Teledyne gfc 7000ta Measure Reference Ratio, Measurement Fundamentals with GFC Wheel

Theory of Operation Model GFC7000TA Carbon Dioxide Analyzer

IR unaffected by N2 in Measurement Cell

IR IS affected by CO2 in Reference Cell

IRM

Source

R

GFC Wheel

Δ H

Photo-Detector

Figure 9-3: Measurement Fundamentals with GFC Wheel

As the GFC wheel spins, the IR light alternately passes through the two cavities. When the beam is exposed to the reference cell, the CO2 in the gas filter wheel strips the beam of most of the IR at 4.3μm. When the light beam is exposed to the measurement cell, the N2 in the filter wheel does not absorb IR light. This results in a fluctuation in the intensity of the IR light striking the photo-detector (See Figure 10- 3) that results in the output of the detector resembling a square wave.

9.1.3.2. The Measure Reference Ratio

The Model GFC 7000TA determines the amount of CO2 in the sample chamber by computing the ratio between the peak of the measurement pulse (CO2 MEAS) and the peak of the reference pulse (CO2 REF).

If no gases exist in the sample chamber that absorb light at 4.3μm, the high concentration of CO2 in the gas mixture of the reference cell will attenuate the intensity of the IR beam by 60% giving a M/R ratio of approximately 2.4:1.

Adding CO2 to the sample chamber causes the peaks corresponding to both cells to be attenuated by a further percentage. Since the intensity of the light passing through the measurement cell is greater, the effect of this additional attenuation is greater. This causes CO2 MEAS to be more sensitive to the presence of CO2 in the sample chamber than CO2 REF and the ratio between them (M/R) to move closer to 1:1 as the concentration of CO2 in the sample chamber increases.

Teledyne Analytical Instruments 189