Gas Leak Detection by Dilution of Atmospheric Oxygen

Article Gas Leak Detection by Dilution of Atmospheric Oxygen

Armin Lambrecht *, Eric Maier, Hans-Fridtjof Pernau ID , Thomas Strahl and Johannes Herbst

Fraunhofer Institute for Physical Measurement Techniques IPM, Heidenhofstr. 8, D-79110 Freiburg, Germany; [email protected] (E.M.); [email protected] (H.-F.P.); [email protected] (T.S.); [email protected] (J.H.) * Correspondence: [email protected]; Tel.: +49-7665-41904

Received: 31 August 2017; Accepted: 30 November 2017; Published: 5 December 2017

Abstract: Gas leak detection is an important issue in infrastructure monitoring and industrial production. In this context, infrared (IR) absorption spectroscopy is a major measurement method. It can be applied in an extractive or remote detection scheme. Tunable laser spectroscopy (TLS) instruments are able to detect CH4 leaks with column densities below 10 ppm·m from a distance of 30 m in less than a second. However, leak detection of non-IR absorbing gases such as N2 is not possible in this manner. Due to the fact that any leaking gas displaces or dilutes the surrounding background gas, an indirect detection is still possible. It is shown by sensitive TLS measurements of the ambient background concentration of O2 that N2 leaks can be localized with extractive and standoff methods for distances below 1 m. Minimum leak rates of 0.1 mbar·L/s were determined. Flow simulations conﬁrm that the leakage gas typically effuses in a narrow jet. The sensitivity is mainly determined by ambient ﬂow conditions. Compared to TLS detection of CH4 at 1651 nm, the indirect method using O2 at 761 nm is experimentally found to be less sensitive by a factor of 100. However, the well-established TLS of O2 may become a universal tool for rapid leakage screening of vessels that contain unknown or inexpensive gases, such as N2.

Keywords: gas sensor; leak detection; tunable laser spectroscopy; infrared absorption; oxygen; nitrogen; methane

1. Introduction Gas leak detection is an important issue in production processes to prevent safety hazards and to ensure functionality of many products. For natural gas pipeline surveillance, many instrument developments aim at fast and sensitive remote detection of gas leaks. For these purposes gas cameras using modified thermal imaging systems, handheld tunable laser spectroscopy (TLS) instruments, and even helicopter-based systems are available [1–5]. Commercially available handheld TLS instruments in the NIR are able to detect CH4 leaks with column densities >1 ppm·m via remote detection. They can be used from a distance up to 30 m and have measurement rates up to 10 Hz [3]. Determination of leak rates is difficult for remote gas detectors since sufficient information about the gas dispersion in the surrounding environment is usually not available. Recently, a TLS remote detection device was able to identify a CH4 leak flux of 15 mL/min from a distance of 37 m [6]. However, all these systems rely on infrared absorption of the target gas, which is the case for hydrocarbons, for example. Consequently, leak detection of non-infrared absorbing gases such as N2,H2, Ar, etc., is not possible in this fashion. Sensitive leak testing of vessels, tubes, vacuum parts, refrigeration systems, etc., is usually performed via helium (He) leak testing. This detection method is based on a mass spectrometer (MS) that is tuned to He. In most cases, the examined object has to be filled with a He-containing gas. In such a leakage scenario, the He is detectable by pumping (“sniffing”) the leakage gas into the MS through a handheld nozzle located close to the leak. Very small leaks (below 10−7 mbar·L/s) can

Sensors 2017, 17, 2804; doi:10.3390/s17122804 www.mdpi.com/journal/sensors Sensors 2017, 17, 2804 2 of 14 be detected by this method, but a close contact to the object is required. Apart from that, He leak testing from a distance is not possible. Further disadvantages are that the testing is a slow and manual task. A less expensive alternative to He is hydrogen or forming gas that is used in combination with sensitive electrochemical, metal oxide-, or Pd-based detectors. However, the use of hydrogen could be a safety hazard. Acoustic leak detection is another common method. This method is based on the principle that the leakage gas generates a sound wave which can be detected by a sensitive microphone. Therefore, the object needs to be pressurizing in order to obtain a turbulent flow at the orifice which is required to generate (ultra-) sound waves [7]. This may be circumvented by using an active ultrasound source within the object, but discrimination of the different sound paths could be difficult. The classical leak detection technique for rather coarse leaks is the well-known “air bubble” method. A vessel under investigation is pressurized with gas (e.g., air) and immersed into a liquid (e.g., water). Air bubbles originating from the leak indicate the leak location and leak size. Minimum leak rates ≥10−3 mbar·L/s can be detected by this method which is comparable to acoustic leak detection [8]. Obviously, this method is not practical or feasible for large objects. Furthermore, the method is, generally, a slow and manual task. However, this concept may also be transferred in the gas phase. Therefore, a background gas in the atmosphere substitutes the liquid such that the background gas is displaced and diluted by the leakage gas. Thus, remote leak detection of any target gas—also of non-IR-active gases—should be feasible by a sensitive tunable laser spectroscopy (TLS) measurement of the background gas concentration. Ubiquitous and IR-active background gases are, for instance, CO2,O2, or H2O, which should be different from the target gas. In order to detect a leak-induced ambient background concentration change, the distance has to be fixed or measured simultaneously [9]. This can be achieved with state-of-the-art laser rangefinders. In the next section, we show that O2 is the optimum candidate for a suitable background gas. Then the experimental setup is explained. Because of the visibility of the employed laser we use the term “LeakEye” for our technique. Computational fluid dynamics (CFD) simulations of the experimental setup are performed using COMSOL Multiphysics®(COMSOL Inc., Burlington, MA, USA). Then the results of experiments on extractive detection—later on, standoff detection—are given. In this work the terms non-tactile and standoff detection are synonymously used, even for rather short distances. The essential difference to extractive detection is that gas flow is not disturbed. We shall always compare experiments with direct (positive) detection of CH4 and with corresponding experiments on indirect (negative) detection of O2.

