Campbell Scientific TGA100 Trace Gas Analyzer Manual - Manual (Page 82)

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 site selection valves. In
this example it is assumed that bit 0 selects site 1, bit 1 selects site 2, etc. The solid vertical lines mark the times when
the valves switch. More information on using the digital output bits to control the site selection valves can be found in
section 5.2.4.
V
alv
e S
wit
ch
V
alv
e S
wit
ch
V
alv
e S
wit
ch
Site 2
Site 3
Site 4
V
alv
e Status
C
onc
entr
ation
Site 2 Valid Site 3 Valid Site 1 Valid
Shift
Samples Shift
Samples Omit
Samples Omit
Samples Shift
Samples Omit
Samples
Site 4 Valid
Figure 5-12. Site Means Shift and Omit Example
The right graph shows the corresponding concentration data. The concentration starts at a high value, 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 site. 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 site. 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 site.
The mean concentration, standard deviation of concentration, and mean pressure are calculated using all of the valid
data for each site (taking into account the shifted samples and the omitted samples). The concentration slope is the rate
of change of concentration in units of ppm per scan, where a scan includes all of the active sites. The concentration
slope allows the user to correct for the fact that the intakes are sampled at different times: site 1 is always sampled first,
then site 2, etc. In some applications it may not matter that the intakes are sampled at different times. However, for
applications involving comparisons between intakes or using calibration tanks for some of the sites, the data should be
interpolated to a common time, usually defined at the center of the site scan. To interpolate each site's mean
concentration to the time at the center of the scan:
s s s
m
N
s N
x x 




 − +
+ = ′
2
2 1
where s is the site number, N is the total number of sites, x
s
is the original mean concentrations for site s, m
s
is the
slope for site s, and is the slope-corrected mean concentration for site s. These corrections are normally very small,
but can be significant if the site scan time (the sum of the Site Samples for all of the sites) is large, the averaging time is
short, and the concentration changes quickly over time. Note that this equation assumes all sites have the same number
of samples. If this is not the case, the equation must be modified accordingly.
s
x′
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