2. Transducers, US beam characteristics
3. Ultrasound, windowing function, resolution
6. Test medium, through-the-wall measurement
1. Commercial features
Is it possible
to deliver UVP Monitor for other frequencies than 2 - 4 - 8 MHz?
Yes, UVP Monitor
can be also delivered with the 1 - 2 - 4 MHz frequency set, and on special request
with 0,5 - 1 - 2 MHz frequency set. Low working frequencies enable measuring
of higher values of on-axis velocity component on greater distance, but at the
same time decrease spatial resolution of measurement.
Is simultaneous
measurement of UVP Monitor and PIV or LDA viable?
UVP Monitor uses ultrasound, and PIV (Particle Image Velocimetry) or LDA
(Laser-Doppler Anemometry) uses light. Both methods therefore do not interfere,
and can be used simultaneously.
What is the
difference between conventional ultrasonic flow-meter and UVP?
The typical ultrasonic flow-meter uses the principle of transit time of
ultrasonic pulse which is carried by the flow. It measures the time difference
from the sound speed between transmitter and receiver with some fixed distance.
This method measures an average velocity in a measured field like a pipe so
that some kind of theoretical assumption is needed to evaluate an integral of
the velocity distribution over a flow channel. This requires some strict conditions
for the flow to be measured such as an entry length upstream the measured position.
On the other hand, UVP measures the velocity profile directly so that the evaluation
of integral is direct, requiring no strict measuring conditions. Moreover, the
theoretical assumption used for the conventional devices are normally valid
for the stationary flow. This eliminates the possibility of measuring a transient
flow-rate after pump start or valve operation. UVP can measure the velocity
profile instantaneously, and thus the transient measurement is also possible
and accurate.
Is there any
UVP Monitor working near my lab?
Probably it is. Please check Met-Flow contact list
to get in touch with the closest commercial representative, or contact Met-Flow
directly to get any specific references.
How much does
UVP Monitor cost?
UVP Monitor price - very roughly, depending on a Model - is between US$
29,000 and US$ 35,000. Price of transducers is from US$ 600 for normal types
to US$ 2,000 for elevated temperature special types. Please note that above
prices are indicative only and are subject to changes; we recommend you ask
your Agent for an exact quote.
2. Transducers, US beam characteristics
Is it possible
to submerge transducers into a large mixing vessel?
Yes, this is possible. All transducers are watertight, and can be temporarily
submerged or installed into vessel as stable measurement installation.
What is maximum
applicable RF voltage on transducer?
During operation, the maximum RF voltage on transducer is 150 Vpp, but
the transducer can withstand 200 Vpp.
What is maximum
transducer cable length?
Transducer cable carries small signals during receiving phase, and it is
not advantageous to use superfluous length of cable in environment with strong
electromagnetic interference (EMI). Without any specification transducer cable
length is 4 m, but the shorter the better. You can get cable length up to 10
m for the asking when ordering, but for lengths above 4 m Met-Flow cannot unconditionally
guarantee transducer performance in environments with strong EMI.
Less problems are met with low transducer frequencies while one has to be careful
with higher frequencies (4 and 8 MHz).
When considering the relay box configuration the previous applies obviously
to the total cable length from UVP instrument to transducer.
Is it possible
to extend or shorten transducer cable?
Cable is integrally molded into transducer, and this is why it is not possible
to extend the cable. But you can shorten the cable from the BNC connector end.
What is ultrasonic
beam divergence?
The beam divergence is determined by the ultrasound wave length and the
initial beam size. For most working frequencies and standard transducers, beam
divergence is approximately ±5°.
3. Ultrasound, windowing function, resolution
What is the
smallest distance of measurement from transducer?
It takes several microseconds to UVP Monitor to switch from transmitting
to receiving mode. This time solely due to electronic switching makes the smallest
measurable distance approximately 5mm from transducer face.
Is it possible
to measure flow in channels smaller than 90mm?
Yes, without any problems. UVP Monitor always stores a 128-point profile.
