correlates
the single profile with a time -shift or correlates velocity spectrums. - transit-time methods - measures flow velocities along the path. Using multi-path and cross-
path techniques, obtains average velocity with greater accuracies.
doppler methods.
Note: For full-pipe applications, the electro-magnetic methods are also very accurate.
Q. What is the difference between the Continuos Wave (CW) and Pulse Wave (PW) Doppler velocity
measurement methods? Which is more accurate?
A.In continuos wave doppler measurement, signals are received from all scatterers within the entire
ultrasonic beam. They have no range discrimination. The returned signals must be evaluated with the
help of analytical methods to determine the characteristic velocity within the measurement distance.
In pulse wave doppler measurement, both, the range and velocity are determined and the signals are
received from a limited sample volume which is controlled by the transmitter. This makes it possible
to assign defined measurement windows for velocity measurements and obtain a velocity spectrum
that provides accurate measurement of the mean velocity.
PW velocity measurement is more accurate than the CW velocity measurement. Also, CW doppler
doesn't give you an accurate average velocity, while using spectrum analysis, PW doppler can obtain
accurate average velocity.
Q. How do you obtain accurate average velocity measurement using Pulse Wave Doppler
measurement?
A. PW doppler measures velocity in a defined cell (window) and reflections from particles in other
areas do not have any influence on the velocity measurement. After obtaining the velocity spectrums,
a spectral correlation can be made to obtain the average velocity. In the Pulse Spectral Correlation
(PSC) Method, 110 power spectrums (representing the distribution of velocities in the flow) are
obtained which are then further analyzed using the 5th order polynomial to obtain the average velocity
of flow.
Q. We often hear about Cross-Correlation (CC) over 16 scan layers. What is the difference between
cross-correlation and Pulse Spectral Correlation (PSC)?
A. In cross-correlation, a first scan of maximum 16 windows is opened and the velocity picture stored
in memory. The 2nd scan of another 16 windows is opened and the velocity picture is superimposed
on the first scan to get the temporal movement of the reflectors. These movements are then
correlated to obtain the time dependent movement of the reflectors, and hence the velocity of the
reflectors. Cross-correlation is made by:
In the Pulse Spectral Correlation, scans of velocity profiles are made and stored in memory as power
spectrums. In total, 110 profiles are stored, which are grouped into 11 defined groups and using a
complex Gaussian process, an Auto-Correlation is performed to obtain the average flow velocity. This
is done by:
Both methods are accurate, but better measurement is obtained using PSC method as opposed to CC
Q. Can we say then that Cross-correlation, Pulse Spectral Correlation and Profilers are the same?
A. Very loosely, cross-correlation has been referred to as Profiler by some. However, a true profiler
would be one where not only the magnitude, but also the vector of the velocity is measured. The
reason you need to know the vector of the velocity is because there are many secondary flows within
the primary flow and they greatly influence the true average velocity of flow.
Therefore, when you talk about Profilers, a profiler should be able to measure both, the magnitude and
vector of the velocity. Also, it should be able to obtain more than just 16 windows for creating a profile
as secondary flows can exist in even a very narrow window of flow.
In our opinion, you can compare the CC to PSC but not to Profiler. Profiler does more than either of
the former.
Q. I have as flow meter that is not intrusive, i.e. I can measure velocity with the sensor outside the flow
and do not have to go and clean the sensor.
A. I think the flow meter you are referring to uses Radar velocity measurement techniques. This
method is very similar to the CW doppler measurement, the only difference being that the velocity is
measured on the surface of the flow.
You have to note that this gives you only a point velocity at the surface of the flow and a correction
factor, k, should be made to account for the average velocity of the flow. This means that you will have
to perform in-situ calibrations to obtain the correction factor, k, which can then be used in:
Q = A * Vs * k
where Vs is the point surface velocity and k is the correction factor.
Note: The k values are not the same for all levels of flow, so you will have to perform lots of
calibrations at different levels and different flow conditions to obtain a series of k values and also
spend lots of time performing data analysis to correct the measured values.
Q. The radar flow meter comes with the so called k factors already integrated into the system and so I
do not have to perform field calibrations. These k values have been obtained by tests and analysis in
a hydraulics laboratory.
A. Yes, that is correct. Also, there are k values that are defined under the ISO 6416 and can be used
for estimation of average velocity. These k values have also been obtained by tests and analysis in
the hydraulics laboratory.
However, to obtain an accurate average velocity reading, a site specific correction factor, k2 is required.
Hence, you are back in the field performing your site specific calibrations.
You should also note that the velocity profiles change with silting conditions and low flows. Although
you have pre-defined k values integrated into your flow meter, these values are values obtained in
laboratory controlled conditions and cannot emulate silting going on in the field, or other roughness
factors that cause a change in the velocity profiles of flow.
If you still insist on using a non-intrusive velocity measurement device and do not want to perform
extensive field calibrations, our suggestion would be to either use SIMK finite element numerical
analysis (download article), or SoftwareQ (download brochure).
Note: With SoftwareQ, you will still have to at least perform one site velocity profile measurement.
Hydro Vision GmbH
Gewerbestrasse 61a
87600 Kaufbeuren,
Germany
Tel: +49 83 41 966-2180
Fax: +49 83 41 966-6030
email: info@hydrovision.de
Gewerbestrasse 61a
87600 Kaufbeuren,
Germany
Tel: +49 83 41 966-2180
Fax: +49 83 41 966-6030
email: info@hydrovision.de
Hydro Vision America
10520 Yonge Street,
Unit 35B, Suite 212
Richmond Hill, Ontario
L4C 3C7 Canada
Tel: +1 905 833 0885
Fax: +1 905 833 0823
email: info@hydrovision.us
10520 Yonge Street,
Unit 35B, Suite 212
Richmond Hill, Ontario
L4C 3C7 Canada
Tel: +1 905 833 0885
Fax: +1 905 833 0823
email: info@hydrovision.us
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