2. Modelling The fundamental relation of absorption spectroscopy is given by the Beer-Lambert law:

−α(υ)·d·c I(υ) = I0·e with α(υ) = S(T, υ0)·g(p, T, υ − υ0) where I(υ) is the frequency-dependent transmitted intensity after passing a measurement cell with an optical path length d, c is the gas concentration, and α(υ) is the frequency-dependent gas specific absorption coefficient, which is given by the product of the temperature-dependent line strength S(T,υ0) at the characteristic absorption frequency υ0, and the pressure- and temperature-dependent line shape function g(p,T,υ − υ0). As a rule of thumb we assume that a 1% reduction of the effective absorption length d by dilution of the background gas by the leakage gas has to be measured with a laser spectroscopic setup. In a typical situation an undisturbed extension of the leaking gas plume up to approximately 1 cm from the surface of the leaking object is assumed. For larger distances strong dilution by ambient air flow is expected. As a result the “LeakEye” technique is practically limited to standoff distances of approximately 1 m. For larger distances the relative measurement effect is too small. Sensors 2017, 17, 2804 3 of 14

Relevant atmospheric background gases, such as O2,H2O, and CO2, with infrared absorption data are available in the HITRAN database [10]. Suitable absorption lines of these gases for typical ambient concentrations, T = 296 K and p = 1013 mbar were selected and laser spectroscopic measurements were simulated using Mathcad (PTC, Unterschleißheim, Germany). For a total optical distance of 200 cm the transmission signal change for a 1% change of the optical distance was determined. For all gases distributed feedback (DFB) laser diodes in the near infrared (NIR) are available. In the simulation laser emission powers were set to 3 mW. Photodiodes with optimum detectivities with respect to noise equivalent powers (NEP) for the selected wavelength ranges were chosen as detectors. A maximum total transmission of 1% was assumed to account for the weak signal for standoff detection. The resulting power changes at the detector ∆P with respect to the relative power changes ∆Prel are shown in Table1.

Table 1. Selection of background gases and detection wavelengths.

Gas O2 H2O CO2 Wavelength(nm) 761 1392 2004 Concentration 21% + 1% H2O 1% 450 ppm + 1% H2O Detectivity D * 1013 1012 1011 (Jones) NEP (@100 kHz, nW) 0.01 0.08 0.8 ∆P(nW) 12 60 2 −4 −3 −4 ∆Prel 4 × 10 3 × 10 1.5 × 10 Suitable + − 1 − 1,2 1 ambient concentration fluctuations; 2 detector noise limitation. The measurement of O2 at 761 nm was selected as the best choice (cf. Table1): although TLS-based hygrometers could serve as an established platform [11], the expected ambient fluctuations for H2O, as well as for CO2, are too high. The first experiments with CO2 show that the measurements are strongly affected by the exhaled air of people in the lab; this effect is much stronger for CO2 than for O2. Additionally, for CO2 at 2004 nm the sensitivity is expected to be limited by detector noise. A further reason for the choice of O2 with respect to for the measurement at 761 nm is that TLS of O2 has become an established industrial measurement technique. For commercially-available process instrumentation, −3 a precision of 10 for O2 concentration measurements is achieved [12,13], and laser radiation at 761 nm is still visible, which is a significant advantage for the adjustment of experimental setups. A further result of the simulation is that for a standoff distance of 1 m a direct measurement of a‘1 cm long CH4 gas column at 1651 nm is expected to be approximately more sensitive by a factor of 1000 than an indirect O2 measurement by the displacement of 1 cm background air. Therefore, for a better comparison between both methods we use 1% CH4 in N2 for direct detection.

3. Experimental Details Gas detection by tunable diode laser spectroscopy is an established technique. We are employing the direct spectroscopy scheme. With this method, the measured absorbance spectra are fitted with calculated gas absorption lines using HITRAN line parameters. Knowing the temperature, pressure, and the absorption length, the gas concentration is obtained [14]. For the measurements a test leak was made. This consists of a shot-blasted 100 × 100 mm2 Al-plate with a central 1 mm diameter orifice. The blasted surface ensures sufficient diffuse reflection of incident radiation. Leakage gas is ejected from the orifice at different flow rates between 1000 mL/min and 1 mL/min. A gas supply hose is connected at the back side of the plate. Flow rates and the leakage gas composition are set using a HovaCAL digital 922 SP flow controller (IAS GmbH, Oberursel, Germany). The leak plate (Figure1) is mounted on a sliding stage to displace the leak position horizontally on the table. For extractive measurements, the gas is sampled by a hose with a 1 mm sampling tip mounted in a distance of 10 mm from the leak plate. The gas is pumped through a measurement cell with an optical path length of 84 cm and a volume of 175 mL. We used pump rates of a diaphragm Sensors 2017, 17, 2804 4 of 14 pump between 100 mL/min and 1000 mL/min. Pressure fluctuations are minimized by an additional bufferSensors 2017 volume, 17, 2804 between the pump and measurement cell. 4 of 14

(a) (b)

Figure 1.1. ((a)) LeakLeak plateplate mountedmounted onon aa linearlinear displacementdisplacement stage.stage. The tiptip ofof anan extractiveextractive samplingsampling probeprobe isis positionedpositioned at 1010 mmmm distancedistance in frontfront ofof thethe plate;plate; andand ((bb)) thethe schematicschematic setupsetup forfor standoffstandoff leakleak detection.detection.

For standoff measurements the laser and detector are positioned at a distance from the leak For standoff measurements the laser and detector are positioned at a distance from the leak plate. plate. The diffuse reflection of the collimated laser beam is collected by an f = 11 cm, 2” focusing lens The diffuse reflection of the collimated laser beam is collected by an f = 11 cm, 2” focusing lens in front in front of a Si-photodiode. Typically, distances of 55 cm and 52 cm of the laser with respect to the of a Si-photodiode. Typically, distances of 55 cm and 52 cm of the laser with respect to the detector lens detector lens from the leak plate are used. The angle between the laser and detector beams is from the leak plate are used. The angle between the laser and detector beams is approximately 17◦. approximately 17°. For static measurement, the leakage flow rates are varied in steps and the leak position is fixed at For static measurement, the leakage flow rates are varied in steps and the leak position is fixed a maximum signal for extractive or standoff detection. For dynamic measurements, the leak plate is at a maximum signal for extractive or standoff detection. For dynamic measurements, the leak plate moved horizontally back and forth with a speed between 0.2 mm/s and 5 mm/s for a fixed leak flow is moved horizontally back and forth with a speed between 0.2 mm/s and 5 mm/s for a fixed leak rate, which is changed after a couple of scans. flow rate, which is changed after a couple of scans. Positive or direct detection is performed with 1% CH4 in N2. For negative or indirect detection we Positive or direct detection is performed with 1% CH4 in N2. For negative or indirect detection used 100% N2. For the CH4 measurements, a 1651 nm pig-tailed single-mode laser diode in a butterfly we used 100% N2. For the CH4 measurements, a 1651 nm pig-tailed single-mode laser diode in a mount was employed (Eblana Photonics Ltd., Dublin, Ireland). For detection, an InGaAs-photodiode butterfly mount was employed (Eblana Photonics Ltd., Dublin, Ireland). For detection, an type G 12182-030 (Hamamatsu Photonics GmbH, Herrsching, Germany) with a 3 mm diameter InGaAs-photodiode type G 12182-030 (Hamamatsu Photonics GmbH, Herrsching, Germany) with a was used. For the O2 measurements we used a C-mount-DFB laser diode emitting at 761 nm 3 mm diameter was used. For the O2 measurements we used a C-mount-DFB laser diode emitting at (nanoplus GmbH, Gerbrunn, Germany) in a custom laser housing and a quadratic 3.6 mm × 3.6 mm 761 nm (nanoplus GmbH, Gerbrunn, Germany) in a custom laser housing and a quadratic Si-photodiode. The lasers were driven with a standard benchtop laser current driver/temperature 3.6 mm × 3.6 mm Si-photodiode. The lasers were driven with a standard benchtop laser current controller (ILX Lightwave LDC 3722, Newport Corporation, Irvine, CA, USA). Laser control, driver/temperature controller (ILX Lightwave LDC 3722, Newport Corporation, Irvine, CA, USA). data acquisition, and evaluation were performed with a proprietary electronics board and LabVIEW Laser control, data acquisition, and evaluation were performed with a proprietary electronics board (National Instruments Corporation, Austin, TX, USA). and LabVIEW (National Instruments Corporation, Austin, TX, USA). 4. Experimental Results 4. Experimental Results

