Ultrasonic flow measurement is not new technology. However, it has recently become something of a hot commodity among other well-represented technologies in the industrial flowmeter market. According to the Ultrasonic Flowmeter Worldwide Outlook, a market analysis forecast through 2005 published by the ARC Advisory Group (Dedham, MA), ultrasonic flowmeters are one of the few field devices for which double-digit growth is projected. The report goes on to say that although this is highly unusual for a mature technology, improvements in the last few years have lead to this significant resurgence. This is the result of experience, combined with recent technology advancement, which form the next generation of flowmeters that are essentially brand new.

The two types of ultrasonic flowmeters applied in closed-pipe flow measurements most often encountered in the process industries use either transit-time or Doppler technology.

Transit-time flowmeters use the difference in time for a sonic pulse to travel a fixed distance in the media, first with the direction of flow and then against it. These flowmeters can have one of two operating modes, time domain and frequency domain. Although they work similarly, transmitting pulses from a transducer to a receiver and back again through the flowing media, time domain meters use the difference in time between the two trips to provide information on the fluid's motion.

Frequency domain transit-time flowmeters use the same sensors as time domain meters, however the signals are read differently. Time is in not read directly. Instead frequency-domain units convert the time information into a frequency. As soon as a sonic pulse is received it is immediately retransmitted to form a pulse rate (frequency) proportional to the transit time. If two such paths, one in each flow direction are used, two frequencies are generated. The difference in frequency is proportional to flow velocity.

Transit-time meters are not limited to measuring flow velocity. Controlotron's (Hauppauge, NY) SonicMass flowmeter combines high-resolution sonic velocity and temperature compensation to convert volumetric flow to mass flow rate. The flowmeter, which is available in a variety of mounting styles and pipe sizes (3/8- to 48-in. dia.), is said to be the first commercially available liquid mass meter based on transit-time technology. Performance accuracy exceeds 0.25% for mass flow of selective fluids and 0.15% for volumetric readings over a specified range of Reynolds numbers.

Doppler meters work differently than transit-time devices, most using continuous transmission of a single sound frequency rather than pulses. The beam is transmitted into the media at some angle to the direction of flow. Bubbles, entrained solids, or eddies in the flow then reflect or scatter the sound back to a receiver. Motion in these inclusions will cause a Doppler (frequency) shift of the returned signal. In short, Doppler-based flowmeters pass the signals between a transducer and inclusions in the flow steam and back, rather than between two transducers.

Each of the inclusions, which have random physical distribution and velocities, reflect sound while in the sonic stream. Hence, their reflected composite signal is a random distribution of frequencies that add up to what appears to the receiver as a single waveform. The difference between the scattered and received frequencies is proportional to the motion of the flow inclusions or the flow velocity.

Other variations of ultrasonic flowmeters are available. There is a hybrid of the two basic technologies intended for use in process (closed-pipe) applications. Ultrasonic flowmeters can be used for determining flow rate in open channels and rivers. The technology, which is also available to measure flow rate in partially filled pipes, determines flow by measuring level in the pipe.

The types of media suitable for measurement ultrasonically are quite extensive for transit-time and Doppler meters alike. Either type requires some prerequisites for successful operation. Applicable media must support the passage of sound, be in a full conduit, and contain no material that will deposit on the inside pipe wall. Flow must be continuous and non-pulsing for either type to function accurately.

There are additional requirements for using Doppler meters. In these cases, the media must provide enough inclusions (suspended bubbles, small solids/particulate, etc.) so that sound energy has something from which to reflect. However, the media must not have so many of these 'scatterers' that sound cannot penetrate the flow.

According to Doug Weerstra, instrumentation chemist at Mesa Laboratories Inc., NuSonics Div. (Lakeland CO), there is an overlap in the amount of suspended solids by volume that both flowmeter types can handle. Transit-time meters work well even with 0-2% by volume of solids in small pipes. Doppler meters work in most flow situations with 0.1-10% solids by volume. As the amount of solids increases, however, instrumentation functionality can suffer.

Doppler meters can also function if eddies exist as the inclusions in the flow. 'However, eddies can be tough to pick up. And since they are created by downstream conditions, they cannot be counted on,' says Mr. Weerstra.

For both transit-time and Doppler flowmeters, the number of sound beams that pass through a pipe can be increased or modified electronically to raise the accuracy of the average velocity reading. Often a single acoustic beam passed between 3 and 9 o'clock (recommended) in a horizontal pipe run provides sufficient accuracy. However, for any ultrasonic flowmeter to provide accurate readings, it must be located in a straight section of pipe at least 10 pipe diameters upstream and three pipe diameters downstream from the nearest flow disturbance (pump, elbow, tee, reducer, etc.).

