Table of Contents

For high-temperature applications, buffer waveguides only a foot or so in length (~300 mm) convectively cool down enough at their “cold” end so that the ultrasonic flow sensor is not troubled by the temperature of a hot (300°C) fluid or hot pipe at the other end. The secret is to isolate thermally and economically, without corrupting the signal.

Larry Lynnworth,
Panametrics, Inc.

The primary problem with ultrasonic measurement of gas or liquid flow at temperatures >260°C lies in the survival of the transducers or sensors that generate and detect the ultrasonic waves. A secondary problem is coupling those waves into and out of the fluid (when using wetted transducers), or coupling ultrasound into and out of the pipe (when using clamp-on transducers). Whether the transducers are wetted or clamp-on, it is also important not to disturb things too much, that is, not to overly disrupt the flow patterns or the temperature. For example, local cooling at the transducer sites could cause solids to precipitate there, clogging the port or blocking the sound beam.

The main focus of this article is on the contrapropagation method of ultrasonic flow measurement (see Figure 1).

In the contrapropagation method, transit times are measured in the direction of flow, and later (or sometimes simultaneously) against the direction of flow. From the two measured transit times we can calculate the flow velocity, V. At the low Mach numbers encountered in liquid flow, the time difference, t, is directly proportional to V. In steam and gas flows the Mach number often exceeds 0.1. Here, t is not exactly proportional to V but V is still calculable from the two transit times. The V is a line average along the ultrasonic path. In order to accurately take into account the flow profile, considerable care is exercised in the choice and weighting of the paths. The most general solutions available today use multipath sampling of the cross section. In custody transfer spoolpieces for large pipes subject to nonideal inlet conditions, it is not unusual to use four or five paths.

The High-Temperature Problem
Figure 1. At ordinary temperatures, ultrasonic contrapropagation flow measurement uses upstream and downstream times-of-flight, each determined to high precision [3,4,6,7]. The transducers may be wetted or clamp-on. Wetted transducers have been preferred over clamp-on when the fluid is of low density (gases), or when multipaths along prescribed geometries are necessary to accurately deal with variable flow profiles and non-ideal installation effects. Clamp-on has the advantages, for liquid flow, of easy installation, no holes, and the possibility of easily going from one pipe to another to obtain a flow survey.

Most types of ultrasonic flowmeters use piezoelectric transducers to generate and detect the waves. Just as special magnetic materials have been developed to retain their magnetic properties at high temperatures, so too have high Curie point piezoelectrics been developed that retain their dielectric and other properties. Lithium niobate, for example, remains piezoelectric to ~1210°C. But when the designer attempts to build a practical high-temperature transducer using this material, several technical problems emerge:

  • Unless excess oxygen is present, lithium niobate loses its own oxygen and ceases to function properly as a transducer.
  • Differential thermal expansion makes it hard to encapsulate the piezoceramic within a metal enclosure such as 316SS or titanium, metals typically used in industrial flow applications.
  • Electroding and damping are problems as well. Without damping, transducers tend to ring and their pulses get smeared out in time. This makes it difficult to accurately distinguish and time the upstream and downstream pulse arrivals.

While other ferroelectric or piezoelectric ceramics exist or are under development that retain their oxygen to the 300°C– 500°C range or even higher, one or both of the other problems remain.

Buffer Waveguides: One Solution
Figure 2. Waveguides with signal integrity combat corruption. The feedthrough PanAdapta (FTPA) design uses sealed bundles of titanium (or other) wire waveguides to convey 200 kHz compressional wave ultrasonic pulses to and from air at atmospheric pressure (A). One version of the FTPA bundle waveguide technology construction is designed for 30 in. pipe ( 750 mm), gas operating temperature of 307°C, and gas pressure slightly below atmospheric pressure. Other FTPA versions have been delivered for steam flow measurements at high temperature and high pressure. The airborne test waveform shows high bandwidth, high SNR, and little crosstalk (B). OKS transducers, welded to a 10 in. steel pipe at a spacing optimized for superheated water at 260°C, yielded the 2 MHz signal shown in (C) when tested in water at room temperature. In this limiting case of clamp-on, where coupling is now permanent, the OKS footprint is long enough to collect the signal over the entire range of refracted angles for this application. The spoolpiece also contains welded-on yokes that function as permanent clamps for removable OKS transducers