4.1. Extractive Measurements with O2 4.1. Extractive Measurements with O2 Using the setup described above we investigated the extractive detection of a N2 ﬂow from the Using the setup described above we investigated the extractive detection of a N2 flow from the leak plate using the indirect O2 detection method. The result of a static measurement is shown in Figureleak plate2. using the indirect O2 detection method. The result of a static measurement is shown in FigureAt 2. the beginning of the measurement we had to adjust the position of the sampling tip in front of theAt leak the plate.beginning The wholeof the setupmeasurement was mounted we had on to an adjust optics the table position in the of lab the (see sampling Figure 1tip). Onin front this tableof the other leak plate. experiments The whole were setup performed was mounted simultaneously, on an optics and table staff in was the nearbylab (see moving Figure 1). around On this in thetable lab. other Obviously experiments they were are disturbingperformed thesimultaneous experiment.ly, and This staff activity was nearby ceased moving after 18:00 around h and in thethe experimentlab. Obviously was they running are automaticallydisturbing the over experime night fornt. approx.This activity another ceased 6 h. After after approximately 18:00 h and 2the h experiment was running automatically over night for approx. another 6 h. After approximately 2 h fewer disturbances by fluctuations of the gas flow distribution (and of the temperature in the lab) occur, distinct N2 flow steps are visible, and flow rates of 5 mL/min can be clearly observed. With the applied pump rate of 100 mL/min it takes nearly 2 min to exchange the gas in the cell. Fluctuations in the N2 gas flow during this time have to be avoided. The observed signal-to-noise-ratio (SNR) indicates that steps of 2 mL/min would be resolvable. Thus, leak rates below 0.1 mbar·L/s could be

Sensors 2017, 17, 2804 5 of 14 detected by this method. Due to the long duration of the individual flow steps in Figure 2 a complete gas exchange in the measuring cell is ensured for a pump rate of 100 mL/min. Sensors 2017, 17, 2804 5 of 14 In Figure 3 a dynamic measurement is displayed. A scan speed of 1 mm/s for the sliding stage was chosen, which seemsfewer disturbances quite by realistic fluctuations of for the gas manual flow distribution leak (and ofsearching. the temperature In in the contrast lab) to the static occur, distinct N2 flow steps are visible, and flow rates of 5 mL/min can be clearly observed. With the measurements the O2applied-concentration pump rate of 100 mL/mindips itat takes the nearly leak 2 min position to exchange the show gas in the considerable cell. Fluctuations fluctuations. For in the N2 gas flow during this time have to be avoided. The observed signal-to-noise-ratio (SNR) 10 mL/min some of theindicates dips that are steps ofcompletely 2 mL/min would bemissing. resolvable. Thus,This leak has rates belowtwo 0.1 reasons: mbar·L/s could First, be the speed of the sliding stage is too fastdetected for by the this method. extractive Due to the longtechni durationque of the even individual at flowa pump steps in Figure rate2 a complete of 500 mL/min. Thus, a gas exchange in the measuring cell is ensured for a pump rate of 100 mL/min. complete exchange of theIn Figuregas 3in a dynamic the cell measurement is not is displayed.possible A scan when speed of 1the mm/s leak for the position sliding stage was is passing by in front chosen, which seems quite realistic for manual leak searching. In contrast to the static measurements of the probe tip. Thisthe behavior O2-concentration is dips an at theessential leak position showdisadvantage considerable fluctuations. of any For 10 extractive mL/min some of method. The other reason is that, for lowthe rates, dips are the completely flow missing. is more This has like two reasons:ly deflected First, the speed by of any the sliding small stage isair too fastturbulence in the lab. for the extractive technique even at a pump rate of 500 mL/min. Thus, a complete exchange of the gas However, for a repetitivein the cell measurement, is not possible when the leakdetection position is passing of a by leak in front with of the probe 10 tip. mL/min, This behavior isi.e., with a leak rate an essential disadvantage of any extractive method. The other reason is that, for low rates, the flow is above 0.1 mbar·L/s, is morefeasible. likely deflected by any small air turbulence in the lab. However, for a repetitive measurement, detection of a leak with 10 mL/min, i.e., with a leak rate above 0.1 mbar·L/s, is feasible.

Figure 2. Extractive static measurement of the O2 concentration using a 0.5 mm diameter tip in Figure 2. Extractive statica distance measurement of 10 mm from the leak of plate. the A pump O2 rateconcentration of 100 mL/min was employed.using Thea upper0.5 mm blue diameter tip in a curve shows the variation of the N2-ﬂux from the leak plate vs. time (right ordinate axis). Flux steps of distance of 10 mm from20, 10, the and 5leak mL/min plate. were used. A pump rate of 100 mL/min was employed. The upper blue curve shows the variation of the N2-flux from the leak plate vs. time (right ordinate axis). Flux steps of 20, 10, and 5 mL/min were used.

Figure 3. Extractive dynamic measurement of the O2 concentration using a 0.5 mm diameter tip at a distance of 10 mm from the leak plate. A pump rate of 500 mL/min is used. The upper red curve shows the position of the sliding stage which moves back and forth with 1 mm/s. The leak position is at 60 mm (right ordinate scale). Three different N2-fluxes of 100, 50, and 10 mL/min are applied as indicated in blue.

