For calibration of a large capacity gas flow meter, a sonic nozzle bank may be used as reference system. International standards (ISO9300-1990) allow installation of a single nozzle in a meter tube as a flow transfer standard. For multiple nozzles in a single tube, the effect of interference between sonic nozzles and the chamber wall must be assessed to predict the discharge coefficient.
Two kinds of sonic nozzle banks were tested. One was assembled from seven sonic nozzles with equal throat diameter (d= 4.3 mm) in a hexagonal pattern. Three installation plates of sonic nozzles were fabricated with distances between nozzles of 16 mm to 81 mm (L/d of 3.7 to 9.5). The other was assembled from three sonic nozzles with different throat diameters (d = 4.3, 8.1, and 13.4 mm). Three installation plates for sonic nozzles were made to vary the distance between nozzles and distance from the chamber wall. Discharge coefficients of all individual nozzles were in agreement with the ISO recommended equation within ±0.2%. The average discharge coefficient of the seven nozzles in a nozzle bank was calculated from the discharge coefficients of the individually measured sonic nozzles. The differences from the directly measured results were within ±0.05% for all test cases with equal throat diameter. Also discharge coefficients of the nozzle bank calculated from those of individual sonic nozzle were the same as the direct measurements within ±0.098% at the 95% confidence level for all tested cases with different throat diameter. For these experiments, the proximity of the tube wall or the interaction of the nozzles was found not to influence the results.
