Campbell Scientific TGA100A User Manual

TGA100A Trace Gas Analyzer Overview

The TGA100A 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 TGA100A is a rugged, portable instrument designed for use in the field. Common applications include gradient or eddy covariance flux measurements of methane or nitrous oxide and isotope ratio measurements of carbon dioxide or water vapor. The TGA100A concentration measurements can be recorded with a Campbell Scientific datalogger, output as analog voltages, or sent to a computer through an Ethernet connection.

OV-1

TGA100A Trace Gas Analyzer Overview

OV1. System Components

Figure OV1-1 illustrates the main system components needed to operate the TGA100A. These system components include:

• TGA100A Analyzer: The analyzer optics and electronics, mounted in an

insulated fiberglass enclosure.

• Computer: A user-supplied computer to display data and set parameters.

• Optional Datalogger (CR5000 shown): Receives concentration data from

the TGA100A through the SDM interface cable.

• Sample Intake (15838 shown): Filters the air sample and sets its flow rate.

• Sample pump (RB0021-L shown): Pulls the air sample and reference gas

through the analyzer at low pressure.

• Suction hose (7123 shown): Connects the analyzer to the sample pump.

Supplied with RB0021 sample pump.

• Reference gas: tank of reference gas, with pressure regulator (supplied by

user).

Computer

Sample Intake

• Reference gas connection (15837 shown): Flow meter, needle valve, and

tubing to connect the reference gas to the analyzer.

Datalogger

1516

1314

1112

910

CR5000 MICROLOGGER

DIFF

CS I/O

2930

2324

2122

H L H

DIFF

CONTROL I/O

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VX3

VX2

VX4

CAO1

CAO2

IX1

IX2

IX3

IX4

IXR

C1C2C3

RS-232

COMPUTER

(OPTICALLY ISOLATED)

>2.0V

<0.8V 5V

SDI-12 12V

SDM-C1

SDM-C2

SDM-C3

12V

SW-12

CAUTION

12V

DC ONLY

CONTROL I/O

POWER

POWER OUT

12V

GROUND

POWER IN

LUG

11 - 16 VDC

pc card

status

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TGA100A Analyzer

SDM Cable

Ethernet Cable

Reference Gas Connection

Suction Hose

OV-2

Reference Gas

Sample Pump

FIGURE OV1-1. TGA100A System Components

OV2. Theory of Operation

OV2.1 Optical System

The TGA100A optical system is shown schematically in Figure OV2.1-1. The optical source is a lead-salt tunable diode laser that operates between 80 and 140 K, depending on the individual laser. Two options are available to mount and cool the laser: the TGA100A LN2 Laser Dewar and the TGA100A Laser Cryocooler System. Both options include a laser mount that can accommodate one or two lasers (up to four lasers can be installed by adding the optional second laser mount). The LN2 Laser Dewar mounts inside the analyzer enclosure. It holds 10.4 liters of liquid nitrogen, and must be refilled twice per week. The Laser Cryocooler System uses a closed-cycle refrigeration system to cool the laser without liquid nitrogen. It includes a vacuum housing mounted inside the analyzer enclosure, an AC-powered compressor mounted outside the enclosure, and 3.1 m (10 ft) flexible gas transfer lines.

The laser is simultaneously temperature and current controlled to produce a linear wavelength scan centered on a selected absorption line of the trace gas. The IR radiation from the laser is collimated and passed through a 1.5 m sample cell, where it is absorbed proportional to the concentration of the target gas. A beam splitter directs most of the energy through a focusing lens and short sample cell to the sample detector, and reflects a portion of the beam through a second focusing lens and a short reference cell to 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 FabryPerot interference.

Reference

Detector

TGA100A Trace Gas Analyzer Overview

Sample

Detector

To Pump

Reference Gas In

Sample Cell

Laser

Sample In

FIGURE OV2.1-1. Schematic Diagram of TGA100A Optical System

Dewar

OV-3

TGA100A Trace Gas Analyzer Overview

OV2.2 Laser Scan Sequence

The laser is operated using a scan sequence that includes three phases: the zero current phase, the high current phase, and the modulation phase, as illustrated in Figure OV2.2-1. 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

During the zero current phase, the laser current is set to a value below the laser’s emission threshold. “Zero” signifies 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 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 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 phase. This stabilizes the laser temperature quickly, minimizing the effect of the temperature perturbation. The entire scan sequence is repeated every 2 ms (500 scans per second).

-1

High Current Phase

(Temperature Stabilization)

Zero Current Phase

(Laser Off)

FIGURE OV2.2-1. TGA100A Laser Scan Sequence

Modulation Phase

(Spectral Scan)

Omitted

2 ms

Laser

Current

Used in

Calculation

Detector

Response

OV-4

OV2.3 Concentration Calculation

The reference and sample detector signals are digitized at 50 kHz (100 samples per scan), corrected for detector offset and nonlinearity, and converted to absorbance. A linear regression of sample absorbance vs. reference absorbance gives their ratio. The assumption that temperature and pressure are the same for the sample and reference gases is fundamental to the design of the

TGA100A. It allows the concentration of the sample, C

TGA100A Trace Gas Analyzer Overview

, to be calculated by:

))()((

DLC

)1(

DLL

−+

where C

= concentration of reference gas, ppm

= length of the short reference cell, cm

= length of the short sample cell, cm

= length of the long sample cell, cm

D = ratio of sample to reference absorbance

The trace gas concentration is calculated for each scan and then digitally filtered to reduce noise.

OV3. Trace Gas Species Selection

The TGA100A can measure gases with absorption lines in the 3 to 10 micron range by selecting appropriate lasers, detectors, and reference gas. Lead-salt tunable diode lasers have a very limited tuning range, typically 1 to 3 cm within a continuous tuning mode. In some cases more than one gas can be measured with the same laser, but usually each gas requires its own laser. The laser dewar has two laser positions available (four with an optional second laser mount), allowing selection of up to four different species by rotating the dewar, installing the corresponding cable, performing a simple optical realignment, and switching the reference gas.

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The standard detectors used in the TGA100A are Peltier cooled, and operate at wavelengths up to 5 microns. These detectors are used for most gases of interest, including nitrous oxide (N

). Some gases, such as ammonia (NH3), have the strongest absorption

(CO

O), methane (CH4), and carbon dioxide

lines at longer wavelengths, and require the optional long wavelength, liquid nitrogen-cooled detectors.

A prepared reference gas must flow through the reference cell to provide a spectral absorption template for the target gas. The concentration of this reference gas is chosen to give approximately 50% absorption at the center of the absorption line. A 200 ft year at the recommended flow rate of 10 ml min

(5.7 m3) tank of reference gas will last over one

-1

OV-5

+ 9 hidden pages

Campbell Scientific TGA100A User Manual

Specifications and Main Features

Frequently Asked Questions

User Manual

OV1. System Components

OV2. Theory of Operation

OV2.1 Optical System

OV2.2 Laser Scan Sequence

OV2.3 Concentration Calculation

OV3. Trace Gas Species Selection