Figure 2. Extractive static measurement of the O2 concentration using a 0.5 mm diameter tip in a distance of 10 mm from the leak plate. A pump rate of 100 mL/min was employed. The upper blue curve shows the variation of the N2-flux from the leak plate vs. time (right ordinate axis). Flux steps of 20, 10, and 5 mL/min were used. Sensors 2017, 17, 2804 6 of 14

Sensors 2017, 17, 2804 Figure 3. Extractive dynamic measurement of the O2 concentration using a 0.5 mm diameter tip at 6 of 14 a distance of 10 mm from the leak plate. A pump rate of 500 mL/min is used. The upper red curve Figure 3. Extractiveshows dynamic the position measurement of the sliding stage which of the moves O back2 concentration and forth with 1 mm/s. using The a leak 0.5 position mm is diameter tip at a at 60 mm (right ordinate scale). Three different N -ﬂuxes of 100, 50, and 10 mL/min are applied as 4.2. Extractivedistance Measurements of 10 mm from with the leakCH 4plate. A pump ra2 te of 500 mL/min is used. The upper red curve indicated in blue. shows the position of the sliding stage which moves back and forth with 1 mm/s. The leak position is 4 2 Forat comparison 60 mm (right4.2. Extractivemeasurements,ordinate Measurements scale). Three with using CH different4 the positiveN2-fluxes detectionof 100, 50, andscheme 10 mL/min with are1% applied CH in as N was performedindicated with inan blue. identicalFor comparison setup measurements, (see Figures using the4 and positive 5). detection scheme with 1% CH4 in N2 was performed with an identical setup (see Figures4 and5).

Figure 4. Extractive static measurement of the CH4 concentration using a 0.5 mm diameter tip at Figure 4. Extractivea distancestaticof measurement 10 mm from the leak plate.of the A pump CH rate4 concentration of 100 mL/min was employed. using Thea 0.5 blue curvemm diameter tip at a distance of 10 mm showsfrom the the variation leak of theplate. gas ﬂux A from pump the leak ra platete vs.of time 100 (right mL/min ordinate axis). was Flux employed. steps of 100, The blue curve 50, 25, and 10 mL/min were used. shows the variation of the gas flux from the leak plate vs. time (right ordinate axis). Flux steps of 100, 50, 25, and 10 mL/min were used.

In the case of the static measurement (Figure 4) strong concentration fluctuations were observed, but a 10 mL/min leak flux could be clearly detected. The observed concentration values do not scale with the employed gas fluxes, probably because of the gas flow distribution (in Figure 2 smaller fluxes were used). The noise equivalent concentration (1 σ, 60 s average) of ≈1.3 ppm for zero gas flux is much lower than the observed concentration values of ≈300 ppm at a gas flux of 10 mL/min. Due to the higher absorption line strength of CH4 the SNR is higher than for O2. Leak detection is limited by the fluctuations. We further observed that increasing the pump rate does not help to reduce these fluctuations and concentration values are smaller due to dilution.

Figure 5. Extractive dynamic measurement of the CH4 concentration using a 0.5 mm diameter tip at a distance of 10 mm from the leak plate. A pump rate of 100 mL/min was employed. The upper red curve shows the position of the sliding stage, which moves ±30 mm away from the leak position at

90 mm (right ordinate scale). The speed is 1 mm/s. Three different N2-fluxes of 100, 50, and 10 mL/min are applied, as indicated in blue.

Sensors 2017, 17, 2804 6 of 14

4.2. Extractive Measurements with CH4

For comparison measurements, using the positive detection scheme with 1% CH4 in N2 was performed with an identical setup (see Figures 4 and 5).

Figure 4. Extractive static measurement of the CH4 concentration using a 0.5 mm diameter tip at a distance of 10 mm from the leak plate. A pump rate of 100 mL/min was employed. The blue curve shows the variation of the gas flux from the leak plate vs. time (right ordinate axis). Flux steps of 100, Sensors50,2017 25,, 17and, 2804 10 mL/min were used. 7 of 14

In the case of the static measurement (Figure 4) strong concentration fluctuations were In the case of the static measurement (Figure4) strong concentration fluctuations were observed, observed, but a 10 mL/min leak flux could be clearly detected. The observed concentration values do but a 10 mL/min leak flux could be clearly detected. The observed concentration values do not scale not scale with the employed gas fluxes, probably because of the gas flow distribution (in Figure 2 with the employed gas fluxes, probably because of the gas flow distribution (in Figure2 smaller fluxes smaller fluxes were used). The noise equivalent concentration (1 σ, 60 s average) of ≈1.3 ppm for zero were used). The noise equivalent concentration (1 σ, 60 s average) of ≈1.3 ppm for zero gas flux is gas flux is much lower than the observed concentration values of ≈300 ppm at a gas flux of 10 much lower than the observed concentration values of ≈300 ppm at a gas flux of 10 mL/min. Due to mL/min. Due to the higher absorption line strength of CH4 the SNR is higher than for O2. Leak the higher absorption line strength of CH the SNR is higher than for O . Leak detection is limited detection is limited by the fluctuations. We4 further observed that increasing2 the pump rate does not by the fluctuations. We further observed that increasing the pump rate does not help to reduce these help to reduce these fluctuations and concentration values are smaller due to dilution. fluctuations and concentration values are smaller due to dilution.

Figure 5. Extractive dynamic dynamic measurement measurement of of the the CH CH4 4concentrationconcentration using using a a0.5 0.5 mm mm diameter diameter tip tip atat a distancea distance of of 10 10 mm mm from from the the leak leak plate. plate. A A pump pump rate rate of of 100 mL/min was was employed. employed. The The upper red curve shows the position of the slidingsliding stage,stage, whichwhich movesmoves ±±3030 mm mm away away from the leak position at

90 mmmm (right(right ordinate ordinate scale). scale). The The speed speed is 1 mm/s.is 1 mm/s. Three Three different different N2-ﬂuxes N2-fluxes of 100, of 50, 100, and 1050, mL/min and 10 mL/minare applied, are applied, as indicated as indicated in blue. in blue.

The dynamic CH4 measurement in Figure5 is qualitatively similar to the results displayed in Figure3. Due to the lower pump rate the gas exchange time in the measurement cell is longer than in Figure3. Thus, strong fluctuations and missing peaks are observed. The concentration values are much smaller than in Figure4 because the cell is only partly filled when the leak position passes the tip. For repetitive measurements detection of fluxes of 10 mL/min is feasible, but lower fluxes are hardly observable despite the high SNR. However, for 100% CH4, this result may be scaled to detectable leak rates in the range of 10−3 mbar·L/s.

