Campbell Scientific TGA100 Trace Gas Analyzer Manual - Manual (Page 68)

Figure 5-1
Figure 5-1. Single-Site Gradient Flux Configuration
illustrates a typical flux gradient measurement at one site. Two intake assemblies are mounted at different
heights on the measurement mast. Tubing connects each intake assembly to a gradient valve assembly that selects one
of the intakes at a time. The air sample from the selected intake flows through the sample air dryer, which filters and
dries the air sample. A needle valve at the outlet of the dryer sets the sample flow rate, typically 5 to 10 slpm. Tubing
connects the outlet of the dryer to the TGA100 analyzer, which may be located 200 m (650 ft) or more away. The
TGA100 PC requires shelter from the environment, and can be located up to 500 m (1650 ft) away from the TGA100
analyzer, connected by fiber optic cable. However, for gradient applications the analyzer is normally positioned some
distance away from the intake mast, and the PC is placed near the analyzer for convenience. The sample pump requires
minimal shelter and can be located up to 90 m (300 ft) away from the analyzer, connected by a 1” ID suction hose.
5.1.2 Gradient Calculations
Although the usual goal of the gradient mode is to measure the difference in trace gas concentration at two sample
intakes, the TGA100 does not directly compute this difference. Instead, it calculates the mean concentration at each
level. The user can then subtract one mean from the other to get the difference. The TGA100 also calculates the
standard deviation of the concentration, rate of change (slope) of concentration, and mean pressure for each level. If
dual ramp mode is active, the mean and standard deviation of the ramp B concentration are also calculated for each
level. 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 level. 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 gradient 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 gradient valve to the sample cell. This time delay depends on
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