Campbell Scientific TGA100 Trace Gas Analyzer Manual - Manual (Page 100)

The purge flow (actual flow rate) must be at least twice the sample flow to achieve a very dry sample. The following
equation gives the minimum purge flow (standard flow rate) to meet this requirement:
t
t
p
p
V
P
P
V 2 ≥
Where V is the purge flow rate, V is the total flow rate, is the pressure of the total flow through the dryer, and
is the purge pressure. The flow rates in this equation are standard flow rates (at one atmosphere) and the pressures
are in absolute units. Note that the sample flow (the flow through the analyzer) is the difference between the total flow
and the purge flow.
p t t
P
p
P
The following example illustrates the use of this equation.
• The sample pressure is 50 mbar. This is a typical pressure for the TGA100.
• The total flow rate is 18 slpm. The sample pump is the RB0021, operating at 60 Hz, with a capacity 18 slpm at
50 mbar.
• The pressure of the sample in the dryer is assumed to be 910 mbar, as determined by the altitude and the flow
rate. The site is assumed to be near sea level, with an ambient pressure of 1000 mbar. The pressure drop in the
dryer is estimated as 90 mbar (5 mbar per slpm). The pressure at the dryer outlet is used as a conservative
estimate for the pressure in the dryer.
• The purge pressure is 75 mbar (25 mbar pressure drop allowed in the connection of the purge outlet to the 1"
suction hose).
• The required purge flow is:
3 18
910
75
2 = 





≥
p
V slpm
The minimum purge flow (3 slpm) is used, to leave a flow of 15 slpm through the analyzer. This flow is desired to be as
high as possible, to minimize the attenuation of high frequency concentration fluctuations in the sample intake tubing.
In this example the minimum purge flow is 3.0 slpm, 16.7% of the total flow (18 slpm). Generally the purge flow
should be 15 to 20% of the total flow into the dryer, but this depends on the sample/purge pressure ratio. A higher
purge flow may be required if the ambient pressure is lower than 1000 mbar, the TGA sample pressure is higher than
50 mbar, or if there is a substantial pressure drop in the tubing that connects the purge flow to the vacuum pump.
9.4.3.5 Two-Dryer Configuration
For site means applications, the sample dryer is located between the sampling system and the analyzer. The flow will be
relatively low, usually 2 slpm or less, and the pressure in the dryer will be low, usually near the analyzer pressure. The
split sample configuration does not work well in this case because of the very low pressure in the dryer. A second dryer
is used to provide the dry air to purge the sample dryer. This second (purge) dryer is purged by its own dry sample air,
using the split sample configuration.
As for the split sample configuration, the purge flow (actual flow rate) must be at least twice the sample flow rate.
However, in this case the pressures in the sample and purge flow are nearly the same, so the standard flow rate for the
sample dryer purge must be twice the standard flow rate of its sample. Typically the total flow into the purge dryer is
set to 2.5 times the sample flow. The purge dryer is used in the split-sample configuration, with 20% of its total flow
used to purge itself, and the remaining 80% of its flow used to purge the sample dryer. The figure below illustrates the
two-dryer configuration, with flow rates labeled relative to the sample flow, X.
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