In seismology, workers concerned with earthquake waves refer to P and S waves (primary and secondary waves). The terminology dates back to the 1800s, perhaps earlier, and is understandable if we note that after an earthquake, at a recording site far away, two waves are received, separated in time. First comes the compressional wave, then the shear wave. If the Earth were made of steel, and if the effects of temperature and pressure are neglected, the P waves would travel at ~6 km/s and the slower S waves at 3 km/s. From the difference between arrival times, t, the distance to the source is calculable. In a hypothetical "steel Earth" example, if t were 100 s the range would be 300 km. Data at two or more stations allows us to triangulate and locate the source. In a few moments we will see why the ultrasonic equivalents of P and S waves are important in measuring fluid flow at high temperature. Briefly, the P waves correspond to the waves used in wetted buffers; the S waves, to the waves used in clamp-on buffers (see Figures 2–4 and Photos 1–6.)

More than 50 years ago, a relatively simple way to avoid having high temperatures at the transducer was already known to ultrasonic researchers. This solution was the buffer rod. The rod was usually solid metal, sometimes steel, sometimes stainless steel, and occasionally tungsten. The diameter was typically 12–25 mm. The outside was threaded or grooved to break up unwanted mode conversions. This solution worked well at high ultrasonic frequencies, say 2–
10 MHz, where the rod radius was >3 , being the ultrasonic wavelength in the rod. (Another solution, for the MHz range, is clad buffers [1].) At lower frequencies, however, unless the rod radius is increased, dispersion results. This means that just as white light is broken up by a glass prism in which different wavelengths travel at different speeds, low-frequency ultrasonic pulses would be smeared according to the frequencies present.

If the industrial flow problem concerns gases or other media at high temperature, where only low frequencies (0.1–1 MHz) have a chance of successfully penetrating the fluid, and if the application does not allow a big port, then we must find a small-diameter, low-frequency buffer solution. By "small diameter" we mean that the buffer outside diameter should be in the 12.7–33.4 mm range and preferably not exceed 25.4 mm [2,3].

In 1995, confronted with the problem of buffering ultrasonic transducers subject to the constraints of small-diameter nozzles, high temperatures (up to 450°C), and
500 kHz operating frequency, my colleague Yi Liu and I found ways of taking hundreds of wires and enclosing them inside a corrosion-resistant sleeve to create a rigid dispersion-free bundle. The rigid bundle is sometimes referred to as the BWT (bundle waveguide technology) system (see Figure 3A).

The resulting bundle forms a pressure boundary suitable for ultrasonic flow measurements of hot corrosive gases. It also works for hot liquids such as molten asphalt, viscous liquids at room temperature requiring frequencies below 1 MHz, and cryogenic fluids that have a tendency to flash. The low frequency penetrates the fluids despite their two-phase character. The KEMA-approved transducer is removable from the buffer, without breaking the pressure seal (see Figure 3B). The portion of the BWT assembly within the insulated nozzle operates uncooled, at the fluid process temperature. The external portion can be as long as necessary to convection-cool down to a temperature such as 100°C, well within the range of ordinary piezoelectric materials and constructions.

