Campbell Scientific TGA100 Trace Gas Analyzer Manual - Manual (Page 20)

1.6.4 Absolute Concentration / Isotope Ratio Measurements
The TGA100 can be configured for highly accurate measurements of trace gas concentrations by performing frequent
calibration. The TGA100 has a small offset error caused by optical interference. This offset error changes slowly over
time, with a standard deviation roughly equal to the short-term noise. Offset errors have little effect on flux
measurements by either the gradient or eddy covariance technique, but may be important in other applications. For
measurements of absolute trace gas concentration, the offset error can be removed by switching between a
nonabsorbing gas (e.g. nitrogen) and the sample, using the gradient mode of operation.
Applications such as isotopic ratio measurements require the highest possible accuracy. This is achieved using a
frequent two-point calibration to correct for drift in the instrument gain and offset. High accuracy requires the flow rate
for the calibration gases to be the same as for the sample air. Even though the sampling system can be designed so that
calibration gases flow only when they are used, frequent calibration (every few minutes) consumes a large amount of
calibration gas if high flow rates are used. The site means sampling mode is normally used because it works well at low
flow rates.
Figure 1-8
Figure 1-8. Example CO
2
Isotope Application
illustrates a typical CO
2
isotope application. It is similar to the site means example above, but it also includes
two intakes connected to calibration tanks. A tank of nitrogen or CO
2
-free air is also shown connected to the analyzer to
purge the air gap between the laser dewar and sample cell. This purge is required for CO
2
isotope measurements
because of the high ambient concentration of CO
2
and the need for high accuracy.
Purge Dryer
Sample Dryer
Site Selection
Sampling
System
Digital Control Cable
Sample
Bypass
Dryer Purge
Sample
Sample
Intakes
Purge
Calibration tanks
20