Using Infrared Lasers for Gas Leak Detection

Currently, a great deal of progress is being made in optoelectronics technology, particularly constructed for infrared wavelength region (IR). VIGO Photonics, which offers IR detectors, greatly impacts the progress. The devices that VIGO Photonics produces are used in scientific experiments and are often vital components in industry technologies, particularly in gas analysis systems, using the phenomenon of the optical radiation absorption.

Leak detection of commonly occurring gases typically deals with flammable or explosive gas. For this reason, conventional (i.e., catalytic) leak detection methods are inadequate to successfully identify a leak of a specific gas type. There is no optimal gas transmission or resource tightness. As such, all gas resources or transmission lines must be observed in terms of gas leakage.

In addition to conventional gas leak detectors, infrared and laser spectroscopy methods may be used to identify extremely flammable gases. The major part of the spectroscopy devices is infrared photon detectors. Infrared detectors can be employed in all spectroscopy gas sensing techniques.  Using assorted spectroscopic methods means that comprehensive information on the chemical analysis of the leaking gas may be obtained.

Laser Gas Leak Detection with Infrared

Image Credit: VIGO Photonics

The spectroscopy methods are dedicated to various forms of concentration measurements. VIGO infrared detectors operate in the wavelength range of 2–16 μm. The suitable adjustment of the selected detector sensitivity to the detected gases in the appropriate wavelength range is due to a broad range of wavelengths.

For light particles, gas spectra with divided lines of the oscillation and rotation can be detected. Usually, the oscillation-rotation structure of the spectra is complex in the case of complex polyatomic molecules, and due to expansion, the individual lines intersect and an uninterrupted band is seen.

Some gases that are especially pertinent for emission and process control, is the need to employ the mid-infrared range (MIR, 3 µm - 12 µm). This is because the gas of interest (e.g., sulfur dioxide, SO2) does not possess absorption lines in NIR or, in the case of nitric oxide (NO) and nitrogen dioxide (NO2), the absorption strength is too low in NIR (in MIR it is up to 1000 times stronger).

General laser absorption spectroscopy schematics.

Figure 1. General laser absorption spectroscopy schematics. Image Credit: VIGO Photonics

Tunable Diode Laser Absorption Spectroscopy

Among the techniques of gas analysis, techniques using laser and LED sources are presently the most prevalent. They guarantee a brilliant signal-to-noise ratio and high selectivity of the radiation employed. These solutions are employed to identify one chosen gas. Systems in this subsection, which are the most commonly occurring, such as TDLAS, are characterized by a straightforward design, fast operation, and sensitivity adequate for multiple applications.

To identify the highest parameters, customers select CRDS instruments providing detection of even trace concentrations of gases. However, in exchange, they need high system stability for the duration of the measurement. Tunable Diode Laser Absorption Spectroscopy (TDLAS) is a technique employed for measuring the concentration of a gas or gas mixture.

TDLAS means that a very low concentration of the measured gas (up to ppb) can be measured. Concurrently, this method permits the measurement of temperature, velocity pressure, and mass flux of the gas. The features of the method are quick responses and extreme sensitivity.

TDLAS absorption spectroscopy schematics.

Figure 2. TDLAS absorption spectroscopy schematics. Image Credit: VIGO Photonics

In addition to the laser diode sources, infrared photon detectors are regularly employed in a TDLAS gas analysis. Maintenance of all aspects of TDLAS can occur by the rapid and sensitive photovoltaic VIGO detectors. Table 1 depicts infrared detectors advised for the TDLAS gas detection systems. For any gas absorption line, detector spectral characteristics must be chosen.

