Teledyne T100, Instruction Manual, Page: 236 / 404

Theory Of Operation Teledyne ML - T100 UV Fluorescence SO2 Analyzer
236 06807A(DCNxxxx)
9.4.2. FLOW RATE CONTROL
The T100 uses a special flow control assembly located in the exhaust vacuum manifold
(refer to Figure 9-10) 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.
9.4.2.1. 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.
Figure 9-11: Flow Control Assembly & Critical Flow Orifice