4.3. Standoff Measurements with O2 For standoff detection the setup of Figure1b was used. To ensure the sensitivity of the method, first the distance between laser and leak plate was varied without any additional N2 flux. The observed relative change of the O2 concentration in Figure6 corresponds to the relative change of the optical path length. The small noise of the concentration signal in Figure6 indicates that changes of the path length or the O2-concentration below 1% could be detected. Correction for the distance changes in Figure6 leaves a slow variation of the O 2 concentration with a period of ≈15 min. This is an artifact caused by temperature fluctuations of the laboratory air conditioning system influencing our TLS electronics and not a systematic concentration fluctuation of the lab air. Sensors 2017, 17, 2804 7 of 14

The dynamic CH4 measurement in Figure 5 is qualitatively similar to the results displayed in Figure 3. Due to the lower pump rate the gas exchange time in the measurement cell is longer than in Figure 3. Thus, strong fluctuations and missing peaks are observed. The concentration values are much smaller than in Figure 4 because the cell is only partly filled when the leak position passes the tip. For repetitive measurements detection of fluxes of 10 mL/min is feasible, but lower fluxes are hardly observable despite the high SNR. However, for 100% CH4, this result may be scaled to detectable leak rates in the range of 10−3 mbar·L/s.

4.3. Standoff Measurements with O2 For standoff detection the setup of Figure 1b was used. To ensure the sensitivity of the method, first the distance between laser and leak plate was varied without any additional N2 flux. The observed relative change of the O2 concentration in Figure 6 corresponds to the relative change of the optical path length. The small noise of the concentration signal in Figure 6 indicates that changes of the path length or the O2-concentration below 1% could be detected. Correction for the distance changes in Figure 4 leaves a slow variation of the O2 concentration with a period of ≈15 min. This is an artifact caused by temperature fluctuations of the laboratory air conditioning Sensorssystem2017 influencing, 17, 2804 our TLS electronics and not a systematic concentration fluctuation of the lab8 air. of 14

Figure 6. Distance variations during standoff O measurements in ambient air using the leak plate Figure 6. Distance variations during standoff O2 2measurements in ambient air using the leak plate as ± Sensorsasdiffuse 2017 diffuse, 17 reflector., 2804 reflector. The The leak leak plate plate was was moved moved back back and and forth forth towards towards laser laser and and detector detector for ± for 1 cm1 (red cm8 of 14 (red curve, right ordinate scale). The resulting change of the O concentration assuming a constant curve, right ordinate scale). The resulting change of the O2 concentration2 assuming a constant optical opticaldistanceDirecting distance of the114 collimated ofcm 114 (laser-plate-detector) cm (laser-plate-detector) laser spot (diameter is shown. is shown. 2–3 The mm) green The greenonto curve curvethe results gas results orifice from from a onlater a the later correction leak correction plate of with definedofthe the concentrationN2 concentration fluxes, a static measurement measurement leak test by is by theperformed the factor factor 114 114(Figure cm/actual cm/actual 8). distance.Fluxes distance. of 10 mL/min can be detected with a SNR ≈ 2, corresponding to an O2 concentration resolution of ≈200 ppm in this experiment. However,A further the observed confirmationconfirmation concentration of the feasibility steps do ofnot the linearly indirect follow method the N is2 fluxes: obtainedobtained a ten-fold by thethe increase followingfollowing of experiment:the N2 flux yields a gas cell only with a three 15 mm times length stronger and and concentrationa a diffuse diffuse reflecting reflecting dip. Thisbackplate indicates (DG10-120, that only Thorlabs a small Inc.,part Newton,of the effusing NJ, USA) N2 gas was jet inserted from the into leak the is intersectinglaser beam atwith the the position laser radiation of of the leak for plate higher in in fluxes.Figure CFD 11b).b). Duesimulations to to the the small smallsupport cell cell volumethis volume argument of of≈6.5≈ (seemL6.5 mLaSection rapid a rapid gas4.6). exchange gas exchange is poss isible. possible. Figure 7 Figure shows7 theshows result the of resultan experimentResults of an experimentof exchanginga dynamic exchanging thestandoff air in detection thethe aircell in byexperime the N2 cell. Thent by observedare N2 .shown The reduction observedin Figure in reduction 9.total Similar average in to total theO2 averageconcentrationextractive O 2measurementsconcentration for an optical in for Figure path an optical length 3, the pathleakof 11 plate length4 cm wasagrees of 114moved very cm agreesperpendicularwell with very the well calculatedto withthe interrogating the of calculated Δc = 20.9% laser of ∆×beam. c(3 = cm/114 20.9% ×cm)(3 = cm/114 0.55%. cm) = 0.55%.

Figure 7. Standoff O2 measurement using a diffuse reflecting gas cell instead of the leak plate of Figure Figure 7. Standoff O2 measurement using a diffuse reﬂecting gas cell instead of the leak plate of 1b. The total optical path length (laser-diffuse reflecting backplate-detector) was 114 cm. The optical Figure1b. The total optical path length (laser-diffuse reﬂecting backplate-detector) was 114 cm. Thepath optical length pathin the length cell was in the30 mm. cell was The 30gas mm. in the The ce gasll was in theperiodically cell was periodicallychanged from changed lab air fromto 100% lab N air2.

to 100% N2.

Directing the collimated laser spot (diameter 2–3 mm) onto the gas orifice on the leak plate with defined N2 fluxes, a static leak test is performed (Figure8). Fluxes of 10 mL/min can be detected

Figure 8. Static standoff N2 detection via an O2 measurement. The distance between laser and leak

plate was 55 cm. The N2 flux was stepwise reduced from 100, 50, 25, to 10 mL/min (blue curve, right ordinate scale).

Sensors 2017, 17, 2804 8 of 14

Directing the collimated laser spot (diameter 2–3 mm) onto the gas orifice on the leak plate with defined N2 fluxes, a static leak test is performed (Figure 8). Fluxes of 10 mL/min can be detected with a SNR ≈ 2, corresponding to an O2 concentration resolution of ≈200 ppm in this experiment. However, the observed concentration steps do not linearly follow the N2 fluxes: a ten-fold increase of the N2 flux yields only a three times stronger concentration dip. This indicates that only a small part of the effusing N2 gas jet from the leak is intersecting with the laser radiation for higher fluxes. CFD simulations support this argument (see Section 4.6). Results of a dynamic standoff detection experiment are shown in Figure 9. Similar to the extractive measurements in Figure 3, the leak plate was moved perpendicular to the interrogating laser beam.

