Teledyne M300EM User Manual

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.   

 

 

04288D DCN5752