Emerson Rosemount Analytical 400A Instruction Manual page 22

Model 400A
1-14
The preferred type of fuel depends on the particular application and the
characteristics of the sample gas:
• For measuring low-level hydrocarbons in ambient air, or in other sample
gas with relatively constant oxygen content, 100% hydrogen is
preferable. It provides the highest obtainable sensitivity and the
maximum stability. Zero drift caused by ambient temperature variations
of the fuel cylinder is somewhat lower for 100% hydrogen than for
mixed fuel. (With either fuel, it is desirable to maintain cylinder
temperature constant.)
• For monitoring vehicular exhaust emissions, or other sample gas with
varying oxygen content, mixed fuel is preferable; and a
hydrogen/helium mixture is more desirable than a hydrogen/nitrogen
mixture. With this type of sample, the use of mixed fuel gas minimizes
the error introduced by oxygen synergism. The preferred way to reduce
the effect of internal oxygen is to dilute it with an inert gas. This might
be accomplished by a constant dilution of sample and calibration gases
ahead of the burner but it is simpler and more accurate to provide that
diluent in the form of premixed fuel. Both nitrogen and helium have
been used as a diluent, but helium has proven to be most effective in
improving the equality of response to the various species of
hydrocarbons.
As indicated earlier the flame output signal is optimum when the ratio of
hydrogen flow to inert flow is about 40/60; therefore, this is the chosen
composition for hydrogen/helium premixed fuel.
The sample flow is kept low to maximize the dilution effect while still providing
adequate sensitivity. The burner air flow is normally about four times the fuel
flow, and changes have little effect on signal strength. For a given sample
flow, the signal can be optimized by adjusting the fuel flow rate. Typical flow
rates with premixed fuel are:
Fuel
Sample
Air
With a 40/60 premixed fuel, the above flows amount to 40 cc (8%) hydrogen,
67 cc (13%) inert plus sample and 400 cc (79%) air, which compare closely to
the 30 cc (8%) hydrogen, 45 cc (12%) inert/sample and 300 cc (80%) air
given earlier for straight hydrogen fuel.
Since the sample flow in the case of mixed fuel operation is only about
one-sixth of that with straight hydrogen fuel, it is clear that higher sensitivity is
obtained with straight hydrogen fuel operation. However, in any application
where the sample contains more than one species of hydrocarbon and/or a
varying concentration of oxygen, the mixed fuel operation should be used.
The mixed fuel is recommended, not only for sample containing variable
concentrations of oxygen, but for two specific pure gas applications. The first
is the case of pure hydrogen samples. The other is the case of pure oxygen
samples. If straight oxygen samples are used with straight hydrogen fuel, the
mixture entering the burner is essentially 40% H
produce an unstable signal. The mixed fuel works better. Note that the choice
of fuel determines certain analyzer characteristics, as tabulated in Table 1-4.
100 cc/min.
7 cc/min.
400 cc/min.
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Instruction Manual
IM-103-400A, Rev. 1.0
December 2008
/60% O , which tends to
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