Sensors 2017, 17, 2804 9 of 14

with a SNR ≈ 2, corresponding to an O2 concentration resolution of ≈200 ppm in this experiment. However, the observed concentration steps do not linearly follow the N2 fluxes: a ten-fold increase of the N2 flux yields only a three times stronger concentration dip. This indicates that only a small part of the effusing N gas jet from the leak is intersecting with the laser radiation for higher fluxes. Figure 7. Standoff O22 measurement using a diffuse reflecting gas cell instead of the leak plate of Figure CFD1b. simulations The total optical support path this length argument (laser-diffuse (see Section reflecting 4.6 ).backplate-detector) was 114 cm. The optical

pathResults length of ain dynamic the cell was standoff 30 mm. detection The gas in experiment the cell was areperiodically shown inchanged Figure from9. Similar lab air to to the100% extractive N2. measurements in Figure3, the leak plate was moved perpendicular to the interrogating laser beam.

Figure 8. Static standoff N2 detection via an O2 measurement. The distance between laser and leak Figure 8. Static standoff N2 detection via an O2 measurement. The distance between laser and leak plate was 55 cm. The N2 ﬂux was stepwise reduced from 100, 50, 25, to 10 mL/min (blue curve, right Sensorsplate 2017 was, 17, 280455 cm. The N2 flux was stepwise reduced from 100, 50, 25, to 10 mL/min (blue curve, right9 of 14 ordinate scale). ordinate scale).

Figure 9. Standoff dynamic N2 leak detection via measurementmeasurement ofof thethe OO22 concentration.concentration. The distance between the laser and leak plate was 55 cm. The leak plate was horizontally moved by a sliding stage with a speed of 0.2 mm/s mm/s in in a asawtooth sawtooth manner, manner, as as indicated indicated by by the the lower lower figure. ﬁgure. The The leak leak position position is at is 46at 46mm. mm.

N2 fluxes of 100 mL/min could be dynamically detected and localization of the leak is feasible. N2 fluxes of 100 mL/min could be dynamically detected and localization of the leak is feasible. However, the dynamic detection is significantly significantly less sensitive than than the the static static detection detection in in Figure Figure 88.. Furthermore, aa fixed fixed pattern pattern noise noise was was observed observed by moving by moving the laser the spot laser across spot the across diffuse the reflecting diffuse plate.reflecting Increasing plate. Increasing the laser spot the laser size by spot a factor size by of a three factor did of notthree improve did not the improve result. the result.

4.4. Standoff Measurements with CH4

As with the the extractive extractive technique technique measurements measurements with with 1% 1% CH CH4 in4 inN2 N were2 were performed performed with with an anidentical identical setup. setup. For For the the static static me measurementasurement a similar a similar nonlinear nonlinear behavior behavior was was found found (see (see Figure Figure 10).10). This supports the assumption that the leakage gas jet is only partly intersected by the laser radiation, and a saturation of the signal occurs at high fluxes. The low noise level for zero flux indicates that even lower flux rates than 10 mL/min should be detectable by the static method. A dynamic standoff measurement with CH4 is shown in Figure 11. In contrast to the indirect detection scheme a leak flux of 10 mL/min could be dynamically detected and localized. The low noise between the leak locations indicates that even lower fluxes should be detectable by this method.

Figure 10. Static standoff measurement of 1% CH4 in N2. The distance between laser and leak plate was 55 cm. The flux was stepwise reduced from 300, 150, 100, 50, 25, to 10 mL/min (blue curve, right ordinate scale).

Sensors 2017, 17, 2804 9 of 14

Figure 9. Standoff dynamic N2 leak detection via measurement of the O2 concentration. The distance between the laser and leak plate was 55 cm. The leak plate was horizontally moved by a sliding stage with a speed of 0.2 mm/s in a sawtooth manner, as indicated by the lower figure. The leak position is at 46 mm.

N2 fluxes of 100 mL/min could be dynamically detected and localization of the leak is feasible. However, the dynamic detection is significantly less sensitive than the static detection in Figure 8. Furthermore, a fixed pattern noise was observed by moving the laser spot across the diffuse reflecting plate. Increasing the laser spot size by a factor of three did not improve the result.

4.4. Standoff Measurements with CH4

As with the extractive technique measurements with 1% CH4 in N2 were performed with an Sensorsidentical2017 setup., 17, 2804 For the static measurement a similar nonlinear behavior was found (see Figure10 of10). 14 This supports the assumption that the leakage gas jet is only partly intersected by the laser radiation, and a saturation of the signal occurs at high fluxes. The low noise level for zero flux indicates that Thiseven supportslower flux the rates assumption than 10 mL/min that the should leakage be gas detectable jet is only by partly the static intersected method. by the laser radiation, and a saturation of the signal occurs at high fluxes. The low noise level for zero flux indicates that even A dynamic standoff measurement with CH4 is shown in Figure 11. In contrast to the indirect lowerdetection flux scheme rates than a leak 10 mL/min flux of 10 should mL/min be detectable could be dynamically by the static method.detected and localized. The low noiseA between dynamic the standoff leak locations measurement indicates with that CH4 evenis shown lower in fluxes Figure should11. In contrastbe detectable to the indirectby this detectionmethod. scheme a leak flux of 10 mL/min could be dynamically detected and localized. The low noise between the leak locations indicates that even lower fluxes should be detectable by this method.

Figure 10. Static standoff measurementmeasurement of 1%1% CHCH44 inin NN22.. The The distance between laser and leak plate was 55 cm. The ﬂuxflux was stepwise reduced fromfrom 300,300, 150,150, 100,100, 50,50, 25,25, toto 1010 mL/minmL/min (blue curve, right Sensorsordinate 2017, 17 scale)., 2804 10 of 14

Figure 11. Dynamic standoff measurement of 1% CH4 in N2 with fluxes of 100, 50, and 10 mL/min. Figure 11. Dynamic standoff measurement of 1% CH4 in N2 with ﬂuxes of 100, 50, and 10 mL/min. TheThe distancedistance betweenbetween laserlaser and leak plate was 55 cm. The The leak leak plate plate was was horizontally horizontally moved moved by by a asliding sliding stage stage with with a speed a speed of of0.2 0.2 mm/s mm/s in a in sawtoot a sawtoothh manner, manner, as indicated as indicated by the by lower the lower figure. ﬁgure. The Theleak leak position position is at is 46 at mm 46 mm

