Sonic Nozzle (Critical Flow Venturi) - One of the Most Precise Gas Mass Flowmeters Available Today

Sonic Nozzle, also referred to as Critical Flow Venturi, is recognized as one of the most precise gas mass flowmeters available today, making them ideal for:

• Gas Flow Transfer Standards
• Working Standards or Check Standards
• Flow limiting devices
• Long term gas flow measurement

Sonic Nozzles maintain their calibration stability within 0.2% for more than 30 years. There are no moving parts or sharp wear edges, which would contribute to the erosion of the accuracy. Performance is limited to the primary standard used to calibrate the sonic nozzle. COX Instruments’ sonic nozzles are calibrated to achieve ±0.5% of reading, using Flow Dynamics’ Gas Calibration Laboratory. Accuracies of ±0.25% are offered as an option.

COX Instruments provides overlapping ranges to cover gas flows from 0.05 to 10,000 SCFM in air or nitrogen. Calibrations are offered in a variety of gases, which include oxygen, helium, argon, methane, carbon dioxide, propane, ethane, ethylene and hydrogen. Performance graphs are supplied with every nozzle.

When the gas is flowing through the nozzle at sonic velocity, a linear relationship exists between mass flow and the inlet pressure; therefore, a particular flowrate can be established by applying the proper value of inlet pressure. Normally, sonic flow occurs when the downstream pressure is not greater than one half the upstream pressure. The COX design improves this ratio so that sonic flow can be achieved in many applications when the downstream pressure is as much as three quarters of the upstream pressure. For applications using mass air flow meters that exceed the available flow pressure range of any one sonic nozzle, multiple nozzles can be employed.

The overall accuracy of the sonic nozzle is dependant on the precision of the pressure transducer and, to a lesser degree, the precision of the temperature transducer. The calibration uncertainty includes these measurements; however, you will be using different system pressure and temperature sensors. The uncertainty of these sensors is added to the uncertainty of the calibrated sonic nozzle. These additional temperature and pressure sensors will degrade the gas flow measurement of the system, if quality sensors are not selected. COX recommends pressure sensor accuracies of ±0.02 %, or better, depending on the accuracy requirement of your application.

For technical information on COX Instrument's New Sonic Nozzles, call (480) 948-3789, Fax (480) 948-3610 or email.