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Critical nozzles for gas meter test rigs

The phenomenon, that Nozzles limit a gas flow independent from the suck off power was theoretical approved since 1838. They are excellent suitable as standards for flow measurement of gas meters under special conditions.

Different outside diameters of critical nozzles in size comparison

They are existing two kinds of nozzles in relation to the inline shape, toroidale and cylindrical. The toroidale nozzles are not so difficult in high quality manufacturing. They are preferred in relation to the cylindrical nozzles in spite of nearly similar flow characteristics.

The flow quantity can be determined by using time measurement with a high accuracy. The nozzles have no cycles or will be influenced by resonances. The physical principles are described in DIN ISO EN 9300 and should not be explained in details here.

Up to the beginning of the eighties, critical nozzles were used only down to a minimal flow of approximately 800 to 500 l/h. Nozzles were used mostly for bigger flows only.

The PTB was using mainly single nozzles as standards up to 1979; when the test rig for bigger gas meters was build. Nozzles with flows of 2 up to 5000 m³/h and in combination realised a big range of different flow quantities.

For a development project of the “Energiekombinat Potsdam” with the “ASMW”, the certification authority of the GDR, critical nozzles were used in 1983 first time at test rigs for domestic meters down to a minimal flow of 40 l/h. Nevertheless the very simple computer technology in the past, it was possible to realise a fast automatic error determination with a high accuracy.

In 1991 one of these test rigs was redesigned under supervision of Mr. Fabian and certified by PTB. Since this time, nozzles with smaller flows are accepted as a flow standard. Based on this, the PTB test instruction volume 25 “Measuring instruments for gas – Test rigs with critical nozzles –“ was written as a general guideline.

Nozzles with flow rates of 25 l/h and 6 m³/h

Today a test with critical nozzles seems as a very fast test procedure with a high accuracy. Further developments of the necessary computer and sensor technology generate not only a more universal and faster test benches, they will be cheaper too.

The nozzles are the nucleus of the test benches, but the test will have only a high quality by right using of the technology and an accurate registration of all necessary measurement data’s. Only altogether will give the guarantee for low uncertainties and accurate measurement.