Campbell Scientific TGA100 Trace Gas Analyzer Manual - Manual (Page 80)

7) If multiple sites are used, the Shift Samples must be set individually for each site. The Omit Samples and
Samples/Level parameters are common to all sites, so they must be set large enough to accommodate all sites.
An additional complication may be introduced by switching from one site to another. The site valves will switch from
one site to another at the start of a level scan, when the gradient valve assemblies switch from level 2 to level 1.
Normally the next samples would be in the shift counts, and would be included in the averaging for the previous level
2. However, the switching of the site selection valves may affect these data. Therefore one entire scan (level 1 and 2) is
discarded at the end of every site averaging time. Similarly, the first data for level 1 of the new site may not be valid,
either because of travel time down the tube from the site selection manifold or pressure/flow transients caused by the
valve switch. Therefore the first scan of every site averaging time is also discarded. In some cases the disturbance
caused by the site selection valve switching may extend beyond the first level scan. In this case, extra level scans must
be discarded. The Discard Scans column of the gradient parameter menu can be adjusted to discard as many level scans
as necessary.
5.2 SITE MEANS MEASUREMENTS
5.2.1 Site Means Overview
The site means sampling mode is used to measure the mean trace gas concentration at multiple sites. A typical
application is to measure a vertical profile, as illustrated in
. This figure shows a set of eight intake
assemblies arranged vertically on the intake mast. Each intake assembly has a filter to remove particulates and an orifice
to set the flow rate (typically less than 1 slpm). Each intake has a separate tube to carry its air sample to the site
selection system located near the analyzer, The site selection system selects one of the air samples at a time to flow
through the sample dryer and then into the analyzer sample inlet. Normally the unselected intakes are bypassed to the
sample pump to maintain constant flow in the intake tubing and to minimize valve switching pressure transients. The
valve switching parameters are normally chosen to cycle through all of the intakes in one or two minutes.
Figure 5-11
The site selection system, sample dryer, and analyzer are located close together to minimize the time delay from
switching to a new site until the new sample reaches the analyzer, but these may be located 300 m (1000 ft) or more
away from the intakes. 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 site means applications the PC is
usually 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.
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