Campbell Scientific TGA100A Overview - Manual (Page 9)

TGA100A Trace Gas Analyzer Overview
OV6.2 Flux Gradient
The TGA100A also supports the measurement of trace gas fluxes by the
gradient method. A gradient valve switches the sample between the two
intakes, the valid samples from each intake are averaged and the difference is
computed. The valve control and the calculations can be performed either by a
datalogger or by a computer and I/O module. The computer and I/O module
provide a convenient user interface, but require shelter from the environment.
The datalogger is generally preferred when an instrumentation trailer or other
shelter is not available.
Figure OV6.2-1 illustrates a typical gradient application. 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
PD1000 high flow sample air dryer, which filters and dries the air sample. A
needle valve at the outlet of the PD1000 sets the sample flow rate, typically 5
to 10 slpm. Tubing connects the outlet of the dryer to the TGA100A analyzer,
which may be located 200 m (650 ft) or more away. The sample pump requires
minimal shelter and can be located up to 90 m (300 ft) away from the analyzer,
connected by 1” ID suction hose.
This example shows a gradient flux measurement at a single site. However, the
TGA100A can also support flux gradient measurements at multiple sites by
installing intake assemblies, a gradient valve assembly, and a sample dryer at
each site, and a site selection system near the analyzer. The site selection
system connects one site at a time to the analyzer. Normally each site is
measured for 15 to 60 min before switching to the next site.
FIGURE OV6.2-1. Example Gradient Flux Application
TGA100A
Analyzer
Protecto
Surge
r
TGA100A
PC
Gradient
Valve
Assembly Intake
Assemblies
PD1000
Sample
Dryer
Reference Gas
Digital Signal Cable
Dryer Purge
Sample
Suction Hose Sample Pump
OV-9