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Teledyne API – Technical Manual - Model 300E Family CO Analyzers Theory of Operation
235
11.4. FLOW RATE CONTROL
To maintain a constant flow rate of the sample gas through the instrument, the M300E/EM uses a special flow
control assembly located in the exhaust gas line just before the pump. In instruments with the O
2
sensor
installed, a second flow control assembly is located between the O
2
sensor assembly and the pump. These
assemblies consist of:
A critical flow orifice.
Two o-rings: Located just before and after the critical flow orifice, the o-rings seal the gap between the
walls of assembly housing and the critical flow orifice.
A spring: Applies mechanical force needed to form the seal between the o-rings, the critical flow orifice and
the assembly housing.
11.4.1.1. Critical Flow Orifice
The most important component of this flow control assembly is the critical flow orifice.
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. This pressure differential combined with the action of the analyzer’s pump draws
the gas through the orifice.
As the pressure on the downstream side of the orifice (the pump side) 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
A
REA OF
LOW
PRESSURE
AREA OF
HIGH
PRESSURE
Sonic
Shockwave
Figure 11-8: Flow Control Assembly & Critical Flow Orifice
04288D DCN5752