Teledyne gfc 7000ta operation manual Teledyne Analytical Instruments 195

Theory of Operation Model GFC7000TA Carbon Dioxide Analyzer

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.

CRITICAL

FLOW

ORIFICE

AREA OF	AREA OF
HIGH	LOW
PRESSURE	PRESSURE

	Sonic
	Shockwave
SPRING	O-RINGS
	FILTER

Figure 9-8: 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 GFC 7000TA is designed to provide a flow rate of 800 cm3/min.

Teledyne Analytical Instruments 195