Flow Rate Control - Critical Flow Orifices - Teledyne T200H User Manual

Principles of Operation
8.3.2.1. NO/NO and AutoZero cycles
x
NO/ NO
PHASE VALVE
STATUS
Open to
NO
AutoZero
Measure
valve
Open to
NOX
NO
Measure
converter
Cycle repeats every ~8 seconds
Open to
AutoZero AutoZero
valve
Cycle repeats every minute

8.3.3. FLOW RATE CONTROL - CRITICAL FLOW ORIFICES

274
For the routing of the sample gas flow, the analyzer uses a variety of valves. The
NO/NO valve directs the sample gas either directly to the reaction cell or through the
X
unit's NO converter, alternating every ~4 s. The AutoZero valve directs the sample gas
2
stream to completely bypass the reaction cell for dark noise measurement once every
minute, which is then subtracted as a measurement offset from the raw concentration
signal. The valve cycle phases are summarized in the following table.
Table 8-2: T200H/M Valve Cycle Phases
AUTOZERO
X
VALVE
STATUS
Open to
reaction cell
Open to
2
reaction cell
Open to
vacuum
manifold
The Model T200H/M analyzers use special flow control assemblies (Figure 8-8) located
at various locations within the instrument to maintain constant flow rates for both the O
supply air and the sample gas. 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.
The figures that follow highlight the location of these flow control assemblies:
Teledyne API - Model T200H/T200M Operation Manual
TIME
ACTIVITY
INDEX
Wait period (NO dwell time).
0 - 2 s Ensures reaction cell has been
flushed of previous gas.
Analyzer measures chemilumi-
2 - 4 s
nescence in reaction cell.
Wait period (NOX dwell time).
4 – 6 s Ensures reaction cell has been
flushed of previous gas.
Analyzer measures NO + O3 chemi-
6 – 8 s
luminescence in reaction cell.
Wait period (AZERO dwell time).
Ensures reaction cell has been
0 – 4 s
flushed of sample gas and chemi-
luminescence reaction is stopped.
Analyzer measures background
4 - 6 s
noise without sample gas
FIGURE
Figure 8-2
Figure 8-2
Figure 8-4
3
07270D DCN7141