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Ultrasonic gas meters
Ultrasonic meters operate over a specified velocity range, which is independent of gas temperature, pressure, or composition. Although limits vary from one manufacturer to another, typical guidelines limit the velocity range from about 3 ft/sec to about 70 ft/sec.
The situation with ultrasonic flowmeters and international standards is quite straightforward—there is none. There is, however, an ISO committee currently working to produce a standard: ISO Standard TC30/SC5/WG1.  In the meantime, there are several best practice guidance documents:
- AGA Report 9, Measurement of Gas by Multipath Ultrasonic Meters(1998)
- BSI 7965, The Selection, Installation, Operation and Calibration of Diagonal Path Transit Time Ultrasonic Flowmeters for Industrial Gas Applications.  (2000)
Both of these documents are under review at the moment, and it is anticipated that a new revision will be issued in the near future.
Ultrasonic meter overview
A multipath transit time ultrasonic meter (USM) is basically a device that consists of three main components (Fig. 1):
- The meter body (cylindrical pipe spool)
- Transducer pairs (mounted in the pipe spool)
- An electronic module
USMs derive the volume flow of the gas by measuring the transit times of high-frequency sound waves. Transit times are measured for pulses propagating up and downstream across the gas stream at an angle with respect to the pipe axis. These transit times, together with the meter geometry, are used to calculate the average gas velocity on a particular chord. Multiple paths are used within ultrasonic meters to maximize accuracy in the overall average velocity measurement. These multiple paths also provide a certain degree of immunity to flow profile effects, such as asymmetry and swirl. The level of immunity offered by the multipath USM varies from one design to another, as shown by Grimley.  Despite the fact that the USM offers some immunity to flow profile distortions, they still require upstream straight lengths of pipe. A typical meter tube layout for a USM is shown in Fig. 2.
Theory of operation
As previously stated, USMs measure the transit times of high-frequency sound pulses. The transducers are mounted on the meter body at defined locations. Fig. 3.  shows a schematic arrangement for a single path. The dimensions X and L are precisely determined during the meter manufacture. These measurements, together with the electronic characteristics of each transducer pair, characterize the ultrasonic flowmeter. The transit time for a signal, traveling with the flow, is less than that for a signal traveling against the flow. The difference in these times determines flow velocity.
It is also important to consider any additional uncertainty associated with the through-life stability of the USM. There are several influencing factors, one of which is wall roughness. It has been shown by Zanker that changes in wall roughness can cause significant drift in USM meters that incorporate a center path bouncing configuration to determine gas velocity. With such chord configurations, the USM measures the velocity at the center of the pipe (i.e., the maximum velocity). To arrive at an average velocity, a correction factor based on the Reynolds number and wall roughness is used. Over time, the wall roughness changes, so the correction factor becomes more and more erroneous. This results in serious meter drift. This is just one influencing factor—to quantify all influences relies on a significant passing of time together with data gathering, so responsibility has to be placed on the manufacturer to demonstrate the meter’s through-life stability.
Advantages and disadvantages
There are a host of benefits offered by ultrasonic technology when compared with traditional measurement techniques such as the orifice or the turbine meter. The main benefits for ultrasonic flowmeters are shown in Table 1.
Ultrasonic meters operate over a specified velocity range, which is independent of gas temperature, pressure, or composition. Although limits vary from one manufacturer to another, typical guidelines limit the velocity range from about 3 ft/sec to about 70 ft/sec. Pressure ranges may impact the configuration of the meter because special ultrasonic transducers are sometimes specified for either high or low pressures. Caution should also be given to applications with carbon dioxide levels in excess of about 25% because CO2 may absorb the ultrasonic signals.
- ISO Standard TC30/SC5/WG1, Measurement of Gas Flow in Closed Conduits—Ultrasonic Meters. 2005. Geneva, Switzerland: ISO Technical Committee.
- Measurement of Gas by Multipath Ultrasonic Meters, Report No. 9. 1998. Washington, DC: AGA.
- BSI 7965:2000, The Selection, Installation, Operation and Calibration of Diagonal Path Transit Time Ultrasonic Flowmeters for Industrial Gas Applications. 2000. London: BSI.
- Grimley, T. 2000. Ultrasonic Meter Installation Configuration Testing. Paper presented at the 2000 AGA Operations Conference, Denver, 7–9 May.
- Zanker, K. 1999. The Effects of Reynolds Number, Wall Roughness and Profile Asymmetry on Single and Multipath Ultrasonic Meters. Paper presented at the 1999 North Sea Flow Measurement Workshop, Gardermoen, Norway, 25–28 October.
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