Flow Rate Control; Critical Flow Orifice; Figure 10-8: Flow Control Assembly & Critical Flow Orifice - Teledyne 100E Instruction Manual

Model 100E Instruction Manual

10.3.2. Flow Rate Control

The Model 100E uses a special flow control assembly located in the exhaust vacuum
manifold (Figure 10-7) to maintain a constant flow rate of the sample gas through the
instrument. This assembly 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.

10.3.2.1. 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 external 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.
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. Also, because the
045150102 Rev XB1
CRITICAL
FLOW
ORIFICE
AREA OF
HIGH
PRESSURE
SPRING
FILTER
Theory Of Operation
AREA OF
LOW
PRESSURE
Sonic
Shockwave
O-RINGS
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