ABB Measurement and Analyser AO2000-LS25 Laser Analyser

General Purpose and Explosion Proof

Easy Measurement

This is a precision instrument It is a precision instrument using state-of-the-art electronics and laser technology.

Installation and maintenance The instrument requires care and preparation for installation and maintenance. Failure to do so may damage the instrument and void the warranty.

1.2 Measurement principle

The analyser is an optical instrument based on tunable diode laser absorption spectroscopy (TDLAS).

The analyser is an optical instrument based on tunable diode laser absorption spectroscopy (TDLAS) and is used for continuous on-site monitoring of gases in chimneys, pipelines, process chambers or similar.

The analyser measures the average gas concentration along the line-of-sight path using a transmitter/receiver configuration (mounted in a relative diameter installation).

The measurement principle is infrared single line absorption spectroscopy, based on the fact that each gas has a different absorption line at a specific wavelength.

The measurement principle is shown in Figure 1-1. The laser wavelength is scanned over the selected absorption lines of the gas to be measured.

The absorption lines are carefully selected to avoid cross-interference from other (background) gases. Cross interference from other (background) gases.

The detected light intensity varies with the wavelength of the laser. In the optical path between the transmitter and receiver, the target gas molecules absorb the laser wavelength.

Absorption of target gas molecules in the optical path between the transmitter and receiver. In order to improve the sensitivity, wavelength modulation technique is used.

In order to increase the sensitivity, a wavelength modulation technique is used: the laser wavelength is slightly modulated while scanning the absorption line.

The detector signal is spectrally decomposed into harmonic frequency components of the laser modulation frequency.

The second harmonic of the signal is used to measure the concentration of the absorbed gas. Both line amplitude and line width are Both line amplitude and line width are extracted from the second harmonic line shape.

Therefore the measured concentration is not sensitive to changes in the line shape (line amplitude and line width). The measured concentration is not sensitive to line shape changes (line width and line width) caused by background gases.

Note: The analyser measures only the concentration of free molecules in a given gas and is therefore not sensitive to molecules bound to other molecules in complexes and molecules attached to or dissolved in particles and droplets.

Care should be taken when comparing results with those obtained by other measurement techniques.

1.3 Instrument Description

The analyser consists of 3 separate units (see Figure 1-2):

 Transmitter unit with decontamination

 Receiver unit with decontamination

 Power supply unit

The transmitter unit consists of the laser module, the temperature stabilised diode laser, the collimating optics and the main electronics in a coated aluminium case.

The receiver unit contains a focusing lens, photodetector and receiver electronics in a coated aluminium case.

Both the transmitter and receiver have an environmental protection rating of IP66. The standard optical window is pressure resistant up to 5 bar (absolute pressure).

The monitor is mounted by assembling the transmitter and receiver with the supplied purge and calibration unit and mounting to a DN50 process flange (see Figure 3-1).

Optical alignment is both simple and reliable, and the purge device prevents dust and other contaminants from being deposited on the optical window.

A block diagram of the AO2000-LS25 is shown in Figure 1-3.

The power supply unit converts 100-240 volts AC to 24 volts DC (24 volts DC can be supplied directly to the transmitter unit if available).

The power supply box is connected to the transmitter box by a cable. 4-20 mA input signal from external gas temperature/pressure sensor

can be connected to the screw terminals in the power supply box or directly to the cable connector on the transmitter unit.

The receiver electronics are connected to the transmitter electronics via a cable.