Yokogawa In-Situ Gas Analyzer TDLS200 - Manual (Page 116)

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IM 11Y01B01-01E-A 6th Edition :Feb 13, 2013-00
<8. VALIDATION AND CALIBRATION>
Instrument Span Drift: The analyzer does not contain measurement components that can wear, decay
and generally drift in one direction. These are items that are used in conventional analyzers that determine
the requirement for routine span gas calibrations. With TDLS analyzers, the measurement drift is limited to
essentially optical elements only and these effects are fixed values. The amount of drift does not change
over time as the actual optical elements do not change. The changes within the optical elements that cause
the drift relate to dimensional changes under varying ambient temperatures. These small changes are
quantified during factory testing to ensure they are within allowed instrument specification and they do not
change with time.
Calculating Oxygen Concentration: Given that the optical path length, gas temperature and gas
pressure are known, and fundamental optical drift has been
predetermined, the only aspect of measurement that can now
affect the true peak area is the quantity of oxygen molecules in
the optical path. The true peak area measurement is ensured
because the laser scans wavelength from a non-absorbing
region (see data points 50-90) through the oxygen absorption
region (see data points 91-159) and on to another non-
absorbing region (see data points 160-200). The two regions
of non-absorbing spectra allow for a base-line to be drawn
below the absorption curve and subsequent true peak area
calculation.
On-Line Validation; By knowing the above measurement
principles it is possible to understand that the TDLS200 is
capable of performing on-line validations to verify analyzer
measurement performance. When the analyzer is initially
calibrated off-line using protocol certified gas standards, the
area of the absorption peak is assigned a calibration coefficient. This coefficient is then used in a series
of equations to correct for optical pathlength, gas temperature and gas pressure. Obviously, when factory
calibrating the analyzer, the measurement conditions are different to that of the process. Typically the
analyzers are calibrated with a 28.6” long optical path and ambient temperature and pressure (75deg F
and 14.7psiA). The relationships of pathlength, temperature and pressure are well known and proven and
therefore can be used to correct for other conditions such as a furnace with 60” optical path operating at
600 deg F and 14.6psiA.
When an on-line validation is performed, the analyzer is adding a known area to the absorption peak. The
oxygen concentration of the validation gas is known (typically 20.9% O2), the optical path length is known
(the validation cell has a 4” OPL), the gas temperature is known (ambient is measured by the analyzer) and
the gas pressure is also known (the validation cell vents to atmosphere). Therefore, the analyzer knows what
additional peak area should be measured during a validation cycle.
Left
Baseline
Region
Right
Baseline
Region
Oxygen
Absorption
Region