The first industrial application of the BWT transducers was in the Shell Per Plus refinery near Rotterdam (see Photo 1). Buffers housed in stainless steel, titanium, or special alloys were used initially in about 15 locations. Some included startup complications where first nitrogen, then hydrogen-rich (low-density) gas had to be measured from as low a pressure as possible (~10 bar) up to an operating pressure of 209 bar, at temperatures >200°C, and furthermore withstand 450°C during upset conditions. The BWT bundle buffer worked well, so its use was expanded to many other locations in the plant, and then to the next plant Shell built in the same area, Moerdijk, for its MSPO-2 project. The BWT buffered transducers were subsequently used in steam tests conducted by Dow in Terneuzen, the Netherlands, at T = 340°C, 34 bar, ±45 m/s steam flow. By mid-1999 there were over 100 BWT systems installed in gas and liquid flow measuring applications around the world (see Table 1).

Clamp-On Transducers
Figure 3. Wetted buffers use a bundle of wire rods to guide compressional ultrasonic waves to hot (or cryogenic) media. The schematic shows a "single buffer" bundle sealed inside a shell (A). A flange can be welded to the shell to create an FTPA version. A double buffer has been used to 209 bar, where the temperature during an upset could reach 450°C (B).

Around 1996, EDF (Électricité de France) initiated a project at Panametrics to develop high-temperature clamp-on transducers that could be used to measure the flow of superheated water in steel pipe (see Photo 2). The temperature was 260°C; the pressure, 60 bar. Clamp-on transducers for contrapropagation flowmeters generally introduce the ultrasonic waves obliquely. In theory, one way to do this could be to use the BWT bundle at an angle. However, as the compressional wave velocity was 5 km/s in the steel BWT buffer, the refracted angle in the hot water would be too small for most applications. The small refracted angle is a consequence of the Snell law of refraction, applied to hot water where the sound speed is only about 1 km/s.

Instead, suppose we select a shear wave as the incident wave, in order to have a slower incident velocity (3 km/s, such as in the seismology example) and a proportionately larger refracted angle in the hot water. The shear wave theoretically offers the possibility of a refracted angle larger by a factor of ~5/3 than that achievable using incident compressional waves at the same oblique angle, say 60°. Only one set of problems now remained: how to produce shear waves, obliquely incident, having a shape easily timed to nanosecond or subnanosecond precision, and at frequencies from 0.5 MHz to 4 MHz?
Photo 1. Bundle waveguide technology has been applied to measuring the flow of hot hydrocarbon gases. (Photo courtesy of Shell, Per Plus project, the Netherlands.)

The answer turned out to be another waveguide. This time, instead of using many thin wires, we used one thin plate. The plate has certain topological features for generating reference echoes or for measuring temperatures at corresponding points, but overall it resembles a small hockey stick (see Figure 4). So "hockey stick" became its name and OKS its abbreviation. The OKS waveguide in Photos 3–6 has a length of ~250 mm and a thickness of 6.4 mm. Of this length, the 75 mm nearest the pipe is insulated. The rest, like the BWT system outside the nozzle, is cooled by convection so that the temperature to which the piezoceramic is exposed is <100° C.

Examples of appropriate applications for the OKS are given in Table 2, organized as a function of pipe diameter. The liquids were superheated water (260°C–271°C, 60 bar) if not otherwise identified. Table 3 (page 46) summarizes BWT and OKS buffered waveguide transducers and offers guidelines on where to use them.

Applications of BWT bundled waveguide wetted transducers
in hot gases, hot liquids, and other fluids
Hot hydrocarbon gas
  • Hydrocarbon gas, 1–210 bar, normally 220°C, design 450°C, in thick-wall CS/SS pipes, 8–16 in.
  • Gas above atmospheric pressure, at T = 165°C–200°C
  • Dow Terneuzen, 4 in., 340°C, 34 bar, V = ±45 m/s bidirectional pulsed steam flow in sootblower; FTPA (Feedthrough PanAdapta) one-piece buffer, flanged near its midpoint, and of sleeve o.d. = 1/2 in. (12.7 mm) or 1 in. (25.4 mm)
Hot liquid
  • Short or long residue, e.g., heavy hydrocarbon mixture, temperature to 270°C and up to 12 in. pipe; very viscous
  • Water, 220°C contaminated with entrained gas
  • Liquid of high viscosity at ordinary temperature
  • Cryogenic liquid that flashes occasionally

