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In time-of-flight ultrasonic gas flowmeters, the reflection of ultrasonic energy causes significant measurement uncertainties if a fast response to changing gas flow rates is required. The reflections can be attenuated by using a sufficiently high ultrasonic frequency. The direct signal is, however, also attenuated and this also makes accurate measurement difficult. The selection of the best frequency range to be used for various pipe diameters is given and it is shown that, for a large pipe diameter, the choice is very restricted.