Using small sonic nozzles as secondary flow standards

When using sonic nozzles as secondary laboratory standards they must be cal ibrated. Standard methods of calculating discharge coefficient values are n ot useful with small nozzles of uncertain geometries. Calibration against p rimary standards is discussed. Alternatively, one nozzle can be directly co mpared with another by several techniques using ratios of pressure and temp erature 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 mac hining or by shrinking glass tubes. Sapphire cutting heads, which may be bo ught, can be used as sonic nozzles. An example of a promising but unsuitabl e form of nozzle having a square throat is given. The pressure dependence o f 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.