Campbell Scientific TGA100A Overview - Manual (Page 10)

TGA100A Trace Gas Analyzer Overview
OV6.3 Site Means Sampling Mode
The TGA100A’s site means mode is similar to the flux gradient mode in that it
controls switching valves and calculates mean concentrations for each intake.
The difference between the two sampling modes is that the gradient mode
considers the sample intakes in pairs, switching several times between an
upper and lower intake before moving to another site, but the site means mode
considers all of the intakes as one group. It cycles through all of the intakes in
sequence. Applications for the site means mode include concentration profile
measurements and trace gas flux measurements using the mass balance
technique. Similar to the gradient mode, the valve switching and calculations
can be performed either by a datalogger or by the TGA100A computer and I/O
module.
Figure OV6.3-1 illustrates an eight-level vertical profile. The eight intake
assemblies are arranged vertically on a single measurement tower. These
intake assemblies include a filter to remove particulates and a critical flow
orifice to set the sample flow (typically less than 1 slpm). A separate tube
connects each intake assembly to the site selection system, which selects one
of the intakes at a time. All of the unselected intakes are connected through the
bypass tube to the 1” suction hose leading to the sample pump, keeping air
flow at all times in all intake tubes. The flow from the selected intake goes
through a PD625 low flow sample dryer to the TGA100A analyzer. A second
PD625 is used to provide dry air to purge the sample dryer.
TGA100A
PC TGA100A
Analyzer
Digital Control
Cable
Sample
Site
Selection
Sampling
System
Bypass
Sample Pump
Sample
Purge Dryer
Sample Dryer
Reference Gas
Dryer Purge
Sample
Intakes
FIGURE OV6.3-1. Example Profile Application
OV-10