Operating principle

LDS 6 is a gas analyzer employing single-line molecular absorption spectroscopy. A diode laser emits a beam of near-infrared light, which passes through the process gas and is detected by a receiver unit. The wavelength of the laser diode output is tuned to a gas-specific absorption line. The laser continuously scans this single absorption line with a very high spectral resolution. The result is a fully resolved single molecular line which is analyzed in terms of absorption strength and line shape. The influence of cross-sensitivities on the measurement is negligible, since the quasi-monochromatic laser light is absorbed very selectively by only one specific molecular line in the scanned spectral range.

Basic design of the LDS 6

Configuration examples:

A feature of the in-situ analytical procedure is that the physical measurement takes place directly in the stream of process gas, and usually also directly in the actual process gas line. All process parameters such as gas matrix, pressure, temperature, moisture, dust load, flow velocity and mounting orientation can influence the measuring properties of the LDS 6 and must therefore be systematically investigated for each new application.

A feature of the standard applications defined in the ordering data of the LDS 6 is that the typical process conditions are well-known and documented, and that the guaranteed measuring properties can be proven by reference installations. If you cannot find your application among the standard applications, please contact Siemens. We will be pleased to check your possible individual application of the LDS 6. You can find an application questionnaire on the LDS 6 product sites on the Internet.

Typical transmitted light setup of LDS 6, in-situ

To avoid contamination of sensor optics on the process side, clean gaseous purging media such as instrument air, N₂ or steam are used. Purging air tubes on the sensor heads, which slightly penetrate into the process gas stream, define the effective measuring path length.

The LDS 6 can measure in both the transverse and longitudinal directions of the process gas flow. In certain cases, the process conditions make it necessary to condition the sample gas stream in a bypass line with respect to process temperature, pressure and/or optical path length. Further treatment of the process gas, such as drying or dust precipitation, is usually unnecessary.

Typical transmitted light setup of LDS 6, in bypass

A flow cell is available by special application for the LDS 6 which has been specially optimized for use with the LDS 6 and its transmitted-light sensors with respect to handling and measuring performance. It is designed to reduce surface effects, and is therefore also highly suitable for polar gases like ammonia. This flow cell is available in heated and non-heated versions. Wheel mounted and wall mounted versions are available.

Measuring configuration of LDS 6 with heated flow cell

General information

LDS 6 is connected to the measuring points by fiber optics. The laser light is guided by a single-mode fiber from the central unit to the transmitter unit of the in-situ sensor. The sensor consists of a transmitter and a receiver; the distance between them defines the measurement path. In the receiver box, the light is focused onto a suitable detector. The detector signal is then converted into an optical signal and transmitted via a second optical fiber to the central unit, where the concentration of the gas component is determined from the detected absorption signal.

LDS 6 usually measures a single gas component by means of the absorption capacity of a single fully resolved molecular absorption line. The absorption results from conversion of the radiation energy of the laser light into the internal energy of the molecule. In the working range of the LDS 6, both rotation-vibration transitions and electronic transitions - such as with O₂ - can be triggered.

In some specific cases, two components can be measured simultaneously if their absorption lines are so close to each other that they can be detected within the laser spectrum by one single scan (for example water (H₂O) and ammonia (NH₃)).

Absorption spectra of water and ammonia

Typical measurable gases for LDS 6 are:

• Oxygen (O₂) for low and high pressure
• Hydrogen fluoride (HF) + water
• Hydrogen chloride (HCl) + water
• Ammonia (NH₃) + water
• Water vapor (H₂O)
• Carbon monoxide (CO)
• Carbon dioxide (CO₂)
• CO + CO₂

By using an internal reference cell normally filled with the gas measured, the stability of the spectrometer is permanently checked in a reference channel.

By doing so, the continuous validity of the calibration is ensured without the need to carry out external recalibration using bottled calibration gases or reference gas cells.

Typical spectral bandwidth of an absorption line compared to the bandwidth of the laser light.