Clamp-On Coupling

More than 20 years ago, H. Karplus at Argonne National Laboratories demonstrated soft metal foil as an effective long-term (>5 yr.) couplant in ultrasonic flowmeters, in his work to 600°C on liquid sodium. The best choice for the foil material for a particular application is dictated by technical and cost factors. If the foil's melting point is too close to the operating temperature, intergranular corrosion is a technical concern.
Photo 2. In this first OKS field trial, conducted in 1996 in France by EDF, the clamp-on transducers are installed on a pressurized water conduit from which the insulation was temporarily removed. (Photo and temperature data in Figure 4 courtesy of EDF.)

In tests of short duration (1 hr), satisfactory economical couplants might be aluminum foil or anti-seize grease. For permanent coupling (usually desirable because of the inconvenience of recoupling at high temperature and perhaps in a radioactive area), foil metals other than aluminum are generally preferred. A low melting point foil was found to be easy to install and use at cryogenic temperatures, taking advantage of the OKS to solve a clamp-on flow problem that previously had been quite troublesome. Until the foil and OKS were introduced, this job at a Korean liquefied gas facility (–150°C) required recoupling at frequent intervals, especially following thermal cycling.

The metal foil is normally supplied as a strip 6.5 mm wide by 100 mm long, bent to conform to the OKS foot shape and secured with two small strips of electrical tape around the foot just before use. After installation, the tape strips are either removed or, at high temperatures, simply burn away, but they've done their job. The foil is already clamped in place, using a 1000+ kg force that is readily obtained by torquing the pressure screw that is part of the clamp in Photo 5.

Again, for quick trials, anti-seize couplant suffices; because coupling pressure can be modest, a "room temperature" clamp such as shown in Photo 3 suffices, aided by an aluminum block that centralizes the OKS. To minimize crosstalk we have found it best to place OKS transducers on opposite sides of the pipe. Sometimes this is an inconvenience, and sometimes there can be undue responsiveness to crossflow. If the latter is a problem, then two sets of transducers are used in an X configuration. The average is the desired V. The difference yields the crossflow velocity VX .

Examples of OKS clamp-on applications at various pipe sizes*
 Nominal Pipe Size
  Liquid; comments
 In.                    mm
 3                    75
  1. Belgium (Thiange 3), running 2+ yr., three points (XMT868); water temperature T = 285°C; OKS were calibrated at Eau de Paris at room temperature in order to be qualified before installation in the plant
  2. U.S. nuclear utility, two blowdown pipes, CS, sch 40, 25–400 gpm, running since plant restarted in May 1999; OKS selected because of turndown ratio and ease of retrofit; OKS system calibrated at Alden Research Laboratory at room temperature in order to be qualified before installation in the plant
 4                    100
EDF installed two pairs of OKS, where water temperature was 300°C
 6                    150
  1. Emergency backup in case previously installed intrusive devices fail (plant in Pacific Northwest)
  2. Cryogenic application in Korea (KOGAS); liquefied propane gas, –150°C
  3. EDF: water temperature was 300°C
8                    200
Flexicoker plant in TX: SS321 pipe; OKS replaced Venturi, which tended to clog
10                    250
  1.  Forsmark Unit 1 in Sweden; heavy wall, SS, 2 MHz, paths available at ±45°, better than 1% agreement with Venturi, but ultrasonics provides much wider turndown ratio; installed September 1998
  2. Atochem at Serquigney (Normandy); oil, 330°C, working >6 months
  3. Hybrid GC spoolpiece yielded 0.5% precision (R&D multipath project)
  4. Secondary flow swirl sensors, multipath no-hole spoolpiece, water tests at room temperature, liquid level R&D project in support of industrial clamp-on liquid level application in 12 in. and 28 in. pipes, in harsh environment, brought to our attention by PN Services
14-18                     350-450
EDF conducted trials lasting eight months or longer, on various large steel pipes, in nuclear and/or fossil-fueled plants; modern clamps apply the coupling pressure in a similar way, using one of the pipe riser clamps, two-piece split collars, welded-on yokes; the pipe riser clamp accommodates one or more yokes
>100                     >2500
Canadian nuclear plant's downcomer, which has a large, thick-walled approximately cylindrical shape, annular passageway ~3 in. between inner and outer wall; outer wall is CS, ~2 in. (50 mm) thick; AECL installed OKS transducers in late summer 1998 and measured not only the water flow but also determined that a second phase (entrained steam) was not present
 * One customer's field service personnel have used OKS on ~50 different applications. In these cases, none of which is included in the above list, the test generally is quick,¾30 min. Pipe sizes ranged from 4 in. to 16 in. Hydrocarbon liquids ranged in temperature up to ~360°C. Permanently installed example from St. Croix, VI, running since January 1997; heavy vacuum gas oil, 275°C, SG = 0.77,
* = 0.5 cSt, carbon steel pipe, 10 in. sch 40, c3 ~ 700 m/s, measured (with DF868) Q = 1260 gpm, using ¶ = 1 MHz, and 2 traverses (vee path).

