TGA200 Trace Gas Analyzer Overview 9/11 C o p y r i g h t © 2 0 1 1 C a m p b e l l S c i e n t i f i c , I n c .
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TGA200 Overview Table of Contents PDF viewers: These page numbers refer to the printed version of this document. Use the PDF reader bookmarks tab for links to specific sections. 1. System Components ...................................................2 2. Theory of Operation ....................................................3 2.1 Optical System..........................................................................................3 2.2 Laser Scan Sequence ................................................
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.
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).
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.
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.
TGA200 Trace Gas Analyzer Overview 2.3 Concentration Calculation The reference and sample detector signals are digitized, corrected for detector offset and nonlinearity, and converted to absorbance. A linear regression of sample absorbance vs. reference absorbance gives the ratio of sample absorbance to reference absorbance. The assumption that temperature and pressure are the same for the sample and reference gases is fundamental to the design of the TGA200.
TGA200 Trace Gas Analyzer Overview 4. Multiple Ramp Mode The TGA200 can be configured to measure two or three gases simultaneously by alternating the spectral scan wavelength between nearby absorption lines. This technique requires that the absorption lines be very close together (within about 1 cm-1), so it can be used only in very specific cases. The multiple ramp mode is used to measure isotope ratios in carbon dioxide by tuning each ramp to a different isotopolog.
TGA200 Trace Gas Analyzer Overview 6. Specifications 6.1 Measurement Noise The typical 10 Hz-concentration measurement noise, given in Table 6-1, is calculated as the square root of the Allan variance with 100 ms averaging (i.e., the two-sample standard deviation). This is comparable to the standard deviation of the 10 Hz samples calculated over a relatively short time (10 s).
TGA200 Trace Gas Analyzer Overview TABLE 6-3. Typical Isotope Ratio Measurement Noise Gas Isotope Ratio Wavenumber (cm-1) 10 Hz Noise Calibrated Noise (‰) Carbon Dioxide δ13C only CO2 2293.881 0.15 ppm 0.03 ppm 13 δ C 2294.481 0.5 ‰ 0.1 ‰ CO2 2310.686 0.5 ppm 0.1 ppm 13 δ C 2310.347 1.5 ‰ 0.3 ‰ 18 2310.206 1.5 ‰ 0.3 ‰ Carbon Dioxide δ13C and δ18O δ O 6.
Concentration (ppm) TGA200 Trace Gas Analyzer Overview Time (seconds) Amplitude FIGURE 6-1. Concentration time series showing response of TGA200 Frequency (Hz) FIGURE 6-2. Frequency response of the TGA200. The blue line is the measured response, the red line is the model response, the green line is the ideal response, and the red line is the response time of an ideal mixing volume with residence time of 14 ms.
TGA200 Trace Gas Analyzer Overview 6.3 Physical Specifications Length: 211 cm (83 in) Width: 47 cm (18.5 in) Height: 55 cm (21.5 in) Weight with peltier-cooled detectors: 78.6 kg (173 lb) Weight with LN2 detectors: 84.
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