While Doppler-based ultrasonic systems are capable of measuring velocity profiles directly without the need for a correction coefficient, and transit-time ultrasonic systems are widely used for clean liquids such as ultrapure water, both measurement methods have limitations. A new hybrid ultrasonic technology postures as a solution to the drawbacks of industry’s two most popular ultrasonic measurement concepts.

Figure 1. Block Diagram Of New Hybrid Ultrasonic Flowmeter

Several recent reports have touted the ability of the ultrasonic-Doppler velocity profile (UVP) method for industrial flow measurement. UVP has proven to be an efficient method of measuring velocity profiles directly without the need for a correction coefficient, a function typical of certain other flowmeter types such as ultrasonic transit-time meters. As a result, it can provide highly accurate flowrate measurement even for undeveloped flow. However, the UVP method is not without limitations.

To generate intense echo signal with Doppler shift, UVP requires bubbles and/or particles to serve as tracers of velocity field in the measured liquid. In addition, UVP’s measurable maximum velocity is limited to a rather low value as compared to other flowmeter types, due in large part to its inherent sampling theorem. For example, while the maximum velocity of an electromagnetic flowmeter is 10 m/s, it may be limited to five m/s for a UVP-based device. Further, when applying UVP to nonintrusive flowmetering, it is not possible to diametrically measure velocity profiles because of the acoustic noises surrounding the ultrasonic transducer. Clamp-on UVP systems are affected by various types of acoustic noises.

Due to the aforementioned limitations of UVP systems, transit-time ultrasonic meters are typically used for industrial nonintrusive measurements. In general, transit-time meters are a good fit for clean liquids, such as ultrapure water measurement for semiconductor applications, where ultrasonic waves can easily penetrate the liquid. In such scenarios, there is no limit for the maximum velocity without aeration. However, transit-time meters require a conversion factor of average velocity of a fully developed flow. And, although digital signal processing technology has helped transit-time technology better handle aeration, bubbly and/or opaque liquids remain a problematic medium for this type of device.

In an effort to combine the strengths of both UVP and transit-time ultrasonic systems and eliminate the weaknesses of these technologies, a new type of ultrasonic flowmeter has been developed. The device, which is essentially a hybrid of UVP and transit-time technology, is nonintrusive. It features a pair of clamp-on type ultrasonic transducers, which lock on to opposite sides of a pipe surface. Like a UVP device, it enables diametric measurement of velocity profiles; and like a transit-time meter it can measure time difference. The measurement method is determined automatically based on the conditions of measured liquid and magnitude of velocity.

Hybrid Ultrasonic Flowmeter

Figure 2. Detector Of New Hybrid Ultrasonic Flowmeter

Currently, transit-time and Doppler systems dominate the ultrasonic flowmeter market. Transit-time meters offer relatively high accuracy (±1% of rate), but they are not suitable for liquids that include a lot of bubbles and/or particles. Doppler meters are better at handling liquids with a lot of bubbles and/or particles, but they are less accurate (±3 to 5% of rate) than transit-time meters. Hence, the need for a hybrid ultrasonic meter.

The measuring principle of transit-time meters requires ultrasonic pulses to be transmitted and received between a pair of transducers obliquely and alternately. In turn, the bidirectional transit time and time difference induced by the carry effect of fluid motion are measured, and flowrate is calculated on the assumption of fully developed and axis-symmetric flow. The principal formula for the transit-time method is:

Q=(πD 2/4)(1/K){C/(2sinÉ∆)}{É¢T/(T 0-É—)} (1)

Where:

Q = Volumetric Flowrate

D = Inside Diameter

K = Conversion Factor of Average Velocity

C = Sound Velocity of Measured Liquid

É∆ = Incident Angle Into Liquid

É¢T = Transit-Time Difference

T 0 = Transit-Time When Flow Is at Rest

É— = Transit-Time In Pipe Wall and Transducers’ Wedge

The K corresponds to correction coefficient on the assumption of fully developed laminar or turbulent flow.

Figure 3. Configuration Of New Hybrid Ultrasonic Flowmeter

Meantime, the UVP method uses the Doppler effect, assuming that bubbles and/or particles contained in measured liquid are moving at the same velocity as the measured liquid. Ultrasonic pulses are transmitted into the liquid. Echo signals scattered by reflectors are received by the same transducer. The propagation line is divided into small “channels.” Velocity profiles are obtained by connecting frequency changes in each channel. Flowrate is calculated by integrating the velocity profiles. The principal formula is:

Q= ÅË v(x)dS (2)

v(x)= {C/(2sinÉ∆)}{f d(x)/f 0} (3)

x=(Ct)/2 (4)

Where:

v(x) = Velocity at Position x

f d(x) = Doppler Shift by Reflector at Position x

f 0 = Basic Excitation Frequency

t = Round Transit-Time Between Transducer and Reflector at Position x

Configuration Of The Flowmeter
Transmitter

Figure 4. Switchover Algorithm

The hardware block diagram of a hybrid ultrasonic transmitter is shown in Figure 1. It consists of a measurement board, a control board, a man-machine interface, and a power supply board. The measurement board supplies transmission signals with ultrasonic transducers, amplifying received signals, converting them into digital data, and then calculating velocities and flowrate by digital signal processing. The control board measures the temperature of the transducer’s wedge and controls key input — DC 4-20mA, DOs, serial port for RS-485/RS-232C, and LCD. The man-machine interface sets parameters and indicates flowrate, total, and diagnostics. The power supply board is the system’s power source.

