>> It started with flow probes, venturitubes, orifice plates and jets, float element flow measurement instruments, mechanical volume counters (in the broadest sense), and turbine blade counters. Until the 1950’s these were practically the only flow measurement instrument technologies available for industrial and commercial use.

Later came measuring instruments which incorporated electrical or electronic signal conversion and computation. Today the instruments on the market operate on a variety of principles; magnetic-inductive, ultrasonic, cyclone frequency, laser, or even on a correlation or caloric principle. But even other effects have given new solutions to the market-place such as Coanda volume counters and Coriolis mass flow meters.

The high cost of raw materials and increasingly high product quality standards and environmental protection are the driving factors of continued product development in flow measuring technology. However, even the most modern measuring instruments are only used for industrial applications when they offer better technological advantages or can be obtained and operated at a lower cost. Modern test benches used to certify or calibrate a flow meter also contribute to a continued improvement and advancement of the measurement’s accuracy and the measuring range itself. Most of the instrument manufacturer’s have such test benches, as this is the way that they are able to test and further their own product development.

Spectacular developments in new technologies cannot be seen at the present time. The current period is characterized by a large number of product offerers who have made inroads. The patent for the Coriolis mass flowmeter is unspecific in it’s wording which has caused a flood of different geometric designs for which the straight pipe version is preferred. This technology has received increasingly wider acceptance for several reasons: Direct mass flow is measured even for large diameter pipes, there is very low measuring inaccuracy, the measuring range is large and the technology has been pushed in the field of gas measurement (initially for high pressures).

Most apparent however, is the rapid growth of the ultrasonic measurement technology. The advances in the development of ultrasonic transducers and the application of modern switching circuits and methods of analog and digital signal conditioning have made it now possible for these instruments to be included in the sophisticated area of gas pipeline measurement. In 1996, in Germany the first ultrasonic gas counter for custody transfer was certified.

The outstanding features of this gas counter are that the measurement which is made without loss of pressure, it incorporates bi-directional operation and also has “diagnostic characteristics”: The instrument is normally built in a multi-path arrangement which insures a no-fail operation, because if one path happens to fail the remaining ones will not. The transducer can be immediately exchanged after giving the appropriate warning signal – under pressure and without interruption of the gas transport. Today standard safe practice calls for two different counters installed in series for example turbine blade counters or vortex counters. In the future it should be possible to replace the two different measurement method gas counters with just one ultrasonic counter.

New types of US-transducers with respect to principle of operation, size and energy consumption – such as that of the FLOWSIC 600 – are also shifting the application of ultrasonics, to small diameter pipes or for use in remote regions with a self-sufficient energy supply for example via solar panels.

Our author, Dr.-Ing. Heinz Gehlhaar, is an expert in gas pipeline measurement and is a member of the DVGW-TK (German association of Gas and Water Experts/ Technical Committee for Pipeline Measurement, Bonn). He leads research projects for pipeline measurement for the Association for Technology and Technology Transfer at the Technical University in Dresden (Tel. 0351-463-34212; email: gehlhaar@mal.mw.tu-dresden.de)