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Teledyne API Model 201A NH
3
Analyzer Instruction Manual, 04689, Rev. B1
converts some of the ammonia to NO, thus the TN channel data contains a small
ammonia signal. Secondly, the ammonia converter does not convert all of the ammonia
or ambient NO2 to NO. Finally, the high temperature in the ammonia converter causes
the oxygen and nitrogen in ambient air to react, creating a background of NOx which
needs to be factored into computing the result. Fortunately, all of the above
shortcomings have been overcome in the M201A by careful calibration, and calculation
of the result. .
Periodically, the AutoZero valve switches allowing the analyzer to read zero
background. The AutoZero readings are subtracted from all of the other readings. This
improves zero baseline stability.
As with many chemical reactions the conversion of ammonia in the presence of other
oxides of nitrogen is complicated.
In the molybdenum converter operating at 315
o
C, the following significant reactions are
taking place:
Mo + NO
2
→ MoO
3
+ NO ~100% Eff
NH
3
+ Mo + O
2
→ MoO
3
+ NO <10% Eff
Notice in the second reaction some of the ammonia is converted to NO by the
molybdenum metal.
The M501NH NH3 Converter operates at 825
o
C. At this high temperature, several
reactions occur:
NO → NO Efficiency = ~100 %
NOx → NO Efficiency = ~85%
NH3 → NO Efficiency = ~85%
N2 + O2 → NO Efficiency = <1%
As can be seen from the above reactions, the calculation of the ammonia concentration
and overall calibration of the instrument must be done carefully if accurate ammonia
concentrations are to be measured. The calibration procedure is covered in Section 2.
1.1.1 Special Considerations for Ammonia Measurement
Ammonia is a difficult gas to measure due to its chemical characteristics. Because of
its large dipole moment, the gas tends to adsorb on surfaces and diffuse into many
materials. The following precautions should be observed when designing an ammonia
sampling system and connecting the M201A analyzer to the system:
Materials:
Do NOT use TEFLON tubing or fittings in contact with NH3 sample or span gas.
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