Campbell Scientific TGA100 Trace Gas Analyzer Manual - Manual (Page 83)

5.2.3
Real Time Display
When the site means mode is active, vertical lines are drawn on graph 1 and graph 2 to mark the time of critical events
(see Figure 5-12). When a sampling system valve switches at the start of a new 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 site means calculations.
At the end of the valid data for a site (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 and
scan 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. The first site scan at site means mode startup is usually a partial scan, and is 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 that are included in the final calculations are shown in a lighter color.
5.2.4 Controlling a Site Means Sampling System
There are many types of site means sampling systems, and the TGA100 software has several options to help the user
control them. An example (see
) is used to illustrate these options. It has 8 intakes arranged in a vertical
profile. Each intake is to be sampled for 15 s, to cycle through intakes 1 through 8 every two minutes. The data are to
be averaged for 30 minutes before writing the results to the site means file. The sampling system has eight valves, each
of which is operated by a solid state relay (SSR). The control signal to each SSR must be a low voltage to turn on the
SSR and energize the valve (to send that intake to the TGA).
Figure 5-11
To configure the TGA100 for this example site means sampling system:
1) In the site means sampling mode, all of the digital output bits (0 through 15) are called the Site I/O Bits or Site Bits.
They are used to control the valves assemblies that switch the air sample between intakes. For this example,
connect digital output bits 0 through 7 to the control inputs of SSRs 1 through 8 (connect bit 0 to SSR 1, bit 1 to
SSR 2, etc.)
2) Go to the Site Means Mode Parameters screen, and set the Output Interval parameter to 30. The TGA will cycle
through the eight sites for 30 minutes before calculating the results and writing them to the mmddhhmm.sm file.
The parameter screen for this example is shown in
. Figure 5-13
Figure 5-13
3) In the rows for sites 1 to 8, set the Site Sample parameter to the number of samples to collect at each site (150).
This will connect the site 1 intake to the analyzer for 15 s and then switch to the site 2 intake for 15 s, to cycle
through sites 1 through 8 every two minutes.
4) In the rows for sites 1 to 8, set the Omit Samples parameter to the number of samples to omit (50).
5) In the rows for sites 1 to 8, set the Shift Samples parameter to the number of samples to shift (0).
6) Set the Site I/O Bits as shown in . This will activate bit 0 during the site 1 time, bit 1 during the site 2
time etc.
7) Set the Pulse Samples parameter to "-". This is a flag to tell the software to hold the control bits active during the
entire sampling time for each site (continuous excitation, not pulsed excitation).
6) Exit the Site Means Mode Parameters screen and go the Miscellaneous Valve Control screen. Set the Invert digital
output bits parameter to xxxxxxxx11111111, where ‘x’ means it can be either 0 or 1. This will invert output bits 0
through 7 (they will have a low voltage when active and a high voltage when inactive).
When the site means mode is started by pressing <S> from the real time screen, bit 0 will be set to a low voltage for 15
s (activating the site 1 valve to allow flow from the site 1 intake), and then bit 1 will be set to a low voltage for 15 s
(activating the site 2 valve to allow flow from the site 2 intake), etc. Note that if the site means mode is off, bits 0
through 7 will be inactive, none of the valves will be open, and there will be no flow through the TGA100 sample cell.
When the site means mode is on, there will always be flow from one of the intakes.
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