Critical or sonic flow has a long historical background. However, substantial
use of critical flowmeters did not begin until the 1960s. Critical flowmetering had
acceptance first in the aerospace field and has now become popular in the natural
gas industry as a meter proving method and in laboratories as a flowmeter calibration
device. Acceptance was slow because it was incorrectly believed that a pressure
drop across the element equal to approximately 50 percent of the inlet
pressure was required to achieve critical flow. Actually, overall pressure drops as
low as 5 percent of inlet pressure may be sufficient to produce critical flow in a
properly designed Venturi.

Devices using this principle are sometimes called sonic nozzles, choked nozzles,
critical nozzles, or critical flow Venturis. A nozzle is a converging device and
a Venturi is a converging-diverging device. However, in common usage these terminologies
have become mixed in relation to the critical flow device.

The critical flowmeter has an excellent theoretical basis and no moving parts. It
is fairly immune to physical damage and eliminates the inaccuracies associated
with the differential pressure measurement used with other primary elements. This
makes the critical flowmeter an excellent transfer standard.

Applications

Critical flowmeters may be used to measure or control the rate of flow of gases.
Typical uses are:

• as a calibration standard to calibrate other flowmeters
• as a stable transfer standard to perform inter-laboratory comparison tests
• as a device to provide an approximately constant volumetric flow rate
  with a varying inlet pressure
• to provide a constant mass flow rate when filling a vessel from a larger
  supply source