Summary of BWT and OKS buffered waveguide transducers and guidelines on where to use them; some limits.* In field-proven applications, pipe diameters have ranged from 1 in. to ~1 m.
 BWT bundle
Ultrasonic wave type and speed Compressional, c 5000 m/s Shear, c 3000 m/s
Clamp-on or wetted Wetted Clamp-on 
Max. temperature 500°C 500°C (360°C for 250 mm OKS size)
Min. temperature –200°C –200°C
Max. fluid pressure 220 bar (3200 psig) >100 bar (>1450 psig)**
Material at the hot end  316SS, Ti, Haynes' Hastelloy, other alloys Usually 1018CS, sometimes 304SS or 316SS
Fluid media  Gas, steam, liquid, two-phase but still sonically conductive Liquid; liquid plus small percentage of gas phase
Frequencies available in 1999 0.1, 0.2, 0.5, and 1 MHz 0.5, 1, 2, and 4 MHz
What to avoid (limits known or of concern as of year end 1999;check again after Y2K) Multiphase highly scattering media (unless Transflection stroboscopic scattering mode applies); pipe <10 mm might be a problem unless axial path flowcell is practical Gases; same as limits listed for BWT bundle; field performance not yet established on pipes smaller than 3 in.( 75 mm)
 * This table does not list accuracy, mainly due to the difficulty of conducting calibrations at high temperatures such as 260°C. BWT and OKS transducer systems have been qualified for particular high-temperature applications based on accuracy demonstrated at ordinary temperature. This is typically followed by evaluation in the field at high temperature, sometimes for periods of one or more years. Wetted and clamp-on buffered accuracy are comparable to that obtained with their non-buffered transducer counterparts, usually to within a factor of two or better.

**Higher pressures mean thicker pipe wall. This could impose a crosstalk-induced limit, if the fluid were so attenuating that crosstalk noise N, around the pipe, exceeded the strength, S, of the fluid-borne signal.

Sometimes the pair of OKS transducers is to be installed at room temperature and the metal foil couplant isn't soft enough to couple at reasonable coupling pressure. A solution in this case, first demonstrated by my colleagues P. Kucmas and P. Fredriksson in a 1998 startup at Forsmark Unit 1 in Sweden on 10 in. stainless steel heavy wall pipe, was to wet the metal foil with a drop of glycerine couplant. This conventional ultrasonic couplant works fine at room temperature and evaporates later, at high temperature, leaving no residue. The same glycerine solution was used during a scheduled shutdown in a 1999 "cold" startup on 3 in. schedule 40 carbon steel blowdown pipes at a U.S. nuclear plant. When the temperature later reached 271°C, coupling was provided by the soft metal foil.
Figure 4. Temperature gradients on two "hockey stick" transducers were measured by EDF when the superheated water temperature was 260°C. Note how cool the transducers are (piezoceramic regions) for the situation shown in Photo 2.

