Sample Gas Air Flow; Flow Rate Control; Figure 6-3. Internal Pneumatic Flow - Basic Configuration - Teledyne T400 User Manual

6.2.1 S
AMPLE
The flow of sample gas through the T400 analyzer is produced by an internal pump that
draws a small vacuum on the downstream side of a critical flow orifice thereby creating a
controlled airflow through the analyzer's absorption tube and other components. This
requires the analyzer gas inlets be at or near ambient pressure usually managed by
placing a vent line on the incoming gas line (see connections in Section 2.3.2).
By placing the pump down stream from the sample chamber, several problems are
avoided.
• First, the pumping process heats and compresses the sample air complicating the
measurement process.
Additionally, certain physical parts of the pump itself are made of materials that might
•
chemically react with the sample gas.
• Finally, in certain applications where the concentration of the target gas might be high
enough to be hazardous, maintaining a negative gas pressure relative to ambient means
that should a minor leak occur, no sample gas would be pumped into the atmosphere
surrounding analyzer.
SAMPLE
GAS INLET
SPAN GAS
INLET
ZERO AIR
INLET
DRY AIR
INLET
EXHAUST
GAS
OUTLET
PUMP
Figure 6-3. Internal Pneumatic Flow – Basic Configuration
6.2.2 F R
LOW
To maintain a constant flow rate of the sample gas through the instrument, the T400 uses
a special flow control assembly located downstream from the absorption tube and in the
exhaust gas line just before the pump (see Figure 10-7). 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
• A spring: Applies mechanical force needed to form the seal between the o-rings, the
144
G A F
AS IR LOW
Flow / Pressure
Sensor PCA
Sample Gas
Flow Control
C
ATE ONTROL
gap between the walls of assembly housing and the critical flow orifice.
critical flow orifice and the assembly housing.
Teledyne API – Model T400 O
Particulate
O
3
Filter
SAMPLE
PRESSURE
SENSOR
O3 FLOW
SENSOR
Analyzer
3
INSTRUMENT CHASSIS
Scrubber
Measure/
Reference
Valve
083730400A DCN8130