Campbell Scientific TGA100 Trace Gas Analyzer Manual - Manual (Page 69)

the volume of the sample tubing and the sample flow rate, and can be several seconds. The user must set the Shift
Samples parameter to accommodate this time delay. If the sampling system includes multiple sites, the Shift Samples
may be different for each site, because the flow rate and/or sample tube length (volume) may be different.
The second complication is that when the air sample from the new level reaches the sample cell, some data must be
omitted from the calculations because of mixing in the tube as the sample travels from the gradient valve to the sample
cell, and because it takes a finite time to replace all of the air in the sample cell. The number of samples to omit is also
entered by the user.
The time delay and mixing are illustrated in . This figure shows the valve status and concentration as they
would be displayed on the real time screen in gradient mode. The left graph shows the valve status, indicating which of
the digital output bits are on. At the left edge, the valve status has a value of 1, indicating only the least significant bit
(bit 0) is on. At the solid vertical line near the left edge, the valve status changes to 2, indicating bit 0 has turned off and
the next bit (bit 1) has turned on. These digital output bits are used to control the valves; therefore when the valve status
is 1, the air sample enters through the level 1 intake, and when the valve status is 2, the air sample enters through the
level 2 intake. The solid vertical lines mark the time when the gradient valve switches. More information on using the
digital output bits to control the gradient valve assembly can be found in section 5.1.4.
Figure 5-2
Figure 5-2. Gradient Mode Shift Samples and Omit Samples
V
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V
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V
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Level 2
Level 1
Level 2
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ation
Level 2 Valid Level 1 Valid Level 1 Valid
Shift
Samples Shift
Samples Omit
Samples Omit
Samples Shift
Samples Omit
Samples
The right graph shows the corresponding concentration data. The concentration starts at a high level, but it does not
change when the valve switches because of the delay as the air sample travels down the sample tube to the analyzer.
During this time the air sample flowing through the sample cell is still a valid sample from the previous level. The data
from the valve switch until the end of the shift samples (indicated by dotted vertical lines) are included in the
calculations for the previous level. Data from the end of the shift samples until the end of the omit samples (marked by
the dashed vertical line) are omitted. Data from the end of the omit samples to the end of the shift samples after the next
valve switch are included in the mean for the present level.
An additional complication is 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 assembly switches 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 invalidate 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 also be invalid,
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
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