Using small sonic nozzles as secondary flow standards

When using sonic nozzles as secondary laboratory standards they must be calibrated. Standard methods of calculating discharge coefficient values are not useful with small nozzles of uncertain geometries. Calibration against primary standards is discussed. Alternatively, one nozzle can be directly compared with another by several techniques using ratios of pressure and temperature and sometimes flow. Several factors change the nozzle coefficient. Estimates of the changes due to pressure and humidity are given. Adiabatic cooling produces temperature changes that affect the nozzle coefficient by changing the throat area. Depending on the nozzle holder the inlet gas can also be cooled with an effect on the flow. Nozzles may be made by metal machining or by shrinking glass tubes. Sapphire cutting heads, which may be bought, can be used as sonic nozzles. An example of a promising but unsuitable form of nozzle having a square throat is given. The pressure dependence of these is discussed. The use of nozzles in arrays, for automatic operation as flow standards, is described. (C) 2000 Elsevier Science Ltd. All rights reserved.