Portable optical spectroscopy device for analyzing gas samples

Abstract

A portable optical spectroscopy device is disclosed for analyzing gas samples and/or for measurement of species concentration, number density, or column density. The device includes a measuring chamber with the gas sample to be analyzed, a light source with at least one laser diode for emitting a laser beam along a light path running through the measuring chamber at least in certain regions, means for modulating the wavelength of the light beam emitted by the light source, and an optical detector device having a first optical detector and at least one second optical detector. At least a part of the light emitted by the laser diode is detected after the light has passed through the measuring chamber m-times, and at least a part of the light emitted by the laser diode is detected with the at least one second optical detector after the light has passed through the measuring chamber n-times, where n>m applies.

Claims

1. A portable optical spectroscopy device for analyzing a gas sample and/or for measurement of species concentration, number density, or column density, the device comprising the following: a measuring chamber with the gas sample to be analyzed; a light source with at least one laser diode for emitting a light beam along a light path running through the measuring chamber at least in certain regions, wherein the at least one laser diode of the light source is a semiconductor laser diode, the light source disposed at a first end region of the measuring chamber; control circuitry configured to modulate a wavelength of the light beam emitted by the light source; an optical detector device disposed at a second end region of the measuring chamber, the optical detector device having a first optical detector and at least one second optical detector, the first and the at least one second optical detector being arranged with respect to the laser diode such that with the first optical detector a first part of the light emitted by the laser diode is detected after the first part of the light has passed through the measuring chamber m-times, and a second part of the light emitted by the laser diode is detected with the at least one second optical detector after the second part of the light has passed through the measuring chamber n-times, where n>m applies; and an evaluation unit for evaluating a signal characteristic output by the first optical detector and/or a signal characteristic output by the at least one second optical detector, the evaluation unit including signal processing electronics, wherein a reference cell is provided in which at least one species is contained which is to be detected in the gas sample to be analyzed, wherein the reference cell is positioned such that the first part of the light emitted by the laser diode passes through the reference cell before the first part of the light is detected by the first optical detector and such that the second part of the light emitted by the laser diode passes around the reference cell before the second part of the light is detected by the second optical detector, and wherein the reference cell is arranged at the second end region of the measurement chamber opposite from the light source and between the measurement chamber and the first optical detector, wherein a first reflector is disposed between the light source and the reference cell and is disposed at the second end region of the measurement chamber, the first reflector formed as a partially transmissive reflector such that the first part of the light passes through the measurement chamber only once and then through the first reflector and then through the reference cell before the first part of the light is detected by the first optical detector, the first reflector reflecting the second portion of the light back into the measurement chamber.

2. The device according to claim 1, wherein the device comprises an internal power source certified for use in potentially explosive atmospheres, said internal power source having at least one rechargeable battery and a battery enclosure for use in a hazardous area, said at least one rechargeable battery comprising a 3.65 V high energy lithium-ion cell and a protective cover in order to cover positive and negative terminals of the lithium-ion cell; and/or wherein the device comprises a power circuit with intrinsic safety technology for safe operation of the electrical equipment of the device in hazardous areas by limiting the energy, electrical and thermal, available for ignition.

3. The device according to claim 1, wherein the evaluation unit comprises: signal-processing electronics for acquiring data from the first and the at least one second optical detector and for establishing linelocking and a comparative signal source of at least one absorption feature; and a microcontroller receiving said data from said signal-processing electronics, and wherein the device additionally comprises: a display for displaying said data from said microcontroller, wherein said display can switch between displaying said data in PPM, percent LEL and percent gas; alarms controlled by said microcontroller; and/or a user button for inputting options and modes.

4. The device according to claim 1, wherein the first optical detector and/or the at least one second optical detector are/is selectively operable for harmonic detection of the light emitted by the laser diode or for direct detection of the light emitted by the laser diode; and wherein the evaluation unit comprises at least one phase detector for a phase-locked loop circuit allocated to the first optical detector and/or the at least one second optical detector.

5. The device according to claim 1, wherein the at least one laser diode is adapted to selectively emit light in a first frequency spectrum or light in at least one second frequency spectrum, wherein the first frequency spectrum is matched to a first absorption line of a species to be detected in the gas sample to be analyzed, and wherein the second frequency spectrum is matched to a second absorption line of the species, wherein the first absorption line is stronger than the second absorption line.