As another approach to finding a permanent couplant, consider welding. The OKS, typically made of CS1018 but in principle manufacturable of any standard engineering material such as 316SS, can be held in place against the pipe with one of the standard clamps and tack welded, after which the clamp is removed. Full-penetration fillet welds along both sides of the OKS foot then provide complete coupling that is unlikely to decouple over the life of the pipe (see Figure 2C). Weld-on OKS transducers and welded-on yokes (instead of removable clamps) might make the OKS system easier to use in some applications, but welding to an operational pipe is not allowed in many cases. Code and safety issues must be addressed for the particular case at hand.

Hybrids--Calibratable Spoolpiece with Removable OKS Transducers

Hybrids combine features of wetted and clamp-on transducers. Hybrid spoolpieces could use welded-on OKS transducers or welded-on yokes. Recall that the BWT wetted buffers, originally developed for high-temperature flow problems, ended up solving some problems at room and cryogenic temperature as well.

First, consider that the KEMA-approved transducer portion of the BWT system is removable while the buffer itself remains in the pipe nozzle as the permanent pressure boundary. This is not unlike the PanAdapta precision plug concept shown on page 168 in [4]. Some 1800 were in use by mid-1999, mostly for ordinary temperature. Referring again to the BWT buffer system in Figure 3B, the KEMA portion is sort of a clamp-on. (In fact it is normally epoxied in place at the end of the BWT buffer, but can be removed using two wrenches and later reinstalled or replaced.)
Photos 3,4,5,6. Each clamp-on OKS ("hockey stick") buffers (thermally isolates) its piezoceramic from high (or low) temperatures. A commonly used strap-on clamping fixture can be used to quickly check the feasibility of measuring liquid flow at high temperature by means of OKS transducers that are coupled temporarily
(1 hr) with a high-temperature couplant (3). The close-up photo shows three "footprints": flat, contoured to pipe o.d., and weld-prepped (4). Two-piece, single-yoke clamps are the standard solution for permanent installations (5). By welding on a second yoke at ±45° with respect to the centerline of one of the clamp sections, dual-diameter multipath high-temperature clamp-on is achieved (6).

Second, consider the introductory remarks about multipath for a more general solution under bad flow conditions. Let us now observe that each pair of thin OKS transducers shown in Figure 2A and Figure 4 can be arranged to interrogate a narrow, well-defined region. Four OKS pairs can therefore provide a Gauss-Chebyshev (GC) quadrature solution to integrating flow over the cross section of a calibratable spoolpiece. This concept, illustrated in Figure 5 and Photo 7, was tested in water at room temperature by my colleague B. Dean last year and yielded an error band of ±0.33% in its first test over the Reynolds number range of 0.47 million to 1.3 million [5].

This accuracy by itself is not remarkable. But it was achieved with no holes for the multipath transducers. All the transducers were removable clamp-on OKS types. Given that the internal notches for this design are triangular and penetrate radially only about 5 mm or less, depending on location, it seems reasonable to surmise that they disturb the flow less than the conventional ports of 30 mm or so associated with transducers of 25.4 mm, for example. Although this hybrid spoolpiece was tested only at room temperature, its design is not limited to room temperature. In those tests, incidentally, B. Dean used another form of the OKS (see Photo 4, center), clamped in a plane perpendicular to the spoolpiece axis, to measure swirl. In that configuration the OKS transducers are referred to as swirl sensors. Recognizing that the swirl sensors interrogate in a thin sheet perpendicular to the pipe axis, much as the OKS hybrids interrogate in the thin GC planes, there is a possibility for sensing liquid level in the swirl sensor plane. Laboratory swirl and liquid level test results are shown on page 380 in [3].

