Campbell Scientific TGA100 Trace Gas Analyzer Manual - Manual (Page 67)

5 SAMPLING SYSTEM CONTROL
The TGA software can control sampling system switching valves and process the 10Hz concentration data to calculate
the mean concentration for multiple air sample intakes. There are two different sampling system control modes:
gradient mode and site means mode.
In the gradient mode, the air sample intakes are always considered as pairs (two levels at one or multiple sites). It is
called the gradient mode because it is normally used to measure the trace gas flux by the flux-gradient method. The two
intakes are called level 1 and level 2 because they normally are located at the same location (site), but at two different
heights (levels). However, there are other applications for this sampling mode. For example, the intakes could be
located at the inlet and outlet of a chamber to measure chamber flux by the mass balance method, they could sample
from a pair of sample bags collected with a relaxed eddy accumulation sampling system, or they could sample a pair of
calibration tanks (zero and span). In all of these examples the measurement of interest is the difference in concentration
between two intakes. To minimize errors in the measured concentration difference, the gradient mode switches
relatively quickly between the level 1 and level 2 intakes, cycling through both intakes many times at one site before
moving on to the next site (if applicable). This reduces errors in the concentration difference caused by drift over time
in either the TGA100 offset error or the actual trace gas concentration A typical gradient mode sampling scenario might
switch between level 1 and level 2 every 10 s, and to spend 30 min at one site before switching to the next site.
In the site means mode, there is no inherent pairing of the air sample intakes. Each intake is considered a separate site.
A typical application for the site means sampling mode is a vertical profile measurement, with air sample intakes
located at several heights on a tower. The site means mode cycles through each of the intakes (sites) in order. A typical
site means sampling scenario might spend 15 s at each of eight sites, cycling through all eight sites every two minutes.
The mean concentration for each site could be output every cycle, or these data could be averaged over a longer time
period, typically 30 minutes, as determined by the user.
With either sampling mode, the user must configure the TGA100 software parameters to:
• Give appropriate valve switching intervals
• Account for the time delay from when the sampling system valve switches until the air sample from the newly
selected intake fills the TGA100 sample cell
• Set the digital output bits as needed to control the sampling system
• Perform averaging over the desired time interval.
The following sections describe how to configure the TGA100 for gradient or site means operation.
5.1 GRADIENT MEASUREMENTS
5.1.1 Gradient Overview
The goal of the gradient sampling mode it to measure the difference in concentration measured at two sample intakes. It
is called the gradient mode because it normally is used to measure the trace gas flux by the gradient method, and the
two intakes are called level 1 and level 2 because normally they are located at the same location, but at two heights.
However, there are many other applications for this sampling mode. For example, the intakes could be located at the
inlet and outlet of a chamber to measure chamber flux by the mass balance method, or they could sample from a pair of
sample bags collected with a relaxed eddy accumulation sampling system, or they could sample a pair of calibration
tanks (zero and span). In all of these examples the measurement of interest is the difference in concentration between
two intakes.
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