Campbell Scientific TGA100 Trace Gas Analyzer Manual - Manual (Page 66)

transmitting most of the optical power to the sample detector and reflecting less than 10% onto the reference detector,
and 2) the reference detector is adjusted (see section 4.5.2) to give a relatively low response. The reference detector
may have a small amount of nonlinearity, but this tends to be cancelled by setting the sample detector linearity
coefficient so that the sample detector matches the reference detector, as described below.
1) Change the plumbing configuration to flow the reference gas through both short cells in parallel. This can be done
by disconnecting the tubing at the inlet to the reference cell and inserting a tee and two short tubes connected to the
inlets of the reference cell and the short sample cell. Similarly, disconnect the tubing at the outlet of the reference
cell and insert another tee and two more tubes connected to the outlets of the reference cell and the short sample
cell. This will split the reference flow to go through the two short cells in parallel. The sample inlet (long sample
cell) should be connected (in the normal way) to a source of air or nitrogen. This plumbing configuration should
result in both detectors seeing the same amount of absorption.
2) Go to the parameter change menu and set the following parameters:
• Reference gas concentration (ppm): 1000
• For dual ramp mode, also set the ramp B reference gas concentration to 1000.
• Length of long sample cell (cm): 0
• Length short sample cell (cm): actual value
• Length reference cell (cm): actual value
3) Return to the real time screen and observe the mean concentration in Graph 1. The short sample cell and the
reference cell contain the same reference gas; therefore, the measured concentration should equal the reference gas
concentration (set to 1000 above). Sample detector nonlinearity will cause the measured concentration to be
underestimated. If the measured concentration is too low, increase the value of the sample detector linearity
coefficient (in the dynamic parameters column) until the measured concentration is 1000. For dual ramp mode,
also observe the mean ramp B concentration in Graph 2, and adjust the ramp B sample detector linearity coefficient
until the ramp B concentration is equal to 1000.
4) Restore the plumbing to its normal configuration and set the reference gas concentration and cell length parameters
back to their proper values.
4.6 Calibration
The predominant sources of error in the TGA100 concentration measurement are the offset error caused by Fabry-Perot
interference, and gain errors caused by errors in reference gas analysis, or by different pressure or temperature in the
reference and sample cells. For eddy covariance or gradient flux applications the offset error cancels out and only the
gain errors are significant. For measurements of absolute concentrations, the offset errors are also significant. Therefore
the appropriate calibration procedure depends on the application. All applications will benefit from the basic span
calibration described in the next paragraph. It should be performed after the TGA has been set up as discussed in
sections 4.1 through 4.5.
The TGA calibration may be checked by switching the sample inlet between two calibration tanks. Normally one tank
should have near ambient concentration and the other calibration tank should have zero concentration, but for
applications measuring very high concentrations, it is preferable to bracket the expected measurement range. For
example, if measuring isotope ratios in ambient CO
2
, calibration tanks with ~300 ppm and ~600 ppm may be preferred.
Configure the calibration tank connections to supply the same flow rate, and to give the same sample cell pressure, as
for the trace gas measurements. The difference in the measured concentrations for the two tanks should be equal to the
true difference between the two calibration tanks. If it is not, adjust the Reference gas concentration (ppm) parameter
on the Concentration Calculation screen proportional to the measured error:






−
−
=
2 1
2 1
M M
T T
C C
Orig New
Where C
New
is the corrected reference gas concentration, C
Orig
is the original reference gas concentration, T
1
and T
2
are
the true concentrations in the calibration tanks, and M
1
and M
2
are the calibration tank concentrations measured by the
TGA.
66