Campbell Scientific TGA100 Trace Gas Analyzer Manual - Manual (Page 79)

inlet end of the sample tube and approximately 7 s travel time. The travel time is nearly the same for the two tubing
sizes, so either one should give acceptable results. The smaller tubing has the advantage of being near the optimum
Reynolds number, making the travel time relatively insensitive to changes in flow rate, but it is at the very limit of being
able to carry the full 4 slpm. It may not allow the 4 slpm flow rate if the tubing is really slightly longer than 85 m, or if
the experiment site is above sea level, which reduces the ambient pressure. The larger tubing will avoid these potential
problems, but will be subject to larger changes in travel time if the flow rate changes. Two possibilities that may be
considered for this gradient example are:
• Use an intermediate tubing size, such as 0.157" (4.0 mm). This would easily carry the 4 slpm flow, and it
would have slightly improved travel time compared to the 0.170" ID tubing.
• Use a larger pump, such as the RA0040. This would allow almost twice the flow of the RB0021. The larger
tubing size (0.170" ID) would now be near its optimum flow of 7 slpm, and would have a travel time of 5.0 s.
5.1.7.3 Setting Timing Parameters
The gradient mode timing parameters (Samples/Level, Shift Samples, and Omit Samples are normally set by iteratively
adjusting the parameters at the Gradient Mode Parameters screen, and then observing the 10 Hz concentration in graph
1 or 2 on the real time screen as the gradient mode is running. However, it is helpful to start this process with
reasonable values for the timing parameters, which can be estimated as follow:
1) Estimate the travel time, in seconds, from the tubing diameter and length and flow rate (see section 5.1.7.2).
Multiply this value by 10 (there are 10 samples/sec) and enter this as the number of Shift Samples for each active
site. Following the first example above, for a single site gradient, with 15 slpm flow rate and 200 m of 0.375" ID
tubing, the travel time is estimated as 13 s. The Shift Samples parameter is set to 130.
2) Divide the Shift Samples parameter by two to estimate the Omit Samples (65).
3) Multiply the Shift Samples by two (260) and then "round it up" to the first number that divides evenly into 600
(600, 300, 200, 150, 120, 100 etc.) In this case, the Samples/Level is set to 300.
These guidelines are intended only to give reasonable starting values. In order to finalize the gradient mode timing
parameters there must be an obvious concentration difference between levels. If the natural gradient is very low, an
artificial gradient can be created by filling a large plastic bag with air and then adding a small amount of the reference
gas (or other source of the target gas). Alternately the bag may be filled with nitrogen. The bag can then be placed over
one of the intakes and sealed with a cable tie. This should give a noticeable concentration difference for a few minutes
(until the bag is emptied). The timing parameters can be evaluated and adjusted as follows:
1) Start TGA real time operation and turn on the gradient sampling mode (press <G> from the real time screen).
2) Display 10 Hz concentration in graph 1 or graph 2.
3) The TGA will plot a solid line when the valve switches, a dotted line at the end of the shifted samples, and a
dashed line at the end of the omitted counts, similar to the illustration in section 5.1.2. If no vertical lines are
displayed, press <v> to turn on the vertical line display feature.
4) Observe the position of the transition from one level's concentration to the other, relative to the vertical lines. The
concentration must be constant from the valve switch (solid line) until just after the dotted line that marks the end
of the shift samples. Exit the real time screen and adjust the Shift Samples parameter on the Gradient Mode
Parameters screen as needed.
5) After the Shift Samples are adjusted, set the Omit Samples. The concentration must reach the value for the new
level just before the vertical dashed line that marks the end of the omitted samples. Exit the real time screen and
adjust the Omit Samples parameter on the Gradient Mode Parameters screen as needed.
6) After the Shift Samples and Omit Samples are set, adjust the Samples/Level as needed. In setting the value of this
parameter, there are two contradictory criteria. First, it is best to switch between the two intakes frequently to
minimize errors (noise) in the concentration difference caused by drift in the TGA100's offset error and changes in
the trace gas concentration. Conversely, the Samples/Level should generally be at least twice as long as the Omit
Samples to provide sufficient samples for averaging. The TGA software also requires that the time spent at each
level must be long enough to accommodate the omit counts and the shift counts. Following our example from
above, assume the Shift Samples parameter is adjusted from 130 to 120, and the Omit Samples parameter is
adjusted from 65 to 50. The minimum value for the Samples/Level parameter to accommodate the shift and omit
samples is 170. This can be "rounded up" to 200 samples to give three complete level scans every two minutes.
This will satisfy the requirement for an integral number of level scans at each site as long as the Site Time for each
site is set to an even number of minutes.
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