6. The device according to claim 4, wherein the evaluation unit is operable: in a first operating mode, in which the at least one second optical detector is operated for the harmonic detection of the light emitted by the laser diode and a signal path recorded by the at least one second optical detector is evaluated; in a second operating mode, in which the at least one second optical detector is operated for the direct detection of the light emitted by the laser diode and the signal path recorded by the at least one second optical detector is evaluated; in a third operating mode, in which the first optical detector is operated for the direct detection of the light emitted by the laser diode and a signal path recorded by the first optical detector is evaluated; and in a fourth operating mode in which the at least one laser diode is driven in such a way that light emits in a frequency spectrum which is tuned to a selected absorption line of a species to be detected in the gas sample to be analyzed, wherein the selected absorption line is weaker than an absorption line, onto which the frequency spectrum of the light emitted by the at least one laser diode is in the first to third operating mode, and wherein in the fourth operating mode the at least one second optical detector is selectively operated for harmonic or direct detection of the light emitted by the at least one laser diode and the signal path recorded by the at least one second optical detector is evaluated.

7. The device according to claim 1, wherein the at least one second optical detector is arranged such that its receiving axis runs parallel to a receiving axis of the first optical detector.

8. The device according to claim 1, wherein a second reflector is arranged at the first end region of the measuring chamber which interacts with the first reflector arranged at the second end region of the measuring chamber such that the second part of the light emitted by the laser diode passes several times through the measuring chamber by reflecting off of the first reflector and the second reflector until the second part of the light is detected by the at least one second detector.

9. The device according to claim 1, wherein the device is handheld and configured as a portable optical spectroscopy apparatus for measurement of gas concentration, and wherein the device can be held with a single hand.

10. The device apparatus according to claim 1, further comprising a pump that continuously pumps the gas to be analyzed through the measuring chamber; and/or further comprising a telescoping sample probe; and/or further comprising a rigid probe for measurement of underground gas concentrations.

11. A portable optical spectroscopy method for measurement of species concentration, number density, or column density, the method comprising the steps of: holding a portable optical spectroscopy device in an area to be measured, said portable optical spectroscopy device being a device according to claim 1; emitting light from the light source of the portable optical spectroscopy device through the measuring chamber of the device; receiving light via the first optical detector and the at least one second optical detector of the device; and evaluating a signal characteristic output by the first optical detector and the at least one second optical detector of the device; wherein the device has a plurality of operational modes measuring a same absorption feature or different absorption features of the species, and wherein said device switches between modes depending on measured absorbance.

12. The method according to claim 11, wherein: a first operational mode is selected from the group consisting of wavelength modulation spectroscopy, frequency modulation spectroscopy, two-tone frequency modulation spectroscopy, cavity ringdown spectroscopy, and rapid-scan direct absorption spectroscopy; a second operational mode determines absorbance from a measurement of width of the absorption feature; a third operational mode comprises direct absorption spectroscopy; a fourth operational mode determines absorbance from a measurement of width of the absorption feature performed by the first optical detector and the at least one second optical detector; and a fifth operational mode determines absorbance from a measurement of width of the at least one absorption feature at different wavelengths tuned to different absorption lines of a species to be detected in the gas sample.

13. The device according to claim 1, wherein the laser diode is a vertical-cavity surface-emitting laser diode.

14. The device according to claim 1, wherein the species to be detected in the gas sample to be analyzed is methane gas.

15. The device according to claim 1, wherein the wavelength of the light beam emitted by the light source is controllably modulated by periodic oscillation of current applied to the laser diode by the control circuitry.

