Teledyne 9110EH - Nitrogen Oxides Analyzer - Manual (Page 180)

Theory of Operation Model 9110EH Instruction Manual
Figure 10-5: External Pump Pack
Finally, the M9110EH requires a steady, high underpressure, which cannot be achieved reliably
over extended periods of time with small vacuum pumps. The external pump used for the
M200E 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.
10.2.2. Sample Gas Flow
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
X
(and possibly the drying of the gas if the optional
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 M9110EH 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 10.2.9 for a detailed
description of critical flow.
The M9110EH uses a 0.010” (10 mil) diameter orifice to achieve a flow rate of about
450-500 cm³/min. With nominal pressures of 28 and 4 in-Hg-A for the sample and reaction
cell pressures, respectively. The necessary ratio of sample to reaction cell pressure of 2:1 is
largely exceeded and accommodates a wide range of possible variability in atmospheric
pressure and pump degradation. Once the pump degrades and vacuum pressure exceeds
14 in-Hg-A, the ratio between sample and vacuum pressures is less than 2:1 and, hence, a
critical flow rate can no longer be calculated. At this point, the instrument will display an
invalid sample flow rate measurement (XXXX).
166 M9110EH Rev 0