4.5.4.5. FurtherFurther ExperimentsExperiments TheThe leakleak plateplate setupsetup waswas alsoalso adaptedadapted forfor measurementsmeasurements withwith aa commercialcommercial heliumhelium leakleak testertester (SmartTest(SmartTest HLT HLT 560, 560, Pfeiffer Pfeiffer Vacuum Vacuum GmbH, GmbH, Asslar, Asslar, Germany) Germany) in the sniffingin the sniffing mode. Formode. this purposeFor this thepurpose nozzle the of nozzle the sniffing of the sniffing probe was probe mounted was mounted at the at position the position of the of extractive the extractive tip intip Figure in Figure1a. Dynamic1a. Dynamic measurements measurements similar similar to to those those of of Sections Sections 4.1 4.1 andand 4.2 werewere performed performed with with 10% 10% He He in N2. Despite the superior sensitivity and dynamic range of the helium leak tester, localization of a 10 in N2. Despite the superior sensitivity and dynamic range of the helium leak tester, localization ofmL/min a 10 mL/min flow from flow the from leak the plate leak plate was wasmore more diffic difficultult than than expected. expected. In Inorder order to to prevent prevent a rapidrapid increaseincrease of of the the He-background He-background concentration, concentration, the experiments the experiments were performed were performed under a fumeunder cupboard. a fume cupboard. The nozzle of the sniffing probe had to be at a closer distance from the leak plate (i.e., 6 mm compared to 10 mm in Figure 1a). Compared to CH4 or N2, He is diffusing sideways more rapidly from the orifice, which tends to make leak localization more difficult than with the other techniques.

4.6. CFD Simulations Obviously the sensitivity of the measurements is limited by the gas flow distribution and its fluctuations. At the open atmosphere influence of wind and soil properties around a leak will be severe. Even in the lab, air conditioning, ventilators of electronic devices, and the presence of lab personnel result in local air turbulence which affects the measurements. For a better understanding of the gas flow distribution for the leak plate (see Figure 1) CFD simulations using COMSOL (turbulent multiphase flow) are performed (Figure 12). For a flow of 100 mL/min the gas is ejected in a narrow laminar flow jet. The simulation model utilizes the rotational symmetry of the setup so the simulation is effectively done in 2D and rotated around the center axis. The model was designed similar to the COMSOL combustion model [15], but without any reaction. The simulated volume above the orifice in Figure 12 has a diameter of 12.5 cm and a height of 17.8 cm. A radial influx of air (80% N2 and 20% O2) is allowed in the model. As in the laboratory experiment no influx (tight wall) is allowed from the region with a vertical displacement smaller than 0 mm (indicated by the solid black line within Figure 12). All other boundary conditions of the simulated volume to the universe are set to “no pressure drop”. The surrounding pressure is set to 1 bar. To validate the simulation results a two times larger diameter and height has also been used, but the results did not differ from the results shown in Figure 12. The temperature of all components is set to 20 °C.

Sensors 2017, 17, 2804 11 of 14

The nozzle of the sniffing probe had to be at a closer distance from the leak plate (i.e., 6 mm compared to 10 mm in Figure1a). Compared to CH 4 or N2, He is diffusing sideways more rapidly from the orifice, which tends to make leak localization more difficult than with the other techniques.

4.6. CFD Simulations Obviously the sensitivity of the measurements is limited by the gas flow distribution and its fluctuations. At the open atmosphere influence of wind and soil properties around a leak will be severe. Even in the lab, air conditioning, ventilators of electronic devices, and the presence of lab personnel result in local air turbulence which affects the measurements. For a better understanding of the gas flow distribution for the leak plate (see Figure1) CFD simulations using COMSOL (turbulent multiphase flow) are performed (Figure 12). For a flow of 100 mL/min the gas is ejected in a narrow laminar flow jet. The simulation model utilizes the rotational symmetry of the setup so the simulation is effectively done in 2D and rotated around the center axis. The model was designed similar to the COMSOL combustion model [15], but without any reaction. The simulated volume above the orifice in Figure 12 has a diameter of 12.5 cm and a height of 17.8 cm. A radial influx of air (80% N2 and 20% O2) is allowed in the model. As in the laboratory experiment no influx (tight wall) is allowed from the region with a vertical displacement smaller than 0 mm (indicated by the solid black line within Figure 12). All other boundary conditions of the simulated volume to the universe are set to “no pressure drop”. The surrounding pressure is set to 1 bar. To validate the simulation results a two times larger diameter and height has also been used, but the results did not differ from the results Sensorsshown 2017 in, Figure 17, 2804 12 . The temperature of all components is set to 20 ◦C. 11 of 14

Figure 12. CFDCFD simulationsimulation using using COMSOL COMSOL for for a fluxa flux of 1%of 1% CH 4CHin4 N in2 effusingN2 effusing from from an orifice an orifice of 1 mm of 1diameter mm diameter with 100with mL/min 100 mL/min into ambientinto ambient air. Anair. axialAn axial cut alongcut along the gasthe gas flow flow direction direction through through the

thecentrosymmetric centrosymmetric laminar laminar flow flow distribution distribution shows shows iso-concentration iso-concentration lines of lines the CH of 4thevolume CH4 fraction.volume fraction. The CH4 concentration is reduced right at the orifice by dilution from air from the side, similar to a jetThe pump. CH4 concentration Within the jet is the reduced maximum right concentrationat the orifice by decreases dilution rapidlyfrom air and from is lowerthe side, than similar 0.4% toat a jet 20 pump. mm distance. Within Thethe jet full the width maximum at half concentrat maximumion at adecreases 20 mm distance rapidly fromand is the lower leak than plate 0.4% is less at athan 20 mm 10 mm. distance. These The findings full width were experimentally at half maximum confirmed at a 20 bymm scanning distance the from gas jetthe with leak a plate commercial is less thanextractive 10 mm. instrument These (IRwin™,findings were Inficon experimenta GmbH, Cologne,lly confirmed Germany). by scanning the gas jet with a commercialWhen theextractive gas flux instrument is increased (IRwin™, from 25 Inficon mL/min GmbH, to 200 Cologne, mL/min Germany). the jetgets longer, but the shapeWhen is almost the gas unchanged. flux is increased Variation from of 25 the mL/min flux parameter to 200 mL/min within the the jet CFD gets simulation, longer, but asthe well shape as isthermal almost imaging unchanged. experiments Variation with of the CO flux2 jets, parame show,ter qualitatively, within the aCFD similar simulati behavior.on, as For well higher as thermal fluxes, imagingturbulence experiments sets in. For smallwith CO fluxes2 jets, the show, distribution qualitatively, is deflected a similar by environmental behavior. For air flowhigher [16 ].fluxes, turbulence sets in. For small fluxes the distribution is deflected by environmental air flow [16]. As already mentioned, the results of standoff measurements are strongly affected by the intersection of the laser beams and the leak flux distribution. Thus, only qualitative results can be obtained.