If a sufficient straight run cannot be 'found' or built into the process piping then multi-beams can be used to cancel out the effects of the disturbed flow. An additional beam(s) can be placed at 'other angles of the clock' and their signals combined to provide more accurate representation of average flow velocity. Keep in mind, sensor configurations can be quite different depending on the size of pipe and flow conditions encountered. In short, there just are no typical installations or application rules.

Sensor mounting styles are varied and often depend on where and when the device is placed in service. Mounting types include direct-mounting or non-intrusive. Direct-mounting devices include spool piece meters-so named because the short flanged pipe section that contains the transducers resembles a thread spool-bolted in place and weld-in style devices. Most of these sensors styles are mounted in the early stages of a process piping installation to avoid 'taking the process down' later in a retrofit situation. Weld-in transducers can be hot-tap mounted, however, allowing some flexibility as to when they can be added into a system.

According to Randy Brekke, vp sales at J-Tec Associates Inc. (Cedar Rapids, IA) the greatest advantage of using ultrasonic flowmeters, whether transit-time or Doppler, is that no pipe cutting is required. Ability to use externally mounted transducers provides the control engineer with greatly increased installation flexibility both in when and where the flowmeter is mounted.

Externally mounted transducers can be used on both transit-time and Doppler meters. There are two basic types. Clamp-on types mount on the outside of a pipe where flow velocity is needed. For clamp-on transducers to work, the pipe wall to which it is attached must be capable of passing sound and be clean and smooth. The inside of the pipe must be free of sound-absorbing material, such as dirty grease or scale. Use of an acoustic coupling material between the transducer and pipe (oil, grease, or epoxy) is recommended.

Where clamp-on transducers can not be adapted, wetted flush-mount sensors-some designs resemble spark plugs-must be used to provide a good interface for passing sound energy into the media. In the case of these transducers, no pipe need be cut, but mounting holes must be drilled and tapped into the pipe, requiring process shutdown and/or pipe draining during the process.

For new construction and where process interruptions (scheduled downtime for periodic maintenance, general cleaning and sanitation, etc.) are not a problem, installation of spool-piece devices is simple and straightforward. Siemens Energy & Automation (Grand Prairie, TX) offers the Sitrans F US, an ultrasonic flowmeter intended for use in liquids. This device is offered as spool-piece mounting, available in 1-, 2-, 3-, and 4-in. nominal diameters, and four standard DIN sizes with suitable flange designs. Because it is available in a limited number of pipe sizes, mounting flexibility as compared to the 'one size fits most' clamp-on type is limited.

However, a dedicated metering tube for each size allows the ultrasonic path to be more precisely controlled. In the case of Sitrans F US, its dedicated flow tube design uses built-in reflection points such that the flow velocity along the measuring path corresponds to the average flow velocity for all flow profiles. The patented helical sound path in this unit is said to result in high accuracy for a wide flow range.

Whether a flowmeter is dedicated or portable is more a function of the electronics than the type of sensor mounting. Dedicated devices are specified for a given location and, as such, are obtained from the factory calibrated to a given pipe size and flow range.

Truly portable ultrasonic flowmeters are microprocessor-based devices that can be reranged and recalibrated in the field, allowing them to be moved from one location to another with relative ease. Often used for testing and verification purposes, devices such as J-Tec's Compu-Flow Model JC5 feature an onboard keypad and LCD, clamp-on transducers, and a carrying case. Unlike dedicated units, the portable electronics are not meant to be permanently mounted.

Clamp-on transit-time ultrasonic flowmeters are most often applied to liquids, however, advancements in the transducer and signal processing technology have extended their use to gas applications as well. In the case of the GE Panametrics (Waltham, MA), Model GC868 clamp-on gas flowmeter extends the technology to gas applications for pipes 3 in. or greater in diameter and to pressures over 90 psig.

According to GE Panametrics' application engineer Daryl Belock, transit-time technology was always adaptable to gas flow if its density was high and delivery pressures were in the several-thousand psig range, not a common real-world situation. Getting the instrument to read flow accurately at smaller pipe sizes and lower pressures was the breakthrough, one that was electronics based. Initial investigation of the technology was done in an actual application to prove product feasibility.

Ultrasonic measurement techniques have made steady progress over the years as a viable flowmeter technology. Wide adaptability and ease of installation, two of its most important features, have been greatly enhanced through advancements in electronics and transducer design. And although no flow instrument is universally adaptable, ultrasonic flowmeters make a good run at it.

Comments? E-mail djohnson@reedbusiness.com.