Teledyne GFC-7001T - Trace CO Analyzer - Manual (Page 296)

Theory of Operation Teledyne API – Model T300/T300M CO Analyzer
Teledyne Analytical Instruments 296
Critical flow orifices are a remarkably simple way to regulate stable gas flow rates.
They operate without moving parts by taking advantage of the laws of fluid dynamics.
By restricting the flow of gas though the orifice, a pressure differential is created.
As the pressure on the downstream side of the orifice continues to drop, the speed that
the gas flows through the orifice continues to rise. Once the ratio of upstream pressure
to downstream pressure is greater than 2:1, the velocity of the gas through the orifice
reaches the speed of sound. As long as that ratio stays at least 2:1, the gas flow rate is
unaffected by any fluctuations, surges, or changes in downstream pressure because such
variations only travel at the speed of sound themselves and are therefore cancelled out
by the sonic shockwave at the downstream exit of the critical flow orifice.
SPRING
O-RINGS
FILTER
CRITICAL
FLOW
ORIFICE
AREA OF
LOW
PRESSURE
AREA OF
HIGH
PRESSURE
Sonic
Shockwave
Figure 13-8: Flow Control Assembly & Critical Flow Orifice
The actual flow rate of gas through the orifice (volume of gas per unit of time), depends
on the size and shape of the aperture in the orifice. The larger the hole, the more the gas
molecules move at the speed of sound and pass through the orifice. Because the flow
rate of gas through the orifice is only related to the minimum 2:1 pressure differential
and not absolute pressure, the flow rate of the gas is also unaffected by ambient pressure
fluctuations downstream.
The critical flow orifice used in the GFC7001T/GFC7001TM is designed to provide a
flow rate of 800 cc/min.
13.3.2. PARTICULATE FILTER
The GFC7001T/GFC7001TM Analyzer comes equipped with a 47 mm diameter,
Teflon, particulate filter with a 5 micron pore size. The filter is accessible through the
front panel, which folds down to allow access, and should be changed according to the
suggested maintenance schedule described in Table 11-1.