Teledyne GFC-7001T - Trace CO Analyzer - Manual (Page 294)

Theory of Operation Teledyne API – Model T300/T300M CO Analyzer
Teledyne Analytical Instruments 294
IR shining through both cells is
affected equally by interfering
gas in the Sample Chamber
M/R
is Shifted
Figure 13-5: Effects of Interfering Gas on CO MEAS & CO REF
Thus, the difference in the peak heights and the resulting M/R ratio is only due to CO
and not to interfering gases. In this case, GFC rejects the effects of interfering gases and
so that the analyzer responds only to the presence of CO.
To improve the signal-to-noise performance of the IR photo-detector, the GFC Wheel
also incorporates an optical mask that chops the IR beam into alternating pulses of light
and dark at six times the frequency of the measure/reference signal. This limits the
detection bandwidth helping to reject interfering signals from outside this bandwidth
improving the signal to noise ratio.
The IR Signal as the Photo-
Detector sees it after being
chopped by the GFC Wheel
S
CO REF
CO MEAS
Figure 13-6: Chopped IR Signal
13.2.1.3. SUMMARY INTERFERENCE REJECTION
The basic design of the GFC7001T/GFC7001TM rejects most of this interference at a
300:1 ratio. The two primary methods used to accomplish this are:
 The 4.7μm band pass filter just before the IR sensor which allows the
instrument to only react to IR absorption in the wavelength affected by CO.
 Comparison of the measure and reference signals and extraction of the ratio
between them.
 Pneumatic Operation
CAUTION – G ENERAL S AFETY H AZARD
It is important that the sample airflow system is both leak tight and not pressurized
over ambient pressure.
Regular leak checks should be performed on the analyzer as described in the
maintenance schedule, Table 11-1.
Procedures for correctly performing leak checks can be found in Section 11.3.2.