Detector
Figure 2 shows the outline of a hybrid ultrasonic detector. To provide measurement accuracy over a wide temperature range, a temperature sensor is incorporated into the transducer’s wedge, allowing it to automatically compensate for changes in sound velocity. The detector enables diametric measurement of velocity profiles by locating a pair of ultrasonic transducers opposite each other on the pipe surface. This makes it possible to use the transit-time method to obtain a measurement with the same configuration by adjusting the distance between transducers.

Acoustic absorber units are installed just before transducer units. They are designed to decrease the affect of multiple reflection in the pipe wall. In addition, the excitation frequency of transducers is adjusted to minimize the effect of dispersion due to finite pipe thickness.

Switchover Algorithm

Figure 5. Test Facility for Accuracy Evaluation

The configuration of the hybrid ultrasonic flowmeter illustrated in Figure 3 uses the transit-time method when echo signals are weak due to insufficient reflectors in the measured liquid or when flow velocity exceeds the UVP measurable range. When bubbles and/or particles increase, the UVP method is used.

The appropriate method is determined by the switchover algorithm shown in Figure 4. After setting parameters such as pipe material, pipe diameter, wall thickness, liquid type, flow range, etc., the maximum velocity is calculated and checked to see if it is within UVP measurable range. If the velocity is determined to be within the range, the UVP method will be used. And if the success rate, which is defined as the number of normal channels divided by total channels, is more than 70 percent, the meter will recover the failed channels and output and display flowrate and total. Without UVP measurable range, or if the success rate is less than 70 percent, the measuring method will be switched over to transit time. The meter will signal an alarm if neither UVP nor transit time can measure normally.

Result Of Accuracy Test
An accuracy evaluation of the hybrid ultrasonic meter was performed for stainless steel pipe with an approximate diameter of 100 mm. The test loop is shown in Figure 5. A flow conditioner was installed on the upstream side of the pipe. The detector was installed on the pipe surface seven diameters downstream from the flow conditioner. An electromagnetic flowmeter installed on the downstream side of the pipe was used as the reference meter. The electromagnetic meter was calibrated within ±0.1 percent by the gravimetric method. The test liquid was water and air bubbles were used as velocity field tracers. Air was injected into water on the suction side of upstream pump and broken into small air bubbles by a pump. The mixture ratio of air to water was set to approximately 0.02 to 0.1 percent of flow volume.

The actual flow test was done at the velocity from 0.2 to two m/sec. For the range of more than 0.4 m/sec, the accuracy approached +/-1 percent. For the range of more than one m/sec, it was within nearly +/-0.5 percent. It was also confirmed that the t wo radius velocity profiles coincided with each other in the central region. The test showed that the hybrid meter was able to provide accuracy on par with the electromagnetic flowmeter.

About the Author
Toshihiro Yamamoto is senior manager of instruments research and development for Fuji Electric Systems in Japan. He has more than 30 years of experience in research and development of pressure transmitters and flowmeters, with particular expertise in sensing electronics. Mr. Yamamoto can be reached at yamamoto-toshihiro@fesys.co.jp.

For More Information: www.fic-net.jp/eng/index.html


Acknowlegement
Special assistance was provided in developing this article by the following engineers of Fuji Electric Group: H. Yao, M. Kishiro, N. Hirayama, T. Okudera, O. Kashimura, Y. Ohmuro, K. Hagiwara, S. Suzuki, K. Yamada, A. Miyamoto, N. Tadata, G. Ohgawara. Professor Takeda of Hokkaido University contributed to this article as well.


References

1. Y. Takeda, “Velocity profile measurement by ultrasound Doppler shift method,” International Journal, Heat & Fluid Flow, Vol.17, No.4, pages 313-318 (1986).

2. M. Mori, Y. Takeda, T. Taishi, N. Furuichi, M. Aritomi, H. Kikura, “Development of a novel flow metering system using ultrasonic velocity profile measurement,” Experienced Fluids, Vol. 32, pages153-160 (2002).

3. K. Tezuka, H. Tezuka, M. Mori, Y. Takeda, H. Kikura, M. Aritomi, N. Furuichi, “Development of flow rate and profile measurement using ultrasonic Doppler method (17) Effects of pipe elbows on swirl and Reynolds number changers,” Atomic Energy Society of Japan’s autumn annual conference in 2003, pages 396.

4. S. Wada, H. Kikura, M. Aritomi, Y. Takeda, M. Mori, “Development of flow rate and profile measurement using ultrasonic Doppler method (18) Multiline flow rate measurement on inlet flow after pipe elbow,” Atomic Energy Society of Japan’s autumn annual conference in 2003, pages 397.