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Teledyne API Model 200AU NO
X
Analyzer Instruction Manual, 02293, Rev. F
5-15
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 7.8.
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
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 Pre-reactor
background, range, temperature, and pressure.
The offset (b) term is the total background light with the Pre-reactor term subtracted out.
The Pre-reactor term measures detector dark current, amplifier noise, and ozone generator and
hydrocarbon background.
After every zero or span calibration, it is very important to check the QUALITY of the
calibration. The calibration of the M200AU involves balancing several sections of electronics
and software to achieve an optimum balance of accuracy, noise, linearity and dynamic range.
See Section 7.8 for the calibration quality check procedure.