Teledyne Camera Accessories T100 ユーザーズマニュアル

The T100 uses a special flow control assembly located in the exhaust vacuum manifold 
(refer to Figure 13-10) 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. 

13.4.2.1. CRITICAL FLOW ORIFICE 

The most important component of this flow control assembly is the critical flow orifice.  

Critical flow orifices are a simple way to regulate stable gas flow rates. They operate 
without moving parts by taking advantage of the laws of fluid dynamics. Restricting the 
flow of gas though the orifice creates a pressure differential. 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.  

 

 

06807C DCN6650