Campbell Scientific TGA100 Trace Gas Analyzer Manual - Manual (Page 84)

5.2.5 Site Means Parameters
Many parameters must be set to control the site means sampling system and calculations. These parameters are edited at
the Site Means Mode Parameters screen, shown in Fi
. Each of these parameters is discussed in this section. gure 5-13
Figure 5-13. Example Site Means Mode Parameters Screen
The top row of the Site Means Mode Parameters screen has the Output Interval parameter. This is the interval, in
minutes, for calculating and saving data to the site means file. The output interval must be an integer multiple of the site
scan time. For the example screen shown, sites 1 through 8 each have 150 samples, or 15 seconds. The time required to
cycle through all eight sites (a site scan) is two minutes. Therefore the output interval must be a multiple of two. A
value of 30 is shown, which will result in data written to the site means file every 30 minutes. The data will be
calculated from 15 site scans.
The rest of the Site Means Mode Parameters screen is organized as a table, with a row for each site. The first column
gives the site number, from 1 to 18. The rest of the columns contain the parameters that must be set for each site.
Site Samples is the number of samples to collect at the site. Each site may have a different number of samples. The sum
of the site times must divide evenly into the output interval. Enter 0 for sites that are not used. For the example shown,
sites 1 through 8 each have 150 samples. The time spent at each site will be 15 s. The total site scan time is 8x15=120 s
(2 minutes).
The Omit Samples parameter is also set individually for each site. Once the air sample from the new intake reaches the
TGA100 sample cell, the concentration reading does not change instantaneously because of mixing in the sample tube
and because it takes a finite time to replace the air in the sample cell. Therefore some data must be omitted from the
calculations. The Omit Samples parameter specifies how many samples to omit, starting at the end of the shift counts,
before starting to include data in the average for the next site (see
). Figure 5-12
Shift Samples: When the sampling system switches to the next intake, the concentration at the new level is not measured
immediately. There is a time delay because of the travel time through the tube from the site means sampling system to
the TGA100 sample cell. This time delay depends on the volume of the sample tubing and the sample flow rate, and
can be several seconds. The user must enter the Shift Samples parameter to accommodate this time delay. The Shift
Samples may be different for each site, because the flow rate may be different.
Site I/O Bits: In the site means sampling mode, all of the digital output bits (0 through 15) are called the Site I/O Bits.
They are used to control the site means sampling system that switches the air sample between multiple intakes. The user
enters the desired bit pattern in the Site I/O Bits column; 1 for bits to be active during the site time and 0 for bits to be
inactive during the site time. Note that these bits are affected by the Pulse Samples parameter defined in the last column
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