Campbell Scientific TGA100 Trace Gas Analyzer Manual - Manual (Page 75)

The top row of the Gradient Mode Parameters screen has three parameters that control the timing of the gradient valve
switching for both levels and all sites. The first parameter, Samples/Level, is the time spent at each of the levels (one
and two) before switching to the other level. It is entered as the number of 10 Hz samples, so the actual duration in
seconds will be 0.1 x Samples/Level. This parameter applies to both level 1 and 2 - the time spent at each of the two
levels will always be the same. The Samples/Level must be set to give an integral number of scans of level 1 and level
2 within each site time. For example, if the site time is set to 1 minute, then the Samples/Level may be 300 (one
complete scan of level 1 and level 2 in one minute) or 150 (two scans) or 100 (three scans), etc. The Samples/Level
must also be greater than the sum of the Omit Samples and the Shift Samples. In other words, the valid data for a level
must begin during the time the level is selected (before the valve switches to the other level).
The Omit Samples parameter also applies to both levels and all sites. Once the air sample from the new intake level
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 level (see
). Figure 5-2
Pulse Samples is the last of the parameters that applies to both levels and all sites. It gives the duration (number of
samples at 10 Hz) of the gradient valve switching pulse. For nonlatching valves, enter "-" to keep the level bits
energized during the entire time for each level. This parameter applies to the Level Bits only (bits 0 and 1).
The rest of the Gradient 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 Time is the length of time, in minutes, to spend at the site. If multiple sites are used, each site may have a different
time. Each site time must be an integral number of minutes, and the sum of site times must divide evenly into 1440 (the
number of minutes in 24 hours). Enter 0 for sites that are not used.
Discard Scans is the number of level 1/level 2 scans that are discarded at the start of each site time. One scan must be
discarded at the beginning of each site time (and another scan must be discarded at the end of each site time) because
switching to a new site invalidates the shifted samples. Depending on the design of the sampling system, there may also
be pressure transients at the start of a new site that extend beyond the first level 1/level 2 scan. The Discard Scans
parameter allows more data to be omitted from the calculations, as needed to avoid these transients at the start of a new
site.
Shift Samples: When the gradient valve switches, 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 gradient valve assembly 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. 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.
Level Bits: In the gradient sampling mode, digital output bits 0 and 1 are called the Level Bits. They are used to control
the gradient valves assemblies that switch the air sample between level 1 and level 2. The user may enable these output
bits by entering "11" in the Level Bits column or disable them by entering a "0" instead of a "1" for either one or both
bits. If enabled, bit 0 will be active during the level 1 time and bit 1 will be active during the level 2 time. If a bit is
disabled, it will be inactive all the time. Note that the level bits are affected by the Pulse Samples parameter defined
above. The level bits are also affected by the Invert digital output bits parameter in the Miscellaneous Valve Control
Parameter screen, which defines whether an active bit will have a high voltage or a low voltage.
Site I/O Bits: In the gradient sampling mode, digital output bits 2 through 15 are called the Site I/O Bits. They are used
to control the site selection sampling system that switches the air sample between multiple sites. 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 of the
table. They are also affected by the Invert digital output bits parameter in the Miscellaneous Valve Control Parameter
screen, which defines whether an active bit will have a high voltage or a low voltage.
Pulse Samples: The duration (number of samples at 10 Hz) of the site selection valve switching pulse. For nonlatching
valves, enter "-" to keep the bits energized during the entire site time. This parameter applies to the Site I/O Bits only
(bits 2 through 15).
5.1.7 Gradient Mode Setup
This section gives information to help set up a gradient measurement, including choosing the flow rate, tubing length
and diameter, and timing parameter settings.
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