16. A portable optical spectroscopy device for analyzing a gas sample and/or for measurement of species concentration, number density, or column density, the device comprising the following: a measuring chamber with the gas sample to be analyzed; a light source with at least one laser diode for emitting a light beam along a light path running through the measuring chamber at least in certain regions, wherein the at least one laser diode of the light source is a low-power semiconductor laser diode, the light source disposed at a first end region of the measuring chamber; control circuitry configured to modulate a wavelength of the light beam emitted by the light source; an optical detector device disposed at a second end region of the measuring chamber, the optical detector device having a first optical detector and at least one second optical detector, the first and the at least one second optical detector being arranged with respect to the laser diode such that with the first optical detector first part of the light emitted by the laser diode is detected after the light has passed through the measuring chamber m times, and a second part of the light emitted by the laser diode is detected with the at least one second optical detector after the light has passed through the measuring chamber n times, where n >m; an evaluation unit for evaluating a signal characteristic output by the first optical detector and/or a signal characteristic output by the at least one second optical detector, wherein at least one of the first optical detector and the at least one second optical detector is configured to selectively operate in either a first mode capable of harmonic detection of the light emitted by the laser diode or a second mode capable of direct detection of the light emitted by the laser diode, the evaluation unit including signal processing electronics; and a reference cell arranged at the second end region of the measurement chamber opposite from the light source and between the measurement chamber and the first optical detector, the reference cell containing at least one species which is to be detected in the gas sample to be analyzed, wherein the reference cell is positioned such that the first part of the light emitted by the laser diode passes through the reference cell before the first part of the light is detected by the first optical detector and such that the second part of the light emitted by the laser diode passes around the reference cell before the second part of the light is detected by the second optical detector; a partially transmissive reflector disposed between the light source and the reference cell and disposed at the second end region of the measurement chamber, the partially transmissive reflector formed such that the first part of the light passes through the measurement chamber and then through the partially transmissive reflector and then through the reference cell before the first part of the light is detected by the first optical detector, wherein the evaluation unit comprises at least one phase detector for a phase-locked loop circuit allocated to the first optical detector and/or the at least one second optical detector, and wherein the at least one of the first optical detector and the at least one second optical detector is operated in either the first mode or the second mode based on an absorbance of the gas sample to be analyzed.

17. The device of claim 1, further comprising: a telescoping sample probe.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The accompanying drawings, which are incorporated into and form a part of the specification, illustrate one or more embodiments of the present disclosure and, together with the description, serve to explain the principles of the disclosure. The drawings are only for the purpose of illustrating one or more preferred embodiments of the disclosure and are not to be construed as limiting the disclosure.

(2) In the drawings:

(3) FIG. 1 is a schematic drawing of an embodiment of the present disclosure preferred for the use in methane detection; and

(4) FIG. 2 is a cross-sectional view of a measuring chamber of an embodiment of the present disclosure.

DETAILED DESCRIPTION

(5) The present disclosure is directed to a portable optical spectroscopy device for analyzing gas samples and/or for measurement of species concentration. In particular, the disclosure relates to a portable gas leak detector. In a preferred embodiment, the portable gas leak detector is a diode laser sensor that determines gas concentration, preferably methane concentration.

(6) A schematic of an embodiment of the present disclosure is illustrated in FIG. 1. This embodiment preferably comprises a portable optical spectroscopy device for measurement of gas concentration. The device is preferably a handheld device.

(7) The optical spectroscopy device preferably comprises laser light source 1 having at least one laser diode and a fixed length optical path which receives light from the at least one laser diode. The fixed length optical path contains a gas to be detected and preferably comprises multiple pass optical cell or measuring chamber 2. The gas to be detected is preferably pumped continuously through multiple pass optical cell or measuring chamber 2 using a pump.

(8) A first optical detector 3a preferably receives light at an end of the fixed length optical path, after the light passes through the gas to be detected one times. Second optical detectors 3b, 3c receive light at an end of the fixed length optical path, after the light passes through the gas to be detected several times.

(9) The first and second optical detectors 3a, 3b, 3c are connected to signal processing electronics 4. Signal processing electronics 4 determines one or more gas concentrations. Microcontroller 5 preferably receives the concentration data from signal processing electronics 4, and displays the concentration data using a display 6. The display 6 can preferably display the gas concentration data in PPM, percent LEL and/or percent gas.

(10) This embodiment also preferably comprises a reference signal for linelocking and comparing signal sources. To establish a reference signal, this embodiment preferably comprises a reference cell 7 in which at least one species is contained which is to be detected in the gas sample to be analyzed. The reference cell 7 is designed such that at least a part of the light emitted by the laser light source 1 passes through the reference cell 7 after the light has passed through the measuring chamber 2 and before the light is detected by the first optical detector 3a.