5. Discussion

Sensors 2017, 17, 2804 12 of 14

As already mentioned, the results of standoff measurements are strongly affected by the intersection of the laser beams and the leak ﬂux distribution. Thus, only qualitative results can be obtained.

5. Discussion

In this work, we have shown that gas leaks can be detected by the dilution of ambient O2. The fact that the leakage gas reduces the O2 concentration in the vicinity of the leak can be measured. The measurement principle is based on tunable laser spectroscopy, which was realized in an extractive and a standoff device, respectively. The method is demonstrated for N2 as the leakage gas, which is not an infrared absorber. The technique can be applied for other non-infrared absorbing gases, such as H2 or Ar, as well as for any gas, except for the ambient gas itself. Instead of O2, other infrared-absorbing background gases may be used, e.g., CO2 or H2O. However, CO2 and H2O concentrations show strong local and temporal fluctuations which have to be compensated. Consequently, an interesting application field could be an environment with a controlled atmosphere (e.g., in a greenhouse). For static extractive detection N2 fluxes of less than 5 mL/min can be observed, corresponding to a detectable leak rate L ≥ 0.1 mbar·L/s. In consideration of a limited response speed, comparable results are obtained in a dynamical extractive detection scheme. Almost the same sensitivity level can be achieved in the case of static standoff detection from a distance of about 0.6 m. In a dynamic standoff detection scenario, only flows of 100 mL/min could be detected. The localization of the gas leak is feasible within ±5 mm. Obviously, this value depends on system parameters, such as the speed of displacement (leak), pump rate (extractive detection), or the laser spot size (standoff detection). Comparing the direct measurements using 1% CH4 in N2 and the indirect measurements using dilution of the ambient 21% O2 by N2, the obvious result is that the sensitivity for the direct method is higher. On the one hand, the difference in the absorption line strength between CH4 at 1651 nm −21 −1 −1 2 −24 −1 −1 2 (10 cm ·mol ·cm ) and O2 at 761 nm (8 × 10 bcm ·mol ·cm ) is about two orders in magnitude [10]. On the other hand, Si-photodiodes used for O2 have a ten-fold higher detectivity than the InGaAs diodes needed for CH4. This helps to reduce the effect of the large line strength difference. Our experiments indicate almost the same SNR for both methods since the influence of gas fluctuations is dominating. For 1% CH4 in N2 the fluctuations of the signal with leak flux are larger than without flux (cf. Figure 10). This effect is hardly observable for O2 since the ambient concentration shows a higher noise level. Consequently, the detection is practically limited by the fluctuations caused by gas flow −3 instabilities. Therefore, the estimation of leak rates L ≥ 10 mbar·L/s for a 100% CH4 standoff measurement seems to be quite optimistic. Looking at these fluctuations could be another promising method of leak detection that needs to be investigated in further experiments. Furthermore, it is shown that the extractive method is more sensitive than the standoff one. In addition, the extractive method can be improved by optimization of the gas cell and pump rate. For O2, a compact Herriott gas cell with an optical path length of 5 m was reported in [17]. The sensitivity level of the above mentioned ultrasound method may be achieved by using such cell designs and leak rates <10−2 mbar·L/s should be detectable. Even higher path lengths can be obtained by using resonant cells and corresponding detection schemes [18,19], which are established for high-resolution gas detection. However, for this application small changes relative to a high ambient background have to be detected. Thus, special differential cell designs or frequent fast gas exchange with ambient air as a reference gas will be required to increase the sensitivity. For the short −4 time intervals typical for leak detection, a relative sensitivity in the O2 concentration below 10 may be detectable. This corresponds to a concentration change of 20 ppm within a 21% background concentration level. The sensitivity of standoff techniques strongly depends on the reflection properties of the backscattering surface [20]. A low reflectivity reduces the detected radiation intensity and the SNR. This holds for CH4 detection at 1651 nm, as well as for O2 detection at 761 nm. Good results are Sensors 2017, 17, 2804 13 of 14 achieved with objects of high diffuse reflectivity, e.g., the reflecting plate shown in Figure7. The leak plate of Figure1a has a stronger specular reflection. However, at a scattering angle of 17 ◦ the reflectivity is similar to the value of the Thorlabs DG10-120 plate. In real applications non-cooperative specular reflectors are most difficult because they reflect in any direction. Specular reflection of common objects increases with increasing wavelength. Thus, working at 761 nm is advantageous compared to 1651 nm. Improvement of the standoff technique might be feasible for short distances around 0.2 m to 0.3 m. Depending on the backscattering properties of leaking objects the optical beam diameter and the aperture of the receiving optics may be enlarged. Therefore, the interaction of laser radiation and the leakage gas plume relative to the total absorption path length could be increased such that minimum leak rates ≥10−2 mbar·L/s may be detectable. In conclusion, we have shown the feasibility of gas leak detection by dilution of atmospheric O2. This indirect method has several advantages in spite of its lower sensitivity compared to direct detection of infrared absorbing gases. Since the method is independent of the leakage gas, it can be used as a universal leak sensor. For instance, a combination with another specific infrared gas detector might be possible. Compared to the established ultrasound technique, leaks can be detected which do not generate sound. Detection of visible radiation at 761 nm enables the use of fast, highly-sensitive, and inexpensive standard photodiodes. Large area detectors, special sensor geometries and arrays, are commercially available. Imaging leak detection is also feasible by the use of a laser scanning system in combination with fast data processing. The most important application of TLS in terms of market share represents the measurement of O2. In process industries, TLS instruments from several suppliers are commercially available. As a consequence, component prices are generally lower compared to other TLS applications. In the case of O2 applications, VCSEL laser diodes can be used, which are less expensive than DFB lasers. Furthermore this laser type can be manufactured in large volumes and small packages. In this context, an integration of a laser-based O2 sensor in a mobile phone might be possible.

Acknowledgments: This work is supported by the Fraunhofer Internal Programs under grant no. Discover 830 968. The authors acknowledge the contribution of Fadi Al-Salti performing first experiments as part of this master’s thesis at Fraunhofer IPM in 2016. Author Contributions: Armin Lambrecht and Johannes Herbst conceived and designed the experiments; Eric Maier and Thomas Strahl performed the experiments and analyzed the data; Hans-Fridtjof Pernau performed the CFD calculations and part of the experiments; and Armin Lambrecht wrote the paper. Conflicts of Interest: The authors declare no conflict of interest. The founding sponsors had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

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