Influences on the measurement

Dust load

As long as the laser beam is able to generate a suitable detector signal, the dust load of the process gases does not influence the analytical result. By applying a dynamic background correction, measurements can be carried out without any negative impact. Under good conditions, particle densities up to 100 g/Nm³ can be handled by the LDS 6. Varying dust loads are compensated by scanning the laser over the gas absorption line and the current background. At a scan position next to the absorption line, the instrument can "see" only absorption caused by the dust load where at the line center the signal is composed of the molecular absorption and the continuous, unspecific background absorption. With the wavelength modulation technique, the actual measured transmission is always compared with the baseline. After signal processing, phase-sensitive application delivers a signal only from the molecular line free of background.
The influence of a high dust load is complex and depends on the path length and particle size. The optical damping increases at longer path lengths. Smaller particles also have a large influence on the optical damping. With a combination of high dust load, long path length and small particle size, the technical support at Siemens should be consulted.

Temperature

The temperature influence on the absorption line strength is compensated by a correction factor determined during calibration. A temperature signal can be fed into the instrument from an external temperature sensor. This signal is then used to correct the influence of the temperature on the observed line strength. If the temperature of the sample gas remains constant, it is alternatively possible to carry out a static correction using a preset value.
At high process gas temperatures, generally from approximately 1 000 °C, there may be noticeable broadband IR radiation of gas and dust, or flames may occasionally occur in the measurement path. An additional optical bandpass filter can be set upstream of the detector to protect it and prevent saturation by the strong background radiation.

Pressure

The gas pressure can affect the line shape of the molecular absorption line. LDS 6 uses a special algorithm to adapt the line shape. Additionally, an external pressure signal can be fed to the instrument to provide complete compensation for the pressure influence including the density effect.

Optical path length

The absorption values analyzed by the LDS 6 are typically small. As a result of Beer-Lambert's law, the absorption of laser light depends on the optical path length within the gas. Therefore, the precision in determining the effective optical path length in the process might limit the overall precision of the measurement.
As the sensor optics on the process side normally need to be purged to keep them clean over a long period of time, the thickness of the mixing zone between the purging medium and the process gas and its concentration distribution need to be considered. In a typical in-situ installation directly in the line and with some meters of path, the influence of the purging gas on the effective path length can be neglected.
Path length and dust load are mutually influencing: the higher the dust load in the process, the shorter the max. possible path length.

Maintenance and fault messages

LDS 6 outputs different warnings via relays:

• Need for maintenance (measured value is not influenced)
• Operating error (measured value might be influenced)

Note

Individual requirements for the measuring point can make the utilization of special sensor equipment necessary. The possibilities for adapting the sensors are:

• Different purging media, such as instrument air, ambient air, nitrogen or steam
• Different purging modes on process and sensor sides
• Special materials of purging tubes and/or sensor flanges
• Cooling or heating of the sensors
• Explosion-protected sensor configurations

Essential characteristics

• Integrated calibration adjustment with an internal reference cell
• Negligible long-term drifts of zero and span
• Dynamic background correction for varying dust loads
• Isolated signal outputs, 4 to 20 mA
• User-friendly, menu-driven operation
• Selectable time constants (response time)
• Two user levels with individual access codes for prevention of unwanted and unauthorized operations
• Operation according to NAMUR recommendations
• Monitoring of overall optical transmission
• Remote preventive maintenance and servicing via Ethernet/modem
• Straightforward replacement of the central unit, since connections can easily be removed
• Sensor and central unit housing free of wear and corrosion
• Easy operation with a numerical keypad and menu prompting

Certified versions for emission monitoring

The LDS 6 is available as certified instrument for emission monitoring of NH₃, NH₃/H₂O, H₂O, HCl, HCl/H₂O. The certificates are issued by TUV for Germany and MCERTS for the United Kingdom. For conducting regular calibration and linearity checks, test kits for ammonia, water and HCl should be used. These kits can be ordered separately as instrument accessories. For new analyzer orders, the NH₃, NH₃/H₂O and H₂O kits named "Version 2" must be ordered. For already installed analyzers, please contact Siemens for spotting the correct kit version, or consult the instrument manual.

Verification of calibration

Assembly with certified, maintenance-free calibration gas cell with connections for laser fiber-optic conductors and detector module of cross-duct sensor. Serves to rapidly verify the factory calibration in the field without compressed gas bottles and flow cell.

Calibration verification kits are available for the following sample gases: O₂ (application codes AA, AC), NH₃, CO, CO₂, CO/CO₂. A "Zero gas test kit" is also available. (see "Additional units")

Example of an assembly for verification of calibration