Bubble Flowmeters are the most common device for measuring a volumetric flow rate. The Flow is determined by measuring the time required for a gas stream to move a soap bubble through a specific volume.
Hewlett Packard Digital Flowmeter 9301 1004
Humonics 1000 Digital Flow meter
Humonics Optiflow 520 Digital Flowmeter
J and W Scientific Digital Flowmeter