The national primary standard for gas flow rates from 0.005 to 200m
3/h has been established using three constant volume tank systems and a weighing tank system in Japan. The constant volume tank systems cover a flow rate range of 0.1 to 200m
3/h and the least measurement uncertainty of 0.05% has been achieved for flow rates from 6 to 200m
3/h. The weighing tank system can measure a mass flow rates of gases directly and the least measurement uncertainty of 0.1% has been achieved for mass flow rates from 0.1 to 5g/min (about 0.005 to 0.25m
3/h in volumetric flow rates of air at 0.1MPa).
Super-accurate machining technology has been applied to sonic Venturi nozzles to achieve a flow rate standard without hardware such as a constant volume tank system. The technology has succeed in determining their discharge coefficient at uncertainties better than 0.1% without calibration by the hardware. The discharge coefficient was theoretically calculated by applying the exact solution of laminar boundary layer of the sonic Venturi nozzle by Geropp and calculation result of velocity profile in the core flow of the nozzle by Hall. The result of calculation agreed with experimental measurements within 0.03% for air flow rates from 25 to 200m
3/h. The results were compared with measurements at PTB in Germany and CEESI in U. S. A. and they agreed within 0.1%. Although the results of comparison agreed with the values of the present ISO within it's uncertainty of 0.5%, clear discrepancy between the results and ISO was also recognized. This result suggests that the present ISO standard should be improved.
The effect of property of several gases on the discharge coefficient has been experimentally investigated and compared with the theoretical calculation based on the specific heat ratio of gases. The calculation agreed with experimental results within 1% except for CO
2 and SF
6. As for industrial application of the nozzles, the facilities to test various flowmeters efficiently with the uncertainty better than 0.2% using sonic Venturi nozzle have been developed. Also automatic calibration facility for sonic Venturi nozzles based on the nozzles calibrated beforehand has been developed and the difference between the calibration by the facility and the constant volume tank system were within 0.03% at flow rate from about 6 to 30m
3/h.
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