This is the fourth of a five-part series on Process Sensing. Other installments included pressure sensing (March), smart sensors (May), and temperature sensing (June). The final topic, level sensing, is scheduled for November.

Cltrasonic flowmeters have come a long way in gaining market acceptance. Recent technological improvements are increasing the accuracy of ultrasonic flowmeters and enabling them to handle a broader range of process conditions. Anticipated industry approvals and a wide variety of sensing technologies are further contributing to the market acceptance of this flow technology.

Consequently, ultrasonic flowmeters are encroaching upon the markets of traditional flow technologies, such as magnetic and differential pressure. Ultrasonic flowmeters, however, still have a number of roadblocks to overcome before gaining full market acceptance. Traditional flowmeter suppliers are taking steps to minimize market attrition.

Early misapplications and relatively low accuracies of ultrasonic flowmeters left many users with a bad impression. As ultrasonic technologies have improved and become better understood, users have begun applying them correctly, therefore achieving better results. Recent successes in application of ultrasonic flowmeters are changing this flow technology's reputation and encouraging more users to try them in their applications.

Advantages of ultrasonic flowmeters include low maintenance requirements, non-intrusive nature, lack of moving parts, lack of pressure drop, and ability to accurately measure gas flow. The chief disadvantage to ultrasonic flowmeters is a relatively high initial cost when compared to traditional flowmeters. Overall cost of ultrasonic ownership, however, is often less over the life of the product because of lower maintenance costs and higher accuracy.

In addition, improved technology in transit-time ultrasonic flowmeters has led to greater accuracy over a wider range of process conditions. For example, transit-time flowmeters can handle fluids that aren't totally clean. This improvement enables transit-time flowmeters to measure fluids that formerly could only be measured by more expensive Doppler flowmeters.

Development of multipath, or multibeam, transit-time flowmeters has been another important component toward market acceptance of ultrasonic flowmeters. Multipath "ultrasonics" have more than one pair of transducers reflecting an ultrasonic signal in a closed pipe. Through the use of multiple signals, multipath flowmeters achieve much higher accuracies than single path flowmeters.

The improved accuracy of ultrasonic flowmeters is allowing them to make inroads into the gas measurement market. Although the number of ultrasonic flowmeters in gas measurement is still quite small, some natural gas companies are beginning to implement them.

For example, many natural gas companies are testing ultrasonic meters on noncritical applications such as check metering. This approach allows these companies to become familiar with the technology without risking problems to critical applications. As the technology becomes more widely accepted for gas applications, these users will begin applying ultrasonic flowmeters for custody transfer applications.

Ultrasonic flowmeter suppliers are also working on gaining approvals by industry associations, which leads to wider acceptance and use of the technology. Industry association approvals play a major role in user decisions about what types of flowmeters to apply, especially in the oil and gas and refining industries. Ultrasonic flowmeters already have approvals in many European countries for custody transfer applications. In the U.S., the American Gas Association (AGA, Arlington, Va.) is working on use of ultrasonic flowmeters for custody transfer; report AGA-9 is due before fall 1998.

Ultrasonic flowmeter models for closed-pipe applications are available in both transit-time and Doppler technologies. Both technologies depend on ultrasonic signals being changed by the velocity of a flowing stream.

For transit-time, a transducer sends a signal across the flowstream to the receiving transducer. The signal is reflected back and sent in the opposite direction. The signal travels faster when moving with the flowstream rather than against the flowstream. The difference between these two transit times is used to calculate the flow rate. Panametric's (Waltham, Mass.) PT868 and Krohne's (Peabody, Mass.) UFM 600 T are examples of transit-time flowmeters.

Doppler flowmeters make use of particles or entrained air moving within the flowstream to measure flow. Doppler flowmeters emit an ultrasonic beam that bounces off particles or entrained air. When the ultrasonic beam is deflected, a frequency shift occurs. The flow rate of the liquid or gas in the stream is proportional to the frequency shift. Polysonics' (Houston, Tex.) DDF4088 and the Spectra from Controlotron (Hauppauge, N.Y.) are among examples of Doppler flowmeters.

Ultrasonic open-channel flowmeters work differently than either transit-time or Doppler flowmeters. Open-channel flowmeters are generally used to measure the flow in rivers, streams, open conduits, and partially filled pipes. Open-channel measurement often occurs with the use of weirs and flumes, which are mechanical devices that guide the flow in a stream.

Ultrasonic open-channel flowmeters are typically located on the bottom of a channel and send an ultrasonic beam to the flowstream surface where it is reflected. The length of time this beam takes is proportional to the level of the stream. Stream level information can be used to calculate flow rate because the dimensions of weirs and flumes are known. Milltronics' (Arlington, Tex.) OCM III and the FMU 861 from Endress + Hauser (Greenwood, Ind.) are examples of open-channel flowmeters.

