Campbell TGA200 User manual

TGA200 Trace Gas Analyzer
Overview
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Copyright © 2011
Campbell Scientific, Inc.

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TGA200 Overview Table of Contents
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1. System Components ...................................................2
2. Theory of Operation ....................................................3
2.1 Optical System..........................................................................................3
2.2 Laser Scan Sequence ................................................................................4
2.3 Concentration Calculation ........................................................................5
3. Trace Gas Species Selection......................................5
4. Multiple Ramp Mode....................................................6
5. User Interface...............................................................6
6. Specifications ..............................................................7
6.1 Measurement Noise ..................................................................................7
6.2 TGA200 Frequency Response..................................................................8
6.3 Physical Specifications ...........................................................................10
Figures
1-1. TGA200 system components ..................................................................2
2-1. Schematic diagram of TGA200 optical system .......................................3
2-2. TGA200 laser scan sequence ..................................................................4
5-1. Real-time graphics screen........................................................................6
6-1. Concentration time series showing response of TGA200 .......................9
6-2. Frequency response of the TGA200........................................................9
Tables
6-1. Typical Concentration Measurement Noise ............................................7
6-2. Typical Concentration Measurement Noise for Multiple Scan Lasers....7
6-3. Typical Isotope Ratio Measurement Noise .............................................8
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TGA200 Trace Gas Analyzer Overview
The TGA200 Trace Gas Analyzer measures trace gas concentration in an air
sample using tunable diode laser absorption spectroscopy (TDLAS). This
technique provides high sensitivity, speed, and selectivity. The TGA200 is a
rugged, portable instrument designed for use in the field. It can measure one of
a large number of gases by choosing appropriate lasers and detectors.
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TGA200 Trace Gas Analyzer Overview
1. System Components
Figure 1-1 illustrates the main system components as well as additional
equipment needed to operate the TGA200. These system components include:
•TGA200 Analyzer
•Sample intake (15838 shown): Filters the air sample and controls its flow
rate.
•Sample pump (RB0021-L shown): Pulls the air sample and reference gas
through the analyzer at a low pressure.
•Reference gas: Tank of reference gas, with pressure regulator (supplied by
user).
•Reference gas connection (15837 shown): Flow meter, needle valve, and
tubing to connect the reference gas to the analyzer.
•Datalogger (CR3000 shown)
•Computer (supplied by user): Used to check status and set parameters in
the TGA200.
FIGURE 1-1. TGA200 system components
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TGA200 Trace Gas Analyzer Overview
2. Theory of Operation
2.1 Optical System
The TGA200 optical system is shown schematically in Figure 2-1. The optical
source is a lead-salt tunable diode laser that operates between 80 and 140 K,
depending on the individual laser. The TGA200 LN2 Laser Dewar includes a
laser mount that can accommodate one or two lasers. The LN2 Laser Dewar
mounts inside the analyzer enclosure. It holds 14.5 liters of liquid nitrogen and
must be refilled once per week.
FIGURE 2-1. Schematic diagram of TGA200 optical system
The laser is simultaneously temperature and current controlled to produce a
linear wavelength scan centered on a selected absorption line of the trace gas.
A beam splitter allows most of the energy from the laser to pass through a
1.5 m sample cell, where it is absorbed proportional to the concentration of the
target gas. A lens at the outlet of the sample cell focuses the beam onto the
sample detector. The portion of the beam that is reflected by the beamsplitter
passes through a reference cell similar to the sample cell and is focused on the
reference detector. A prepared reference gas having a known concentration of
the target gas flows through the reference cell. The reference signal provides a
template for the spectral shape of the absorption line, allowing the
concentration to be derived independent of the temperature or pressure of the
sample gas or the spectral positions of the scan samples. The reference signal
also provides feedback for a digital control algorithm to maintain the center of
the spectral scan at the center of the absorption line. The simple optical design
avoids the alignment problems associated with multiple-path absorption cells.
The number of reflective surfaces is minimized to reduce errors caused by
Fabry-Perot interference.
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TGA200 Trace Gas Analyzer Overview
2.2 Laser Scan Sequence
The laser is operated using a scan sequence that includes three phases: the
modulation phase, the zero-current phase, and the high-current phase, as
illustrated in Figure 2-2. The modulation phase performs the actual spectral
scan. During this phase the laser current is increased linearly over a small
range (typically ±0.5 to 1 mA). The laser’s emission wavenumber depends on
its current. Therefore, the laser’s emission is scanned over a small range of
frequencies (typically ±0.03 to 0.06 cm-1).
During the zero-current phase, the laser current is set to a value below the
laser’s emission threshold. “Zero” signifies that the laser emits no optical
power; it does not mean the current is zero. The zero-current phase is used to
measure the detector’s dark response, i.e., the response with no laser signal.
The reduced current during the zero-current phase dissipates less heat in the
laser, causing it to cool slightly. The laser’s emission frequency depends on its
temperature as well as its current. Therefore, the temperature perturbation
caused by reduced current during the zero-current phase introduces a
perturbation in the laser’s emission frequency. During the high-current phase,
the laser current is increased above its value during the modulation phase to
replace the heat “lost” during the zero-current phase. This stabilizes the laser
temperature quickly, minimizing the effect of the temperature perturbation.
The entire scan sequence is repeated every 2 ms. Each scan is processed to
give a concentration measurement every 2 ms (500 Hz sample rate).
Laser
Current
Zero Current Phase
(Laser Off)
Omitted Used in Calculation
2 ms
High Current Phase
(Temperature
Stabilization)
Modulation Phase
(Spectral Scan)
Detector
Response
Modulation Phase
(Spectral Scan)
High-Current Phase
(Temperature
Stabilization)
Laser
Current
Zero-Current Phase
(Laser Off)
Detector
Response
2 ms
FIGURE 2-2. TGA200 laser scan sequence
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