(11) According to this design, the first optical detector 3a also serves for receiving light from the reference cell 7. Since the first optical detector 3a is connected to signal processing electronics 4 which determines concentration of the reference signal based on the amount of the optical detector detects. The reference cell 7 preferably comprises approximately a methane mixture with an optical absorbance of 0.001 to 0.1.

(12) The first optical detector 3a and the second optical detectors 3b, 3c are optionally operable for the harmonic detection of the light emitted by the laser diode or for the direct detection of the light emitted by the laser diode.

(13) The at least one laser diode is preferably adapted to selectively emit light in a first frequency spectrum or light in at least one second frequency spectrum, wherein the first frequency spectrum is matched to a first absorption line of a species (here: methane) to be detected in the gas sample to be analyzed, and wherein the second frequency spectrum is matched to a second absorption line of the species (here: methane), wherein the first absorption line is stronger than the second absorption line.

(14) For analyzing and evaluating the data provided by the first and second optical detectors 3a, 3b, 3c, the device comprises an evaluation unit 10. The evaluation unit 10 preferably comprises the already mentioned signal-processing electronics 4 for acquiring data from the first optical detector 3a and the second optical detectors 3b, 3c and for establishing linelocking and a comparative signal source of at least one absorption feature. Moreover, the evaluation unit 10 preferably comprises the already mentioned microcontroller 5 receiving said data from said signal processing electronics 4. For displaying said data from said microcontroller, a display 6 is provided. The display 6 can preferably switch between displaying said data in PPM, percent LEL and percent gas.

(15) The evaluation unit 10 is operable:

(16) in a first operating mode, in which the second optical detectors 3b, 3c are operated for the harmonic detection of the light emitted by the laser diode and the signal path recorded by the second optical detectors 3b, 3c is evaluated;

(17) in a second operating mode, in which the second optical detectors 3b, 3c are operated for the direct detection of the light emitted by the laser diode and the signal path recorded by the second optical detectors 3b, 3c is evaluated;

(18) in a third operating mode, in which the first optical detector 3a is operated for the direct detection of the light emitted by the laser diode and the signal path recorded by the first optical detector 3a is evaluated; and

(19) in a fourth operating mode in which the at least one laser diode is driven in such a way that this light emits in a frequency spectrum which is tuned to an absorption line of a species (here: methane) to be detected in the gas sample to be analyzed, wherein this absorption line is weaker than the absorption line, onto which the frequency spectrum of the light emitted by the at least one laser diode is in the first to third operating mode, and wherein in the fourth operating mode the at least one second optical detector is optionally operated for harmonic or direct detection of the light emitted by the at least one laser diode and the signal course recorded by the second optical detectors 3b, 3c is evaluated.

(20) In accordance with the present disclosure, a diode laser sensor measures an optical absorption for a methane concentration. The optical absorption is preferably at a wavelength corresponding to a methane absorption line between 1,630 and 1,700 nm. At the specified wavelength, methane has a very narrow absorbance where there are typically no interfering species to absorb. A high sensitivity optical absorption technique known as wavelength modulation spectroscopy and an enclosed multiple pass optical cell are preferably used to obtain sub-ppm sensitivity.

(21) At higher concentrations, where the optical absorption becomes thick, conventional absorption spectroscopy is preferably used. In this embodiment, the methane is continuously drawn through the multiple pass optical cell with a small pump.

(22) In yet another embodiment of the present disclosure, the light source 1 is preferably a diode laser. Laser characteristics preferably comprise approximately 0.1 to 5 mW output power, approximately 5 to 150 mA diode injection current, and approximately 0.5 to 3 V diode drop.

(23) The at least one laser diode is preferably mounted directly on a miniature thermoelectric cooler, which allows for thermal control while minimizing power consumption.

(24) Multiple pass optical cell or measuring chamber 2 preferably comprises two mirrors A, B configured in a Herriott cell design. This design makes the system insensitive to mechanical vibration. More preferably, the base path of multiple pass optical cell or measuring chamber 2 is approximately 5 to 20 cm and the total fixed length optical path provided by multiple pass optical cell or measuring chamber 2 is approximately 50 to 500 cm. In this embodiment, the volume of the measuring chamber 2 is approximately to 50 ml. The pumping speed through the measuring chamber 2 is approximately 5 to 50 ml/sec.

