Campbell Scientific TGA100 Trace Gas Analyzer Manual - Manual (Page 92)

TGA analog input four is configured for a pressure transducer to measure the pressure in the vacuum manifold at the
outlet of the sample and reference cells. Its measured voltage is converted to pressure using the parameters in the
Pressure Calculations screen. The measured pressure may be viewed on the real time screen by selecting Pressure for
display in graph 1, 2, or 3, and it will be written to the 10 Hz file if selected for file output on the File Output Selection
screen.
7.5 PC Analog Inputs
If the optional 7996 I/O board is installed in the PC, up to eight analog signals may be input to the system via the screw
terminal connection board. These inputs are sampled at 10 Hz, synchronized to the TGA100 concentration
measurements. These inputs are configured at the factory to have a -10 to +10 volt range, and the TGA software
calculates the voltage assuming this input rage. The input range may be changed by setting jumpers on the board, as
described in the user manual for the I/O board. However, the TGA software will continue to assume the input range is
+/- 10 V, so the user must correct the measurements if another input range is selected. The standard I/O board has 12-
bit resolution for these analog inputs, although some TGA100s have been provided with 16-bit I/O boards. Refer to the
I/O board user manual for details.
The measurement results can be viewed on the real time screen by selecting the desired channel (e.g. PC Analog 1) for
display in graph 1, 2, or 3. The PC analog inputs will be written to the concentration file if selected for file output on the
File Output Selection screen.
7.6 Analog Outputs
If the optional 7996 I/O board is installed in the PC, the TGA100 will continuously output two analog voltages to the
screw terminal connection board. These voltages correspond to the output variables selected by the user. Any of the
output variables available for display in the real time graphs (see Appendix A) may be selected. The output voltages
have 12-bit resolution, and they will drive up to 5 mA. They are scaled using the equation below:
) (
) (
) (
lo hi
lo hi
lo lo x x
v v
x x v v
−
−
− + =
where x is the value of the selected variable,
is the minimum data value,
lo
x
is the maximum data value,
hi
x
is the analog output voltage, v
is the minimum output voltage, and
lo
v
is the maximum output voltage.
hi
v
These voltages can be recorded by the user's data logger or other measurement system. The voltages may be converted
back to the original data units using the equation below:
) (
) (
) (
lo hi
lo hi
lo lo v v
x x
v v x x
−
−
− + =
The maximum and minimum data value parameters and the maximum and minimum output voltage parameters are set
by the user in the Analog Output parameter screen (see section 3.5.3). The maximum and minimum data value
parameters must be chosen to include the range of interest. If a data value falls outside this range, the corresponding
voltage will not be valid - it will be limited to the maximum or minimum output voltage. The maximum and minimum
voltage parameters must correspond to the voltage input range selected by jumpers on the I/O board. These jumpers are
preset at the factory for +10 V and -10 V, but may be changed by the user as described in the I/O board user manual.
As an example, assume “10 Hz Concentration” is selected for analog output 1, the maximum and minimum data value
parameters are set to 0 and 5, and the I/O board jumpers are configured for +/- 10 V output. The maximum and
minimum voltage parameters must be set to 10 and -10 to correspond to the I/O board jumper settings. If the measured
concentration is 1.8 ppm the corresponding output voltage will be -2.8 V, calculated as follows:
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