Of these three spinoffs--hybrid GC spoolpiece, swirl sensor, and liquid level--R&D activity is currently aimed at the first. Although the OKS transducers are released and on the market, and work with commercially available flowmeter consoles such as the Panametrics PT868, DF868, and SMT868, unreleased R&D items like the swirl sensor or the notched GC spoolpiece with the conventional four parallel planes of measurement and Vee paths, but no holes, are not yet available except on special order. (The '868 instruments are the work of my colleague S.A. Jacobson and his staff.)

To obtain the best results with clamp-on, it is recommended but not absolutely required that the pipe be flat and smooth under the OKS foot. As the OKS transducers are normally manufactured with flat feet, the pipe would ideally have two parallel flats at 180°, say at 3 and 9 o'clock. Flats, however, are not absolutely necessary, as the metal foil can be supplied thick enough to make up the theoretical gap. Those who have used ultrasonic clamp-on flowmeters at room temperature know that the gel or grease couplant supplied by the manufacturer similarly fills up the air gap between transducer and pipe. What's different about the metal foil is that is doesn't dry out, doesn't get washed away, and doesn't run away when it gets hot. Early tests at EDF, for example, during an eight-month high-temperature trial on one pipe, showed no change in coupling. Similar results have been reported for the cryogenic Korean application, which has been running with no maintenance for six months since it was started up early in 1999.

BWT, PanAdapta, and Transflection are trademarks of Panametrics, Inc.
This calibratable no-holes multipath R&D spoolpiece uses the thin bladelike character of OKS transducers to interrogate across four narrow, off-diameter, well-defined regions without the usual ports. For paths reflecting in conventional GC parallel planes, the volumetric flow rate is Q = [0.138 (V1 + V4) + 0.362 (V2 + V3)] pD2/4.


Contributors whose transducer design or field test work is reported here include: A. Arden, D. Bruinzeel, M. Capobianco, E. Chérifi, C. Frail, J. Freeke, G. Jossinet, O. Khrakovsky, R. Koch, Yi Liu, E. Machado, J. Marchi, J. McGregor, J. Matson, R. McCarey, W. Mellish, T. Nguyen, J.K. Park, R. Richard, T. A. Russell, J. Slaughter, D. Thomson, D. Xiao, and M. Zimmerman. The author acknowledges Panametrics's permission to use excerpts from its copyrighted report UR-242S.


1. C.K. Jen et al. 1997. "Clad Buffer Rods for In-situ Process Monitoring," Proc 1997 IEEE Ultrasonics Symp:801-806.

2. Y. Liu et al. 1998. "Buffer Waveguides for Flow Measurement in Hot Fluids," Ultrasonics, 36 (1-5):305-315.

3. L.C. Lynnworth and V. Mágori. 1999. "Industrial Process Control Sensors and Systems," E. P. Papadakis (Guest Ed.), Ultrasonic Instruments and Devices: Reference for Modern Instrumentation, Techniques, and Technology, 23 in the series Physical Acoustics, Academic Press:275-470.

4. L.C. Lynnworth. 1989. Ultrasonic Measurements for Process Control--Theory, Techniques, Applications, Academic Press.

5. B.J. Dean and L.C. Lynnworth. 1999. "Hockey Stick Developments Extend Range, Improve Accuracy of Clamp-On Flow Measurements," Proc 1999 IEEE Intl Ultrasonics Sym, Paper PG-7.

6. R.C. Asher. 1997. Ultrasonic Sensors for Chemical Process Plant, Institute of Physics Publishing, London.

7. L.C. Lynnworth. 1979. "Ultrasonic Flowmeters," Physical Acoustics--Principles and Methods, W.P. Mason and R.N. Thurston (Eds.), Academic Press 14:407-525.

Larry Lynnworth is a VP & PCI R&D General Manager, Panametrics, Inc., 221 Crescent St., Ste. 1, Waltham, MA 02453-3497; 781-899-2719, x-146/307, fax 781-894-5785, lynn

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