Ultrasonic Time of Flight

Time of flight may be used to measure local density fluctuations. In a chemically homogeneous ceramic, changes in velocity result from differences in density and modulus caused by undensified regions. For dry-pressed bodies, sound velocity has been found to relate to density by the equation p = where p is density, v is 

A material absorbs ultrasonic energy at a different rate for different frequencies. A constructive use of this phenomenon is ultrasonic spectroscopy, covered in the next section. Another consequence is that a single spike pulsed into the sample for time-of-flight measurements is distorted during transmission. This makes it difficult to time the interval between echoes precisely. A continuous sine wave is a single frequency, but it interferes with itself upon reflection. A compromise is found in a tone burst, a sine wave applied for a short time. 

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The use of electromagnetic acoustic transducers (EMAT) obviates the coupling problems already referred to, and has been applied successfully to the inspection of boiler tubes. Ultrasonic time of flight diffraction (TOFD), developed by the Harwell Laboratory" , is utilised to fingerprint flaws (cracks) in process plant. Subsequent examination at, say, six month intervals can indicate any growth or extension of the crack. It is claimed that changes in crack height of 0-5 mm or less be estimated. 

Ultrasonic-time of flight Volume 3 Section 6.2.1 Volume No No No Yes Usually No Yes No No 100-530... 

Two or more of these conditions can occur at the same time, resulting in asymmetric axial, radial and tangential velocity vectors. Some flowmeters are more sensitive than others to particular types of flow distortion, e.g. orifice meters are affected by pure swirl more than venturi meters are magnetic flowmeters are unaffected by changes in the radial velocity component whereas ultrasonic time-of-flight meters are highly susceptible thereto swirl and asymmetry have the least effect on positive displacement meters and the greatest effect on variable area meters. 

In an early study, Greenleaf et al. [4] reported reconstructions of ultrasonic velocity from time-of-flight profiles. Since then there has been periodic activity in using ultrasound to determine the transmission properties attenuation or refractive index. 

In general a thickness measurement using ultrasound is done by measuring the time of flight of the ultrasonic pulse and calculating the thickness of the objeet multiplying the time and the known constant sound velocity in the material. 

The PS-4 ultrasonic examination system provides many new features, which allows the operator to perform several inspections simultaneously. Both pulse-echo and time-of-flight-diffraction technique can be applied together with storage of digital A-scan data at the same time. 

The use of ultrasonic energy is different in on/off switches and in transmitters. Switches act on the attenuation of the acoustic signal in the gap between two crystals, while transmitters measure the time of flight of the ultrasonic pulse. 

Fig. 6.3. Counter-propagating time-of-flight ultrasonic flowmeter...

Planar tomography could be complementarily adopted to indicate cracks and determine their depth propagation with high resolution, while time of flight diffraction (TOFD) has been considered not suitable as a surface crack inspection. Ultrasonic equipment such as phased array technique, on the other hand, allows complete weld inspections, improving, for instance, the separation between back wall and defect indication [10]. 

Rhosonics Ultrasonic Concentration Analysers (www.rhosonics.nl) Pulse echo time of flight Concentration determination Yes Yes... 

The cross-correlation technique measures the time of flight of an inherent flow tag passing through two sensors separated by a known distance. The technique has been used successfully to monitor single-phase fluid flows in which turbulent eddies modulate the interrogating ultrasonic beams. This type of correlation flowmeter has also been developed for solid/liquid and gas/liquid flows, in which the density fluctuation, caused by clusters of solids and by gas bubbles, is the prime inherent flow tag. 

Time-of-flight ultrasonic flow meter single-path type. 

Time-of-flight ultrasonic flow meter multipath type. (Adapted from Miller, R. W., Flow Measurement Engineering Handbook, New York McGraw-Hill, Inc., 1989.)... 

MTs = metallothioneins NMR = nuclear magnetic resonance pKa = negative logarithm of acid dissociation constant PMT = photomultiplier tube QITMS = quadrupole ion trap mass spectrometry SBSE = stir bar sorptive extraction SDS-PAGE = sodium dodecylsulfate polyacrylamide gel electrophoresis Se-Cis = selenocystine Se-Cys = selenocysteine Se-Et = selenoethionine Se-Hcy = seleno-homocysteine Se-Met = selenomethionine SEC = supercritical fluid chromatography TBT = tributyltin TETRA = tetramethylarsonium ion Tf = transferrin TMAsO = trimethylarsineoxide TMSe = trimethylselonium ion TOFMS = time of flight mass spectrometry USN = ultrasonic nebulizer. 

Time-of-flight diffraction (TOFD) systems measure the time required for an ultrasonic wave to reach a flaw and return to the transducer. Defect depth and visualization of defect regions can be obtained with this technique. Incident ultrasonic energy propagates differently within a material as follows ... 

In recent years much effort has been devoted to developing advanced techniques to Improve the accuracy of ultrasonic sizing. The PlSC 11 (ref 1) and Defect Detection Trials (ref 2) are examples of the use of these techniques. In particular Time of Flight, ALOC, Holography, Focused Probes and the Synthetic Aperture Focusing Technique. However, to date, these techniques have been used In laboratory environments and It Is sometimes questioned how relevant are these techniques to standard site Inspections since. In general, highly sophisticated computer and Instrumentation hardware Is required. 

A particularly Important example of this Interactive capability occurs in the analysis of time-of-flight diffraction inspection data files. Following display of the set of ultrasonic rf waveforms stored in each file on the colour graphics unit, a screen cursor is used to select points on identified waveforms for display as time-delay ellipses on the appropriate nozzle profile. The intersection point of the arcs generated from the same point on several waveforms corresponds to the location of the defect extremity producing the observed signal. 

The concept of time of flight has also been adopted, similarly to its use in ultrasonic flowmeters for macroscale flows. A heat pulse is generated upstream by a laser or electric coil, and the heated fluid is detected downstream. The time difference is used to determine the velocity and then the volumetric flow rate. However, diffusion of heat in the fluid results in a broadened temperature distribution when the fluid reaches the sensors and hence results in difficulty in determining the transit time accurately. An ion generator can be used to replace the heater for the purpose of determination of the time of flight [6]. 

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