Table 10-3: M200E Gas Flow Rates - Teledyne 200E Instruction Manual

Chemiluminescence nitrogen oxides analyzer

Table of Contents

Model 200E Instruction Manual

upstream pressure, the more gas molecules pass through the orifice. The flow rate of the

gas is also unaffected by small degradations in pump efficiency due to age as long as the

2:1 pressure difference is maintained.

Location

Sample gas inlet of

reaction cell

supply inlet of

reaction cell.

Dry air return of

Perma Pure

dryer

Vacuum manifold ,

auto-zero port.

Vacuum manifold,

IZS exhaust port

Note that the diameter of the critical orifice may change with temperature because of

expansion of the orifice material (ruby) and, hence, the most critical flow orifices in the

M200E are maintained at a constant temperature inside the reaction cell. These are the

sample and O

flows. Table 10-3 shows the flow rates for each of the critical flow orifices of

the M200E. See Appendix B for spare part numbers of these orifices.

In addition to controlling the gas flows, the two critical flow orifices at the inlets of the

reaction cell also maintain an underpressure inside the reaction cell, effectively reducing the

number of molecules in the chamber and therefore increasing the chemiluminescence yield

as the likelihood of third body quenching is reduced (Section 10.1.5.1). The M200E

sensitivity reaches a peak at about 2 in-Hg-A, below which the sensitivity drops due to a

low number of molecules and decreased yield in the chemiluminescence reaction.

The other components of the flow control assemblies are:

•

A sintered stainless steel filter, which prevents particulates from entering the

reaction cell and potentially plug the orifice. Note that very fine sub-micron particles

may still accumulate on that filter and slowly clog up either the filter or the orifice

over time. See Chapter 11 on trouble-shooting sample flow for more information.

•

Two O-rings are located before and after the critical flow orifice to seal the gap

between the walls of the assembly housing and the critical orifice and force all gas

through the orifice opening.

•

A spring applies mechanical force to form the seal between the o-rings, the critical

flow orifice and the assembly housing and to prevent the components to float up and

turn on sudden pressure drops.

044100102 Rev A