Teledyne gfc 7000ta operation manual Sample Gas Flow, Flow Rate Control, Critical Flow Orifice

Theory of Operation Model GFC7000TA Carbon Dioxide Analyzer

9.2.1. Sample Gas Flow

Sample Gas Flow Control

SAMPLE

PRESSURE

SENSOR

CO2

FLOW

SENSOR

Flow Control

Purge Gas

Purge Gas

Pressure Control

Figure 9-7: Internal Pneumatic Flow – Basic Configuration

9.2.2. Flow Rate Control

To maintain a constant flow rate of the sample gas through the instrument, the Model GFC 7000TA uses special flow control assemblies located in the purge gas line at the entrance to the GFC wheel housing and in the exhaust gas line just before the pump (see Figure 10-7). These assemblies 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.

9.2.2.1. 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

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