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Model 360E Instruction Manual THEORY OF OPERATION
10.2.2. Flow Rate Control
To maintain a constant flow rate of the sample gas through the instrument, the Model 360E uses special flow
control assemblies located in the purge gas line at the entrance to the GFC wheel housing and in the exhaust
gas line just before the pump (see Figure 10-7). These assemblies consists 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.
10.2.2.1. 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 though 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 10-8: 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 gas molecules, moving at the speed of sound, 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 degradations in pump
efficiency due to age.
The critical flow orifice used in the Model 360E is designed to provide a flow rate of 800 cm
3
/min.
05232 Rev B3 169