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Teledyne API Model 200A NO
X
Analyzer Instruction Manual, 02246, Rev. G, DCN 5247
5.2.2.5 Formula Values
The slope and offset terms should be checked after each calibration. The values for these terms
contain valuable information about the internal health of the analyzer. The range of acceptable
values and their meanings is given in Section 7.10.
To compute the NO
x
and NO concentrations, the formula for a straight line is used.
Where:
y = the NO
x
or NO concentration
m = the slope
x = the conditioned PMT tube output (normalized PMT voltage)
b = the offset
y = mx + b
In comparison with analog analyzers the slope term is equivalent to the "span pot" and the b term
is equivalent to the "zero pot". Again, like an analog analyzer there is only a limited range of
adjustment allowed for either term, and there are consequences of having the values near the
high or low limits of their respective ranges.
The x term is the conditioned PMT signal. PMT signal is adjusted for the AutoZero background,
range, temperature, and pressure.
The offset (b) term is the total background light with the AutoZero term subtracted out.
The AutoZero term measures detector dark current, amplifier noise, and ozone generator
background. Therefore, in the case of the NO channel the b term should be very close to zero. In
the NO
x
channel the b term is composed mostly of the molybdenum converter background.
After every zero or span calibration, it is very important to check the QUALITY of the
calibration. The calibration of the M200A involves balancing several sections of electronics and
software to achieve an optimum balance of accuracy, noise, linearity and dynamic range.
See Section 7.10 for the calibration quality check procedure.
5-13
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