Campbell Scientific TGA100 Trace Gas Analyzer Manual - Manual (Page 70)

discarded. The Discard Scans column of the gradient parameter menu can be adjusted to discard as many level scans as
necessary. More information on setting up multi-site gradient parameters may be found in section 5.1.7.3.
The mean concentration, standard deviation of concentration, and mean pressure are calculated using all of the valid
data for each level (taking into account the shifted samples, the omitted samples, and the discarded scans). The
concentration slope is the rate of change of concentration in units of ppm per scan, where a scan includes levels 1 and 2.
The concentration slope allows the user to correct for the fact that the two intakes are sampled at different times: level 1
is always sampled before level 2. If there is an overall change in the concentration during the averaging period, there
will be an error in the concentration difference. This error can be corrected by interpolating the level 1 and 2 mean
concentrations to a common time. Interpolating the level 1 data forward in time ¼ of a scan and the level 2 data
backward in time ¼ of a scan will estimate their mean values at the center of the scan:
1 1 1
25 . 0 s x x + = ′
2 2 2
25 . 0 s x x − = ′
where and are the original mean concentrations for levels 1 and 2, and are the slopes, and and
1
x
2
x
1
s
2
s
1
x′
2
x′ are
the slope-corrected mean concentrations. These corrections are normally very small, but can be significant if the scan
time (samples/level) is large, the averaging time is short, and the concentration changes quickly over time. The user
may then subtract these slope-corrected mean concentrations to calculate the concentration difference.
5.1.3 Real time display
When the gradient mode is active, vertical lines are drawn on graph 1 and graph 2 to mark the time of critical events
(see Figure 5-2). When a sampling system valve switches at the start of a new level or site, a solid vertical line marks
the time. A vertical dotted line marks the end of the shift samples, and a vertical dashed line marks the end of the
omitted samples. Data are included in calculations from the end of the omitted samples (dashed line) to the end of the
shift samples (dotted line), and are omitted from the end of the shifted samples (dotted line) to the end of the omitted
samples (dashed line). The vertical line display can be disabled by pressing “V” at the real time screen, and re-enabled
by pressing “V” again. Disabling the vertical line display has no effect on the digital outputs used to control the
sampling system, or on the gradient calculations.
At the end of the valid data for a level (valve switch plus shift samples), the TGA100 software calculates the mean
concentration for that level. This intermediate result is displayed on the real time screen, above graph 1. The site, scan,
and level are displayed as well as the mean concentration. If dual ramp mode is active, the mean concentration for ramp
B is also displayed. If dual ramp mode is active and a standard isotope ratio is defined (nonzero), the mean isotope ratio
will also be displayed. As described above, the first one (or more) level scans and the last level scan in a site averaging
time are discarded. These discarded intermediate results are still calculated and displayed on the real time screen, but
they are shown in darker blue, with an (x) beside them. Intermediate results from level scans that are included in the
final calculations are shown in a lighter color.
5.1.4 Controlling Gradient Valve Assemblies
There are many types of gradient valve assemblies, and the TGA100 software has several options to help the user
control them. The three basic issues are: 1) does the gradient valve assembly contain a single three-way valve or a pair
of two-way valves, 2) does the valve assembly require just a short pulse to change its state (latching type) or does it
require a constant signal to maintain the active state, and 3) does the gradient valve assembly require a high voltage or a
low voltage to activate it. These issues are explained by the following two examples.
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