As the measuring volume length is 0.74 mm for a standard 4 cycles pulse of 4
MHz the smallest distance between these sampling volume is 0.74 mm for a non-overlapping
configuration, and thus the smallest measurable full profile length is in fact
90 mm.
But not all measurement points have to be used, and in case of shorter measured
length the rest of 'unused' points are simply not shown on screen and in data
files. For example, when measurement on 20 mm channel is made, the number of
'used' points is only 20 : 0,74 = 27 points. For practical purposes this is
more than sufficient.
Moreover a minimum of 2 cycles per pulse can be used in case of good echo conditions,
or a higher frequency of 8 MHz proposed by Met-Flow implying shorter wavelength
(4 cycles per 8MHz pulse is equivalent to 0.37 mm sampling volume) can be selected.
What is measurement
resolution and accuracy for low velocity flows?
A data length for the velocity value is 8 bits. The first bit is used for
the sign, which represents flow direction. Velocity values are stored in the
remaining 7 bits. This means that the velocity resolution is 1/127 of the maximum
detectable velocity which can be selected by the UVP. The best accuracy is thus
theoretically 0.8%.
When the maximum detectable length is selected as 748mm for water, the maximum
detectable velocity is 8.77 cm/s which gives the minimum velocity resolution
as well as the threshold velocity as 0.7 mm/sec. Adding to this, there is an
effect of beam size, particle motion, etc. UVP accuracy has been investigated
using a rigid body motion of water in a rotating cylinder, giving overall velocity
error better than 5%, and overall axial position error better than 1%.
What typical
spatial resolution can be expected from UVP method?
Each single-point measurement of velocity out of the 128 points profile
is made in a finite cylindrical volume which depends on the ultrasound beam
size. Its longitudinal resolution corresponds to the burst length (0.74mm for
4 cycles of a 4 MHz US wave) while its lateral resolution corresponds to the
beam diameter and divergence of a respective transducer.
For high spatial resolution one should then choose a high US frequency.
What typical
time resolution can we expect from UVP method?
Measuring speed of UVP Monitor depends of concrete set-up of measurement
parameters. Generally, 30 to 200 full profiles can be measured and saved every
second, corresponding to a sampling time lying between 4 and 30 ms roughly.
Obviously, it is possible to slow down sampling rate to a desired value.
4. Data processing
Can measured
data be accessed by other programs?
Measurement data are saved on hard disk in binary format. This data set
can be converted by a provided function to ASCII format (.xlu format recognisable
by Excel) for further post-processing.
How many measured
profiles can be saved during measurement?
Number of saved data is only limited by available space on computer hard
disk. This usually is just theoretical limitation. Number of profiles which
can be saved is therefore practically unlimited.
Can UVP Monitor
calculate turbulence power spectra?
Yes, with the help of an additional program package.
Is UVP Monitor
capable of recording time history of flow?
Yes, very easily. Single measurements are recorded with frequency comparable
with video recorder rate. Such time 'movie' can then be - in addition to normal
computational processing - played on computer screen as 'movie' of flow.
Moreover in the data file a time stamp can be added to each profile measurement
if needed.
5. UVP applicability
Can UVP Monitor
measure in air?
To measure in air or other gasses is principally impossible with UVP instrument
working with high frequency sound fields, as in those media the acoustic impedance
is much more smaller than in liquid or solids. Moreover, echo would be generally
very weak.
Can UVP Monitor
measure in liquids containing bubbles?
As far as bubbles are small in comparison with ultrasound beam diameter,
bubbles form very good ultrasound scattering centres. If the concentration of
bubbles is too high, there occurs a multiple reflection of ultrasound pulse
among these bubbles and obtained profile might not be correct.
What happens
when ultrasonic beam hits liquid's free surface?
In such case measured profile extends up to the liquid surface, and measurement
points above surface are missing. This effect can also be used for surface level
measurement. It should be noted however that a reflection from the surface returning
to the transducer may under certain circumstances destroy the measured profile.