Table 1. Selected spectral characteristics for the gas absorption line. Source: VIGO Photonics

Gas type Selected absorption line [µm] Suitable VIGO Photonics detector and modules
Water vapor 2.9
CO2 4.26, 9.4 PV/PVA-xTE-5, PV-xTE-10.6
NH3 10.74 PV-xTE-10.6, PVM-xTE-10.6
HCl 3.5 PV/PVA-xTE-4
HF 2.5 PV/PVA-xTE-3


Cavity Absorption Spectroscopy

Due to methane detection spectrometers built on the CRDS method, natural gas leaks can be examined even at a very early stage. Cavity Ring-Down Spectroscopy (CRDS) is an extremely sensitive optical spectroscopic method allowing the measurement of an absolute optical extinction by samples dispersing and absorbing light.

CRDS has been broadly employed to examine gaseous samples which absorb light at specific wavelengths, and, in turn, to establish mole fractions down to the parts per trillion level. Devices such as methane detection spectrophotometers built on the said technique are used for: analyzing defective pipes, monitoring gas transmission lines, forecasting leakages and repairs, emission reduction processes, and risk mapping.

CRDS working principle schematics

Figure 3. CRDS working principle schematics. Image Credit: VIGO Photonics

CEAS working principle schematics.

Figure 4. CEAS working principle schematics. Image Credit: VIGO Photonics

The notion of Cavity Enhanced Absorption Spectroscopy (CEAS) method is centered around a measurement of the decay time of radiation trapped in an optical resonator with a high-quality factor.

Detailed CEAS working principle schematics. Left – built up state, right – ring down state.

Figure 5. Detailed CEAS working principle schematics. Left – built upstate, right – ring downstate. Image Credit: VIGO Photonics

A pulse of laser light is introduced in the CEAS method into an optical cavity (resonator) equipped with highly reflective, spherical mirrors. The pulse is reflected many times in the resonator. After the reflections, some of the laser light exits the resonator because of the lack of 100% mirror reflections. A photodetector registers part of the light coming out of the cavity.

Detected radiation on the output of CEAS

Figure 6. Detected radiation on the output of CEAS. Image Credit:VIGO Photonics

Due to a strong absorption line for this gas at a wavelength of 3.31 µm, methane testing is viable. VIGO Photonics detectors can identify concentrations of this gas below 50 ppb ± 0.05%.

Table 2. Detectability of methane by VIGO Photonics detectors. Source: VIGO Photonics

Gas type Selected absorption line [µm] Analysis technique Suitable VIGO Photonics detector and modules
CH4 3.31 CRDS/CEAS PV-xTE-3, PVA-xTE-3


Practical Applications of Various Gas Leak Detection

Gas leak detection standard methods are employed as principal techniques. They must be applied in end-user practical applications to retain customer-specific requirements. Measurements of gas pipe and natural gas leaks from the ground are the most commonly used practical applications.

Gas leaks can be a static measurement (open path) one that is moving, i.e. from the air. Open path spectroscopy has been employed to calculate hazardously or trace gases from hot point sources including a volcano, industrial, or agricultural facilities. This method is not frequently employed to measure greenhouse gases from field-scale sources.

Open Path Laser Detection.

Figure 7. Open Path Laser Detection. Image Credit: VIGO Photonics

Mobile MWIR Gas Detection

The mobile laser gas detection technique is comprised of detecting and finding a methane natural gas leak.  In order for the gas leakage measurement to be valuable, the vehicle must have started and traveled a specific distance, and then a chemical sensitive detector fitted in the device will trace the leak. In this instance, the methane analyzer is fitted on a specific vehicle, which, because of its signal transmitters, communicates information about the enhanced gas concentration directly to the threat monitoring center.

Mobile laser gas detection.

Figure 8. Mobile laser gas detection. Image Credit: VIGO Photonics

Drones with Laser Spectroscopy Detector

Due to the advancement of modern technologies, drones fitted with gas sensors can safely offer detailed information on a natural gas leak. Real-time unmanned aerial vehicles reach gas leaks that are hazardous for humans (drone operator). Monitoring gas leakage by devices in the air is advantageous due to their small size, meaning they can access more areas, the simplicity of use, and the inexpensive operating costs of these devices.

This information has been sourced, reviewed and adapted from materials provided by VIGO Photonics

For more information on this source, please visit VIGO Photonics


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