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5. PRINCIPLES OF OPERATION
The Teledyne API Model T265 analyzer is designed to measure the concentration
of ozone using the chemiluminescence reaction below. The signal comes from the
light emitted by the gas phase reaction of nitric oxide (NO) and ozone (O
3
) as
follows:
2
*
23
ONOONO
hvNONO
2
*
2
The reaction of ozone with NO results in electronically excited NO
2
*
molecules as
shown in the first equation. The excited NO
2
*
molecules release their excess energy
by emitting a photon hν and dropping to a lower energy level as shown in the
second equation. It has been shown that the number of emitted photons is directly
proportional to the O
3
concentration in the sample stream.
Calibration of the instrument is performed in software and usually does not require
physical adjustments to the instrument. During calibration, the microprocessor
measures the photo multiplier tube (PMT) signal when gases with known amounts
of O
3
are supplied, and stores these results in memory. The microprocessor uses
these calibration values, along with the signal from the sample gas, the pressure
readings for both the reaction cell and sample gas and box temperature in order to
calculate a final concentration.
The product requires a cylinder of NO balanced in N
2
, which acts as the reagent gas.
Both the electronic and pneumatic platforms are built using the current T series
hardware. An additional precision regulator is added to keep the NO pressure
constant, prior to the reaction cell. The internal pump draws sample through the
sample dryer, removing ambient moisture, into the reaction cell. Periodically, an
AutoZero valve switches the sample stream to the vacuum manifold, allowing the
reaction cell to be evacuated and the analyzer to read zero background. The
AutoZero readings are subtracted from the concentration readings, which improves
zero baseline stability.
The analyzer uses two temperature-controlled critical orifices to deliver sample O
3
and reagent NO into the reaction cell. See diagram of the pneumatic design in
Figure 3-4.
07337B DCN6324