In addition to three preceding ultrasonic technologies, new techniques including hybrid meters are directly impacting the market. Hybrid flowmeters are a new variation of ultrasonic flowmeters combine the above technologies. The most common combination is transit-time and Doppler technology. It enables hybrid flowmeters to cover a broader range of process conditions, and to better handle changes in the flow characteristics of the fluid being measured.

For example, if a stream is initially clear and later has particles or entrained air introduced to it, a hybrid flowmeter can shift from transit-time to Doppler technology and continue to measure the flow. Examples of hybrid flowmeters include Panametrics' XMT868 and Controlotron's System 1010.

One of the most significant topics of discussion in today's flowmeter market is the replacement of traditional flow technologies with ultrasonics. While there is no doubt about increasing use of ultrasonic flowmeters, the speed at which they are replacing traditional flowmeters is limited by several factors. First, traditional technologies such as orifice plate, turbine, and positive displacement meters have a large installed base. Users often prefer to stick with a proven technology, and require a compelling reason to move to a new technology. As a result, they often choose the same type of flowmeter when selecting a replacement. Also, many users simply don't understand the new technologies well enough to feel comfortable selecting them.

Second, traditional flowmeters have the advantage of numerous standards written for their use by industry associations. Despite the anticipated publication of AGA-9, industry approval for ultrasonic flowmeters has been slow. Approval of AGA-9 required two years of sustained effort by AGA members. Despite this approval, the American Petroleum Institute (API, Washington D.C.) does not plan to publish the report in conjunction with the AGA. In addition, there are no immediate plans for either group to publish a standard for the use of ultrasonic flowmeters. AGA-9 simply recommends the proper use of ultrasonic flowmeters without being as a binding legal document or standard.

A further factor which limits the replacement of traditional flowmeters by ultrasonic flowmeters is advances in the area of multivariable flowmeters and primary elements.

Pressure transmitter suppliers, have recognized the trend in the market is toward new flow technologies. These suppliers have responded in two ways: introducing multivariable flowmeters that provide a measurement of inferred mass flow, and accelerating the search for improved primary elements to use with their pressure transmitters.

Among the multivariable flowmeters available are pressure transmitters that use differential pressure, static pressure, and process temperature measurements to calculate mass flow. Prominent examples of these instruments include 3095 MV from Fisher-Rosemount (Austin, Tex.) and SMV 3000 from Honeywell IAC (Phoenix, Ariz.). Multivariable magnetic flowmeters have also been introduced.

Honeywell recently introduced a multivariable version of its MagneW 3000 Plus magnetic flowmeter. It uses a process temperature measurement and a density compensation calculation to deliver mass flow. The main advantage offered by these flowmeters is the ability to perform several measurements, such as pressure, temperature, and flow, in one unit. Disadvantages include typical list prices more than double that of traditional flowmeters; and the need to calculate or infer mass flow rates rather than measuring them directly.

In addition to introducing multivariable flow meters, some pressure transmitter suppliers are taking a new look at the role of primary elements in the measurement of flow. In many cases users purchase pressure transmitters from one supplier and primary elements from another. Fisher Rosemount entered this segment through the acquisition of primary element supplier, Dieterich Standard. Dieterich Standard sells the Annubar, an averaging pitot tube that offers considerable accuracy improvements over the traditional single-point pitot tube.

Fisher-Rosemount went on to integrate its 3095 MV transmitter with the Annubar to create the Mass ProBar, a system that computes mass flow from the differential pressure, static pressure, and temperature process values. Uniqueness of this approach is that it is sold in the form of a differential pressure flowmeter. By integrating a sensing element with a transmitter in a single unit, the Mass ProBar resembles most other flowmeters, simplifying installation and calibration procedures.

Ultrasonic flowmeters have many features appealing to end-users. They are nonintrusive, have no pressure drop, and clamp-on versions can be installed without disturbing the process. As users become more aware of considering cost of ownership in purchasing decisions, the reduced maintenance of ultrasonic flowmeters, in particular, will appeal to small engineering staffs.

Despite all the factors leading toward increased use of ultrasonic flowmeters, traditional flowmeters still offer many advantages.

In addition, new developments in primary elements and multivariable transmitters continue to improve the accuracy and applicability of traditional units. As a result of the large installed base, industry approval, user familiarity, and new technological developments in traditional flowmeter technologies, it will take some time for ultrasonic flowmeters to overcome traditional flowmeters in many industries.