Such returning occurs randomly depending on the condition of the free surface.
Can UVP Monitor
measure flow-rate?
In case of circular pipe or square channel it can, through a geometric
integration of the velocity profile. If beam incident angle and pipe diameter/
channel width are known, then UVP Monitor can recalculate measured profile directly
to through-flow. This is true assuming that the flow is well developed at the
measuring position.
Comparative tests has been made with a weight tank calibration system in water,
providing error rate from 0.18 % to 0.59 %. Measurement repeatability was also
very good.
Is it possible
to measure molten chocolate flow?
Yes, this has already been tested, but measuring distance is decreased
and depends on the concentration of seeding particles.
UVP Monitor can also measure in mayonnaise, ketchup, paper pulp, tooth paste,
ferromagnetic fluid, glycerol, oil and petrol, and in other liquids and pastes.
What is the
thickest suspension measurable by UVP Monitor?
Usually it is 10-15% of solid particles. Sometimes even thicker suspensions
can be successfully measured, but experimental testing should be mandatory as
the correct answer really depends on material, size, depth to be measured, etc.
How is it possible
to measure sound velocity in measured media?
If we do not know sound velocity in measured media, it is easy to calibrate.
Use a vessel of known size, and place transducer perpendicular to the wall.
In program, iteratively change speed of sound as long as measured reflection
from the wall corresponds to the real distance from transducer to wall. Then
the speed of sound is set up correctly. Using oscilloscope and observing the
echo gives better and more accurate results.
Can UVP Monitor
measure turbulence of flowing liquid?
Yes, this is being performed automatically. Profile measurements are being
done repeatedly, results are calculated by local averaging, and at the same
time RMS value is also calculated.
What is the
influence of high temperature of the fluid on measurement?
The temperature has an effect on the sound velocity. If the speed of sound
in the fluid has a strong dependence on the temperature, it has to be corrected.
From the practical point of view, the temperature of the fluid affects the condition
of mounting a transducer. The present 'standard' transducers have the maximum
operating temperature of 60°C. If the temperature is higher than this at
the place where the transducer is mounted, special care has to be taken, or
special high-temperature transducers up to 150°C used.
More importantly, in application of UVP to high-temperature flow fields, it
is not the temperature level which might form a problem, but temperature gradient
in the fluid. The temperature gradient has an influence on propagation of ultrasound.
Ultrasound beam can be bent or reflected a little, unless the beam direction
is normal to the temperature gradient. Clearly, UVP can measure velocity profile
as long as the liquid includes reflectors, but the position of the velocity
profile could be distorted a little.
Up till now, UVP has been used in water with temperature difference of ca. 30°C
per 10 cm, and no significant influence on measurement has been found.
Can the UVP
be applied to liquid-gas two-phase flows?
Ultrasound is almost 100% reflected at the interface between liquid and
gas, namely gas bubbles. However, the beam size of the ultrasound is relatively
large so that if the void fraction of the flow is low and flow regime is nearly
dispersed flow where the size of the bubble is smaller than the beam size, the
gas bubbles play the role of a reflector and a velocity profile can be well
obtained. However, if the void fraction is larger than that for dispersed flow,
like chunk flow or annular flow, UVP can only measure the velocity profile for
the liquid part (such as liquid film for the annular flow) between the transducer
and any large bubbles.
6. Test medium, through-the-wall measurement
In what kind
of fluids can the UVP be used?
The present model of UVP has been developed for flow in such liquids as:
How is it possible
to measure flow in vessel or pipe through the wall?
Some combinations of wall material and liquid are well suited, some not
so well suited. The decisive factor is 'acoustic impedance' of wall and liquid
(acoustic impedance is product of density and sound velocity of material). If
acoustic impedance of wall material and liquid are at least similar, through-the-wall
measurement is usually possible without significant problems.
In principle wall acoustic impedance should not be more than twice to three
times test liquid impedance.
What kind of
materials can be used for the container wall on which a transducer is fixed?
