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Questions and Answers about Tunable Diode Laser Gas Spectroscopy (TDLS)

FAQ about TDLS Technology

1 TDLS Technology

1.1 What does TDLS mean? How does it work?

1.2 Which gases can be measured with the TDLS?

1.3. Which gases cannot be measured with the TDLS?

1.4. What kind of laser diodes are used in the TDLS LGD?

1.5 How is the TDLS influenced by external pressure variations?

1.6 How is the TDLS influenced by external termperature variations?

1.7 How is the target gas measuerement influenced by other gases in the gas matrix?

 

    TDLS comparison with other technologies

2.1 What are the advantages of TDLS as compared to FTIR instruments?

2.2 What are the advantages of TDLS as compared to NDIR systems?

2.3 What are the advantages of TDLS compared to Elecrochemical or Catalytic Detectors?

2.4 What are the advantages of TDLS compared to Solid State Sensors (MOS)?



1 TDLS Technology

1.1 What does TDLS mean? How does it work?

 

Tunable Diode Laser Spectroscopy (TDLS) is an infrared absorption measurement.

Many gases have characteristic absorption bands in the infrared wavelength region. In a typical set-up IR light from a light source traverses a cavity and falls onto a photo detector. Interaction of infrared light (absorption) with gas molecules in the cavity (or measurement cell) leads to a decrease of IR radiation intensity on the detector, depending on the gas concentration.

 

The absorption bands of gases with relatively small molecules show a well resolved fine structure, consisting of many individual absorption lines. In TDLS, a single-mode tunable diode laser is the light source for the gas measurement. Its wavelength is chosen to be centered onto one of the fine absorption lines of the target gas. The laser is then tuned by temperature or current to scan this absorption line within a very narrow range (~2 nm) to obtain the gas concentration.

 

Typically one laser is used to measure one gas. Due to the sharpness of the lines (~ 0.1 nm) there is practically no cross-sensitivity with other gases. In the case where the absorption bands of two different gases are interlaced, it is sometimes possible to monitor the concentration of two different gases with one laser.

 

Due to the fact that telecom-type laser diodes are only available at wavelengths up to 2.7 µm, TDLS is rarely done in the wavelength range of the fundamental gas absorption bands (3 to 9 µm). Instead, the first overtones of the absorptions bands are used. These bands are situated conveniently in the range of 0.76 to 2.3 µm and enable the use of affordable near-infrared telecom lasers and detectors.

 

 LGD Axetris Wavelength
Gas absorption band as seen in low resolution (e.g. NDIR). High-resolution observation reveals a fine structure of individual rotation & vibration bands.
 LGD Axetris Gas Absorption Lines  
TDLS scans individual rotation or vibration bands of the molecules in a high-resolution measurement.  

 

1.2 Which gases can be measured with the TDLS?

 

TDLS in general can measure gases that have relatively simple molecular structures where the absorption bands have a distinct and resolved fine structure.

Currently the following gases are available: NH3, CH4, CO2, C2H6, C2H2, H2O and HCl

Further gases that can be measured with TDLS, but that are not currently available by Axetris include:  HF, O2, N2O, H2S, CO, NO, NO2, PH3, HI, HBr, HCN, H2CO, H2CO and C2H4 (further information on request).

 

1.3 Which gases cannot be measured with the TDLS?

 

Gases that do not have near-infrared absorption bands cannot be measured. These includes H2, N2, O3, Cl2, He, Ne, Ar, etc. Also, more complex or ring-shaped molecules cannot be measured because they do not show narrow and well separated rotation and vibration bands, e.g. propane, butane, pentane, ethanol, propanol, acetone, ethyl ether, benzene, toluene, etc.

