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Gas Spectroscopy



NEO has provided gas monitors based on spectroscopic techniques for industrial applications since 1990, when the first Open Path Monitors for measurement of Hydrogen Fluoride (HF) were installed in an aluminium potroom in Norway. Following this successful development NEO has been a pioneer in developing gas monitoring instruments based on spectroscopic techniques utilizing the unique spectral properties of tuneable diode lasers. This work has resulted in a completely new and unique range of gas monitoring instruments for a number of applications in industrial process and emission control. The single mode ( i.e. single frequency) characteristics of the diode laser makes it possible to measure the absorption of a particular gas using only one single absorption line, thus avoiding effectively the interference from other gases in the same gas volume, resulting in a highly accurate instrument capable of measuring very low concentration levels when required.

LaserGas Monitor

The LaserGas products provide solutions to many measurement problems where traditional spectroscopic techniques are inadequate, and they have proved to be very competitive, both with respect to performance, reliability and cost. The LaserGas instruments have been in use by the industry since 1995 and they are marketed on the international market through reputable distributors working directly with the end customers with installation and commissioning as well as after sales services including calibration and preventative maintenance. The first 100 installations were reached in 1998 and the first 1000 installations were completed in 2003.

LaserGas II

A new generation of the LaserGas Monitors was introduced in 2003. This new product family has the same unique performance and reliability as the first generation LaserGas Monitors, but it will first of all simplify installations and maintenance through its simple modular construction. Different applications may require different instrument configurations, and with the new instrument family we will be able to provide the best solution for both in situ and extractive measurements.

How does a LaserGas Monitor work?

LaserGas Monitor, Basic Set-up

The LaserGas monitors are based on tuneable diode lasers (TDL). The laser beam is sent through a measurement volume from a transmitter comprising a laser to a receiver with a light sensitive detector. The instrumentation must be protected from the process gas using windows that are kept cleen by purge air. See the figure below for a schematic diagram of the optical parts.

Basic set-up, LaserGas

The figure above shows the optical components in a set-up where a LaserGas monitor is connected to a stack containing process gas. The laser beam is collimated by a lens and sent through a window and then across the process gas. The laser beam is then sent through the window on the opposite side, collected and focused by a lens onto the light sensitive detector.

Tuneable Diode Lasers

Now we have sent a laser beam through the gas, how can the gas concentration be measured? First of all we must assure that the wavelength of the laser is close to the wavelength of the absorption line we want to use in our measurement. This can be achieved by controlling the temperature of the laser with an accuracy of better than 10 mK (milli Kelvin). Next we want to scan the laser across the absorption line i.e., we adjust the laser wavelength so that it scans across the absorption line. See the figure below:

LaserGas Animation

The figure above shows an animation describing how tuneable diode laser spectroscopy works. We assume we have adjusted the laser temperature so that the wavelength is in the leftmost position. Then we apply a saw tooth current (ramp) to the laser device which results in an increase in the internal laser temperature leading to a change in wavelength. The left figure shows the laser wavelength and the absorption line, - transmission as a function of wavelength. The right part of the figure shows the detector signal as a function of time. The red dots indicate the detector signal corresponding to each laser position. The laser is scanned as a function of time and therefore the detector signal will vary with time or laser wavelength.

More Complications, Second Harmonic Detection

The straightforward direct absoption technique illustrated above is normally not sufficient to achive a sensitivity and detection limit required for many emission monitoring and process control applications.

The solution is to introduce high frequency modulation on top of the saw tooth (ramp) current descibed above and use anlogue mixers in the detector amplifier stage (lock-in amplifier). This is illustrated in the figure below:

Illustration of second harmonic detection

The figure above shows the modulation used in second harmonic detection. S is the signal (upwards), t is the time and f is the laser frequency (wavelength). On the rear left amplitude projection you can see the signal from the absorption line as function of wavelength. On the bottom projection, the t-f plane, you can see the modulation ramp, laser frequency as a function of time. On the rear right projection you can see the detector signal as a function of time when the laser is scanned this way.

At this stage we have an even more complicated detector signal than before. Using analogue mixers synchronous with the laser high frequency modulation in the detector amplifier system we get the second harmonic signal. See the figure below.

Transmission and second harmonic signal

The figure above shows the detector signal (top) and the second harmonic signal (bottom) which is the output of the amplifier system with the mixer. The amplitude of the second harmonic (envelope) is used in the actual calculations of the concentration. The envelope is indicated using a dashed curve.

Curve: Second Harmonic Signal

The figure above shows a real second harmonic signal acquired by one of our LaserGas HF Monitors. This curve is equivalent to the "envelope" in the previous figure and is an example of the raw data that is input to the signal processing algorithms in the LaserGas monitor.

Using the second harmonic detection for gas monitoring has several advantages in addition to the improved detection limit. One important aspect is that the DC-component is removed and therefore the zero offset and drift for such an instrument is negligible.

You can read more about second harmonic detection in this paper from Applied Physics B.

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