Teledyne 7120 - NDIR analyzer - Manual (Page 26)

Photometric Analyzer Start-up and Operation
3-3 Teledyne Analytical Instruments
absorbance.
a) A source of emitted infrared radiation to be absorbed by the gas of
interest.
b) A set of two chambers, one filled and sealed with dry air (to provide a
reference— note, it may also be a flowing reference as in the case of purity
gas measurements), and the other opened to accept a flowing sample gas.
c) A detector specifically tuned to measure only the wavelength of infrared
energy that will be absorbed by the gas being measured. For example,
carbon monoxide requires a detector turned to 4.65 microns, while carbon
dioxide needs one tuned to 4.27 microns.
d) An electronics system to process the changes in the electronic resistance
of the detector, and to convert these changes into specific electronics signals
that deliver a voltage at current linearly proportional to the concentration of
the gas measured. The Teledyne NDIR analyzer employs all of the above
features.
3.3.2 Analyzer
The emitted IR energy is generated by two specially configured miniature
lamps in parabola’s focused and operating at low power of only 0.5 watts.
These lamps are typically rated for 20,000 hours continuous operation
when
run in the DC mode (+5VDC). This collimated energy is directed through
parallel infrared beams which are chopped by a beam interrupter or disc to
effect an alternate on-off sequencing of each beam. The radiant energy
passes through two tubes, one containing a continuously flowing sample gas,
the other a non-absorbing static or flowing background gas (application
dependent) (for reference). As the beam passes through the reference tube,
the energy of the beam is unattenuated and balanced which provides a
standard of source energy output for comparison. The gas of interest is
present in the measurement tube, however, the energy of the beam will be
attenuated at the specific absorption wavelength.
After passing through the two tubes, the radiant beams are reflected and
imaged by a second concave mirror onto a photon detector. A spectral filter,
located just in front of the detector, represents the precise “window” of the
absorption band for the specific gas of interest. Energy outside the band is
eliminated.
The detector converts the optical energy from the radiant beams into an
electrical signal which is modulated in proportion to the gas concentration.