Yokogawa In-Situ Gas Analyzer TDLS200 - Manual (Page 115)

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IM 11Y01B01-01E-A 6th Edition :Feb 13, 2013-00
<8. VALIDATION AND CALIBRATION>
A tunable diode laser emits light energy within a very narrow wavelength range which is controlled by the
analyzer itself, typically no more than 0.1nm across the entire scan region. This therefore allows laser scanning
across just one absorption peak and baseline (zero absorption) regions on either side. The analyzer scans this
region approximately 1,000 times per second while accumulating the spectra in memory. The scanning (i.e.
wavelength adjustment) is controlled by rapid adjustment of the electrical drive current on the diode itself.
Below is the typical ‘Raw Detector Signal’ received from the detect unit. On the left it is shown with no
oxygen present and on the right it is shown with some oxygen present.
Raw Detect Signal, No Oxygen present Raw Detect Signal – Oxygen Present
The TruePeak TDLS200 uses the raw detector signal to produce the actual absorption spectra (see
below, note the vertical scale in Absorbance Units is different between the two spectra) which is then
used to calculate the peak area. It is the peak area that is proportional to the oxygen concentration
with given process parameters. The given process parameters used in the oxygen concentration
calculation are the optical path length, gas temperature and gas pressure. These known parameters
are pre-determined and programmed into the analyzer. If the process gas temperature and/or pressure
are varying under normal operation then these values can be actively input via process transmitters.
In this case, the analyzer uses active values for the gas temperature and/or pressure and a pre-pro-
grammed value for the optical path length.
Absorption Spectra, No Oxygen present Absorption Spectra – Oxygen Present