Campbell Scientific TGA100 Trace Gas Analyzer Manual - Manual (Page 71)

Example 1: The gradient valve assembly uses a pair of nonlatching two-way valves. The level 1 intake is connected to
the level 1 valve, the level 2 intake is connected to the level 2 valve, and the output ports of both valves are connected
by a tee to the TGA100 sample inlet. If the control signal to a valve is low, it will be closed; if the control signal is high,
it will be open. This example is illustrated in
. Figure 5-3
Figure 5-3. Gradient Valve Assembly with Two 2-Way Valves
Level 2 Intake
Level 1 Intake
Level 1 Valve Level 2 Valve
To TGA Sample Inlet
To Bit 0 To Bit 1
To configure the TGA100 for this example gradient valve assembly:
1) 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. For this example, connect digital
output bit 0 to the level 1valve and connect bit 1 to the level 2 valve.
2) In the top row of the Gradient Mode Parameters menu, set the Samples/Level parameter to the number of samples
to collect at each level. In this example, this parameter is set to 100. This will connect the level 1 intake to the
sample tube for 10 s and then switch to the level 2 intake for 10 s.
3) In the top row of the Gradient Mode Parameters menu, 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 level (continuous excitation,
not pulsed excitation).
4) Set the Level Bits for each active site to 11. This will enable bits 0 and 1, so that bit 0 will be active during the level
1 time and bit 1 will be active during the level 2 time. (Setting the Level Bits parameter to 0 disables the bit, so it
will never be active.)
5) Exit the Gradient Mode Parameters menu and go the Miscellaneous Valve Control screen. Set bits 0 and 1 in the
Invert digital output bits parameter to 0. This is the normal mode (not inverted). It will set the bits to a high voltage
when active and a low voltage when inactive.
When the gradient mode is started by pressing ‘G’ from the real time screen, bit 0 will be set to a high voltage for 10 s
(activating the level 1 valve to allow flow from the level 1 intake), and then bit 1 will be set to a high voltage for 10 s
(activating the level 2 valve to allow flow from the level 2 intake). Note that if the gradient mode is off, bits 0 and 1 will
both be inactive, neither intake valve will be open, and there will be no flow through the TGA100 sample cell. When
the gradient mode is on, there will always be flow from the level 1 or level 2 intake.
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