 

1.4 What kind of laser diodes are used in the TDLS laser gas detection (LGD)?

 

The measurement technique is based on telecom-type single-mode lasers. Appropriate laser diodes are Distributed Feedback (DFB) lasers, whose laser structure is arranged parallel to the surface and emit the laser light from the edge of the laser chip, and Vertical Cavity Surface Emitting Lasers (VCSEL's), in which the layers are stacked vertically and the laser light is also emitted vertically to the surface. 
Laser diodes in the near IR are used for all currently available LGD modules. Further developments are planned for the LGD Compact in which quantum cascade lasers are to be used for mid IR range.

 

1.5 How is the TDLS influenced by external pressure variations?

Schematic representation of pressure effect

Pressure broadening affects absorption peak-height only slightly. In first order approximation the collision broadening effect increases with increasing pressure. This enlarges the absorption band and reduces it in strength, however this effect is compensated by the higher number of absorbing molecules/volume, which in turn increases the absorption strength again.

In NDIR measurements the pressure effect is much bigger and directly proportional to the increase in surface of the absorption line (i.e. the complete envelope with dozens of vibration and rotation bands due to the low resolution of the technique).

It is always recommended to do a new span and offset correction when the environmental conditions are changing a lot.

 

1.6 How is the TDLS influenced by external temperature variations?

LGD Axetris Representation of Temperature Effect
Different temperatures have more influence on the peak intensity, the peak width is hardly affected.

All LGD products are calibrated at an ambient temperature of 20°C or hot system to an expected operating temperature.
If the gas to be measured has a different temperature than the gas cell itself, incorrect measurement results may be obtained.

 

1.7. How is the target gas measurement influenced by other gases in the gas matrix?

The background gas matrix of the target gas to be measured influences also the shape of the beam. In normal ambient air the peak of the target gas is broadened and reduces its intensity compared to a pure N2 background. This effect is only a % of reading and negligible in the majority of cases where concentration changes are just in the range of ppm level.
It is always recommended to do a new span and offset correction when the environmental conditions like gas matrix are changing a lot.

2 TDLS comparison with other technologies

2.1 What are the advantages of TDLS compared to FTIR (Fourier-transform infrared spectroscopy) instruments?

 

FTIR analytical instruments can be used for multiple gas measurements in the laboratory and some process applications. However, accurate and fast devices can be expected to cost 10 or 20 times the price of an Axetris LGD with TDLS technology. Standard FTIR instruments have susceptible optics (contamination) and measurements in the lower ppm range are difficult. Furthermore, they are sensitive to vibrations and shock and often need expert operators. Bulkiness and relatively high power consumption are not in favor of easy OEM integration, particularly for semi in-situ, gas-extractive set-ups.

 

2.2 What are the advantages of TDLS as compared to NDIR (Non-Dispersive Infrared) systems?

LGD Axetris Overlap Absorption Bands NDIR Spectroscopy

A NDIR detector will be subject to cross sensitivities when absorption bands of different gases overlap. Furthermore, the detected concentration will change with the intensity of the light bulb, which makes a separate reference channek mandatory for stable operation.

 

Both NDIR and the TDLS LGD monitor the absorption of infrared light by the target gas – however, NDIR only achieves rather weak resolution. NDIR detectors select the appropriate wavelength by filtering the light of a thermal emitter through an interference filter. This filter has a spectral resolution which is roughly a factor 1’000 weaker compared to the sharp laser measurement of TDLS.
While overlapping gas absorption bands can pose a serious problem to NDIR detectors – particularly with ever-present water absorption bands, the high resolution approach of the TDLS LGD makes this a non-issue resulting in Zero Cross-Sensitivity. Even when two absorption bands of different gases overlap, the individual lines of the fine structure of the bands are simply interlaced so that a clear separation of the two gases is always possible by choosing the appropriate line. 

LGD AXAG Example of Interlaced Absoprtion Lines

Blue dots represent the absorption lines for CO2. Although the peaks in NDIR would grossly overlap, laser spectrometry can distinguish between lines as little as 0.25 nm apart (second dot from the left).

