UVP applicability

Can Ultrasonic Velocity Profiling be applied to gas flows?

To measure in air or other gasses is practically 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 biphasic 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.

Can UVP Monitor use any other emitting frequencies than 0.5 - 1 - 2 - 4 - 8 MHz?

No, our UVP Monitors various models has always been designed to work with 0.5 - 1 - 2 - 4 - 8 MHz fixed set of emitting frequencies. The latter allows velocity measurements for most laboratory-scale applications.

Lower frequencies allow for longer distance range due to their better propagation ability showing less acoustic attenuation, and for larger velocity measurement.

For low velocity measurement or small flow dimensions where high spatial resolution is required higher frequencies are used, featuring shorter wavelength.

Is simultaneous measurement of UVP Monitor and PIV or LDA viable?

UVP instrument uses ultrasound, while PIV (Particle Image Velocimetry) or LDA (Laser-Doppler Anemometry) use light. Both methods therefore do not interfere, and can be used simultaneously.

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:

• Water

• Organic liquids: Freon, Petroleum

• Liquid metals: Mercury, Lead-Bismuth-Eutectic

• Ferromagnetic liquid

• Polymeric fluid

• Food materials: Mayonnaise, Ketchup, Coffee, etc.

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:

• Water (1.48 MRay) - Plexiglas (3.15 MRay), PVC (3.27 MRay), thin glass (12 MRay)

• Freon (1.12 MRay) - thin glass (12 MRay)

• Mercury (19.6 MRay) - Aluminium (17.3 MRay), Stainless Steel (45.7 MRay)

• Pb-Bi Eutectic (~20 MRay) - Aluminium (17.3 MRay)

• Petroleum based ferromagnetic liquid (~1.4 MRay)- Plexiglas (3.15 MRay)

• Water based thin slurry (1.48 MRay) - Plexiglas (3.15 MRay)

The following combinations need careful consideration:

• Water (1.48 MRay) - Metal (20 - 50 MRay)

• Mercury (19.6 MRay) - Glass (12 MRay)

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 collagen measurement is impossible. Thanks to its fibrous structure collagen 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.

Ultrasound beam, resolution, window function

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 3 mm from transducer face.

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°.

Is it possible to measure flow in channels smaller than 100 mm?

Yes, without any problems. For example, measuring a 100 points velocity profile with a 4 MHz transducer would generate a profile length of approx. 80 mm, with a 0,74 mm spatial resolution.

Thus, measuring a 20 mm deep channel with 4 MHz would still generate a 23 points profile, which is still sufficient for instance to measure a boundary layer. In case of good echo conditions, a 8 MHz transducer can be used to reach 0,37 mm spatial resolution, doubling the number of points per profile compared with 4 MHz.

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 748 mm 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. Additionally, 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 (0.74 mm 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.

Echo generation

What should be the size and density of the reflecting particle (acoustic reflectors)?

To generate a significant echo a solid or gas particle should have a diameter at least equal to the quarter of the emitted ultrasound signal wavelength (i.e. for 4 MHz in water a minimum diameter of 93 microns is required).

On the other hand, it is known that a solid particle follows the flow motion faithfully when its density is close to the carrying liquid's density, and its size is smaller than 100 microns (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 type of reflecting particles can be used, at which concentration?

In principle, the reflecting particle should have as different acoustic impedance from the measured liquid. 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 add reflecting 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.

UVP-DUO Profiler

Is it possible to synchronize UVP-DUO with external events?

UVP-DUO Profilers have a trigger input, with TTL-level sync signal (0-5 V, min 20 ns), with different modes of triggering. This enables to synchronize measurements of periodic flows (e.g. piston pumps), or to measure transient events (e.g. valve opening).

Is it possible to connect UVP-DUO to a computer?

Although UVP-DUO contains its own embedded PC dedicated to communication purposes, it is actually controlled by a remote PC, via its Ethernet port. The latter controls, acquires, displays and stores the measurement data generated by UVP-DUO.

What operating system is delivered with UVP-DUO embedded computer?

UVP-DUO internal PC runs Windows XP Embedded®, a simplified and robust Windows operating system, similar to OS found in cash machines. The latter OS cannot be attacked by viruses or any other malicious software, and requires no specific updates.

UVP Transducers

Is it possible to submerge Metflow's transducers into a large mixing vessel?

Yes, this is possible. All our transducers are watertight, and can be temporarily submerged or installed into vessel as stable measurement installation. Our standard line TX can withstand up to 3 bars absolute, so theoretically immersed up to 20 meters water column.

What is maximum applicable RF voltage on transducer?

During operation, the maximum excitation voltage applied on the transducer is 150 V peak-peak.

What is maximum transducer cable length?

Transducer cable carries small signals during receiving phase, and it is not recommended to use superfluous length of cable, especially in environment with strong electromagnetic interference (EMI). Our transducer cable length is 4 m standard. You can get cable length up to 20 m optionally, nevertheless for lengths beyond 4 m Met-Flow cannot guarantee the transducer proper functioning, especially in environments with strong EMI.

Low frequency transducers are in principle less problematic, while one has to be careful with our higher frequencies 4 and 8 MHz.

Is it possible to extend or shorten transducer cable?

Although the cable is integrally molded into the transducer casing, it is possible to extend or shorten its length, at a cost.

UVP Software

Can measured UVP data be accessed by other programs?

UVP measurement data are stored in a specific binary file format, .mfprof, generated by UVP software. The latter includes special export functions to convert the stored data to MS Excel compatible formats or to text files, and we provide a Matlab® import utility 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. As a file containing 1000 profiles with 300 velocity points each is approximately 1 kB, the number of profiles which can be saved is therefore practically unlimited.

Can UVP Software calculate turbulence power spectra?

Yes, using its specific post-processing function, included.

Can UVP Software record the time history of flow?

Yes, of course. Each single velocity profiles is stored with a time stamp, so various displays including a time scale are proposed.

Additionally velocity profiles of a file can be replayed offline, in slow motion or in real-time.

Marketing issues

Is there any UVP Profiler in use near my lab?

With hundreds of UVP instruments used, leased and lent worldwide, there might be one. Please contact Metflow directly to get any specific references.

How much does a UVP Monitor costs?

Prices are on request. Please check Met-Flow contact list to get in touch with your closest commercial representative.

What is the difference between conventional ultrasonic flow-meter and UVP?

The most common ultrasonic flow meter use the principle of transit time of ultrasonic pulse which is carried by the flow. They measure the transit time between transmitter and receiver at some fixed distance, compared with a reference time with the flow at rest. This method measures an averaged velocity in a pipe typically, so that some simplification and calibration are required to evaluate the integral of the velocity distribution over the measured line. Additionally some strict measuring conditions such as an entry length upstream the measured location is necessary. On the other hand, UVP method 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.