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Teledyne API - Model T200H/T200M Operation Manual
8.3.2. SAMPLE GAS FLOW
07270D DCN7141
Figure 8-5:
Finally, the T200H/M requires a steady, high under-pressure, which cannot be achieved
reliably over extended periods of time with small vacuum pumps. The external pump
used for the T200H/M has a very long lifetime and duty cycle and provides a very good
vacuum for its entire lifetime. However, the pump is too large to fit into the chassis of the
analyzer.
The sample gas is the most critical flow path in the analyzer, as the medium has to be
routed through a variety of valves and tubes for the measurement of zero offset and
concentrations of both NO and NO
sample dryer is installed). At any point before and in the reaction cell, the integrity of the
sample gas cannot be compromised.
Sample gas flow in the T200H/M analyzer is not a directly measured value, but is rather
calculated from the sample pressure using the flow principle across a critical orifice. In
general, the differential pressure ratio between sample pressure and reaction cell pressure
needs to exceed 2:1 to allow critical flow. The actual flow rate is then only dependent on
the size of the orifice and the upstream pressure. Refer to Section 8.3.3 for a detailed
description of the instrument's method of gas flow rate control.
External Pump Pack
(and possibly the drying of the gas if the optional
X
Principles of Operation
273
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