 

Better Long-Term Stability with Single-Channel Systems: As mentioned above, NDIR detectors need a second measurement channel as reference in order to monitor the light intensity of the bulb that changes intensity over time due to aging. As the reference channel has a separate detector and a separate narrow band interference filter, the difference in these devices compared with the measurement channel and different aging can lead to drift, therefore calibration needs to be relatively frequent.

Lower Power Consumption: The thermal light bulb of a NDIR detector uses electrical power to generate a broad spectrum of intensities, very much like a light bulb in a ceiling lamp. The optical filter is tuned to the gas absorption wavelength and therefore uses only a tiny fraction of the emitted light, leaving the main portion of the used electrical power wasted.

In contrast to this, the laser diode of the TDLS LGD emits 100% of its light at exactly the wavelength of interest, wasting only a minimal amount due to the conversion efficiency of the laser. Therefore, the TDLS LGD will have lower power consumption in most applications.

Use of standard optical glass components: The wavelengths used are in the range of 760 – 2,300 nm, which makes it possible to use glass as a material for windows and lenses. NDIR measurements take place in the mid-infrared where special and often fragile optical components need to be used.

 

2.3 What are the advantages of TDLS compared to Electrochemical or Catalytic Detectors?

Zero Poisoning, No Aging, and No Cross-sensitivity:
Electrochemical detectors are always in direct contact with the gas. They are prone to ageing as the measurement principle is based on a chemical reaction depleting the sensor substance. This leads to short life times. This life time decreases when the gas to be measured is present frequently. The response is rarely specific to a single gas, which may lead to costly false alarms. Issues with 0-drift for multiple reasons are frequent. Therefore, calibrations are necessary in short intervals, which in turn increases the cost-of-ownership for initially low-cost instruments. The sensors are also sensitive to pressure and humidity changes. Pellistors are also subject to poisoning, especially by silicone and halogen compounds. Span loss from aging cannot be monitored and can only be detected during calibration.

As the TDLS LGD is an optical detector without direct contact to the gas, it is not at all subject to poisoning or degradation. Humidity and pressure changes around normal atmospheric conditions have no influence. Calibrations are enduring and recovery from high concentrations is instantaneous.


Continuous Sensor Status Monitoring for Functional Safety:

The European norm IEC 61508 is regarded as "good practice" for security sensors. It requires functional safety from any device, needs redundancy (i.e. 2 redundant detectors within one sensor), and constant monitoring of the sensor status. With an optical sensor, this is achieved by monitoring the light source intensity.

However, electrochemical gas sensors or pellistors cannot comply with this norm as any testing degrades the sensor performance and due to this operators of security sensitive installations (chemical plants, refineries etc) will have to replace electrochemical sensors in the future.


High Dynamic Range:
Most detection technologies can either measure only low concentrations (e.g. electrochemical detectors below 100 ppm) or high concentrations (catalytic sensors in the % range). The high dynamic range of the TDLS LGD (1’000 – 2’000 times the detection limit) makes it very versatile for many applications, without the need to use 2 or 3 different detection technologies in the same instrument to cover the range of interest.

 

2.4 What are the advantages of TDLS compared to Solid State Sensors (MOS)?

 

No cross-sensitivity, no influence from environmental parameters: Solid state sensors are sensitive to humidity and generally have poor selectivity for toxic gases. Also, variations in the oxygen content lead to unreliable readings. Exposure to high gas concentrations can lead to irreversible changes to the 0-gas reading, as well as to the sensitivity. On the contrary, cross-sensitivity to other gases, humidity or oxygen is virtually zero for the TDLS LGD if the bands from the absorption fine structure are carefully chosen. There is no direct contact to the gas to be measured.
Low power consumption: A solid state sensor needs to be continuously at high temperature to be operational, which leads to significant power consumption. In applications where this is an issue, e.g. portable instruments, the TDLS LGD could have the advantage of lower power consumption.

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