Campbell Scientific TGA100 Trace Gas Analyzer Manual - Manual (Page 81)

Purge Dryer
Sample Dryer
Site Selection
Sampling
System
Digital Control Cable
Sample
Bypass
Dryer Purge
Sample
Sample
Intakes
Figure 5-11. Example Site Means Application: 8-level Vertical Profile
5.2.2 Site Means Calculations
The TGA100 calculates the mean concentration, the standard deviation of the concentration, the concentration rate of
change (slope), and the mean pressure for each site. If dual ramp mode is active, the mean and standard deviation of the
ramp B concentration are also calculated for each site. If dual ramp mode is active and the standard ratio is defined
(nonzero), the mean isotope ratio is calculated from the mean ramp A and ramp B concentrations for each site. The
mean isotope ratio is displayed on the real time screen, but it is not saved in the data file.
These calculations account for two complications introduced by the sampling scenario. First, when the site selection
valve switches, the air sample from the new intake does not enter the analyzer's sample cell immediately. There is a
time delay because of the travel time through the tube from the site selection system to the sample cell. This time delay
depends on the volume of the sample tubing and the sample flow rate, and can be several seconds. The user must set the
Shift Samples parameter to accommodate this time delay.
The second complication is that when the air sample from the new site reaches the sample cell, some data must be
omitted from the calculations because of mixing in the tube as the sample travels from the site selection system to the
sample cell, and because it takes a finite time to replace all of the air in the sample cell. The number of samples to omit
is also entered by the user.
The time delay and mixing are illustrated in . This figure shows the valve status and trace gas concentration
as they would be displayed on the real time screen in site means mode. The left graph shows the valve status, indicating
which of the digital output bits are on. At the left edge, the valve status has a value of 1, indicating only the least
Figure 5-12
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