Generally, the ultrasound is reflected at the interface where acoustic
impedance (density x sound speed) changes discontinuously. And thus, when the
transducer is set outside the container wall for non-invasive measurement, a
combination of liquid and wall is limited as follows:
The following combinations need careful consideration:
How can the
applicability of the method in certain media be tested?
The easiest method is the following: fill a beer bottle with test liquid.
Then sink transducer into the liquid and pull it out repeatedly. If measurement
is acoustically possible, on UVP Monitor screen you will see profile movement
corresponding to the transducer movement.
Is there any
medium where UVP Monitor fails to measure?
Yes. For example in colagen measurement is impossible. Thanks to its fibrous
structure colagen features very high absorption of ultrasound so no echoes return
back to transducer. There exist more media like this. Ultrasound measurement
in very high absorption media is impossible, in plastic as well.
How can I measure
water flow in relatively thick cast iron pipe?
It is not practical to transmit ultrasound through a cast iron wall (which
acoustic impedance is about 30 times the one of water) into water, since the
interface would largely reflect incident pulses.
In most cases it is preferable to drill a small 8 mm diameter port into the
pipe (4 MHz transducer with 5 mm active diameter) and insert transducer into
a pipe flush with its inner surface. The transducer can be sealed with an O-ring.
This removes all problems with wall impedance.
7. Seeding
What should
be the size of the reflecting particle (seeding)?
To generate a significant echo a particle should have a diameter at least
equal to the quarter of the ultrasound beam wavelength (i.e. for 4 MHz in water
a minimum of 93 *m of diameter is required).
On the other hand, it is known that a particle as a reflector follows the flow
motion faithfully, when the density is nearly equal to the fluid itself and
the size is smaller than 100 *m (in water). From these facts, the particle is
expected to be larger in size as a reflector and at the same time to be smaller
as a tracer.
What should
the reflecting particle be? - Particle concentration
Ideally, the reflecting particle should have as different acoustic impedance
from the measured liquid, as possible (or as practical). Since the reflecting
particle size is usually much smaller than the ultrasound wave length, signal
is formed by reflections from many particles, and measured profile is affected
by its concentration. When concentration of particles is smaller, some points
of profile may not be measured during a single US pulse. This does not mean
that the accuracy in velocity value becomes lower, but that the instantaneous
profile has some points missing. These points have zero value because of no
reflection, or are set to zero by the algorithm when too weak a reflection is
detected. When the UVP is to be applied to a configuration with low concentration
of particles, the average velocity profile can be reconstructed from many profiles
stored on a disk file. This can been successfully done for the stationary flow.
Is natural particle
contamination of water sufficient for measurement?
Usually natural particle contamination of water is sufficient for UVP Monitor
measurement. If media is very clean or if you want to improve measurement, it
is possible to introduce tracing particles. A little mud in hydraulic models,
or stirring of bottom slurry is usually enough. Other particles (hollow glass
spheres, nylon or polystyrene powder) can be used as well.
8. Hardware
Is it possible
to synchronise UVP Monitor with external events?
UVP Monitor features synchronisation input for TTL-level synchronisation
(0-5V, min 20 ns), and with different modes of triggering. This enables to synchronise
measurements of periodic flows (e.g. piston pumps), or to measure transient
events (e.g. valve opening).
Is it possible
to connect UVP Monitor to a computer?
UVP Monitor itself already contains full PC-compatible computer. This computer
can be connected through standard serial ports or Ethernet network card to other
computers or to computer network.
Can UVP Monitor
use external display?
Yes. While the integrated 8.4" color LCD screen is suitable for outdoors
data logging, for inside laboratory and for data processing it is more convenient
to use larger external display through UVP-XW VGA connector.
What operating
system is delivered with UVP Monitor internal computer?
Currently it is Windows NT 4.0, chosen for its reliability and true multitasking
ability compared with Windows 95/98. In the future, the operating system might
change for most recent high-end operating systems.