(25) In an embodiment of the present disclosure, a portion of the laser beam is split off via a partially transmissive reflector (mirror B) prior to entering the first optical detector 3a.

(26) Below concentrations of approximately 1,000 ppm, at least one of the second optical detectors 3b, 3c and wavelength modulation spectroscopy is preferably employed. A high sensitivity is obtained by conducting spectral measurements at a frequency high enough to greatly reduce laser excess noise. The technique is implemented by rapidly modulating the laser wavelength and performing phase sensitive photodetection at a harmonic of the modulation frequency.

(27) Since a diode laser's wavelength tunes with injection current, a small periodic oscillation of the diode laser current results in wavelength modulation. The gas absorption converts the wavelength modulation to an amplitude modulation of the transmitted beam. A relatively slow sweep of the laser wavelength (current) across the spectral region generates a spectrum. The spectrum resulting from nth harmonic detection appears as the nth derivative of the unmodulated absorbance. In the sensor, second harmonic detection is preferably used. Because diode lasers are generally linear in intensity versus wavelength, second harmonic detection has the added benefit of being a zero baseline measurement. The amplitude of the spectral peak is proportional to absorbance and thus, through Beer's law, proportional to concentration. The peak amplitude is also linear with respect to the beam intensity. Thus, signals are normalized by the light intensity.

(28) At gas concentrations between approximately 150,000 ppm and approximately 1,000,000 ppm, the sample becomes optically thick. In this region, a modified form of absorbance spectroscopy is performed.

(29) According to embodiments disclosed herein, the modified form of absorbance spectroscopy is performed by means of at least one second optical detector, wherein the laser is not modulated; however, wherein the at least one laser diode is driven in such a way that it emits light in a frequency spectrum which is tuned to an absorption line of the species (here: methane) to be detected in the gas sample to be analyzed, wherein this absorption line is weaker than the absorption line, onto which the frequency spectrum of the light emitted by the at least one laser diode is in the other operating modes.

(30) In one embodiment of the present disclosure, the device is further provided with a GPS system for allocating a position to the methane spectra evaluated by the device.

(31) User button allows a user to answer yes/no questions regarding instrument options and modes. This embodiment preferably comprises alarms that are controlled by microcontroller preferably include an audible alarm, a vibrating buzzer, an LED, and an audio headset alarm.

(32) An embodiment of the present disclosure comprises a casing around the instrument. The casing is preferably plastic. An inlet tube is preferably used as a sample probe. More preferably a telescoping inlet tube is used as a sample probe Most preferably, a telescoping inlet tube with a 5 micron filter attached near the input is used as a sample probe. The sample probe preferably connects to multiple pass optical cell or measuring chamber 2. In normal leak surveying, the end of the probe is dragged along the ground.

(33) A rigid probe can optionally be attached to the sample probe for detection of gases underground. The rigid probe enables a user to easily detect gases underground. The present disclosure requires only single-handed operation when used in this manner. A clogged filter warning is provided when the inlet pressure drops significantly below ambient pressure.

(34) The disclosure preferably runs using a wireless energy source, such as batteries. More preferably, the present disclosure runs on four AA size rechargeable nickel metal hydride or preferably Lithium Ion batteries.

(35) In one embodiment of the present disclosure, a calibration, preferably a two point calibration, performed by a user in order to establish span and offset factors. In this embodiment, one calibration point is performed on clean air. The other calibration point is performed on approximately 1,000 ppm methane mixture in air or nitrogen. The calibration mixture is introduced into the instrument using a demand flow regulator. A demand flow regulator feeds gas to the system at the pumping speed, thereby keeping the sample pressure from changing. The calibration is performed in both wavelength modulation and normal absorbance modes. Calibration can be performed as little as once a month.

(36) FIG. 2 shows an exemplary embodiment of the measuring chamber 2 of the disclosed device previously discussed with reference to the schematic illustration in FIG. 1.

(37) The portable device for gas leak detection, consists mainly in a “laser sensor”, a pump to sample the gaseous mixture to analyses, a Li-Ion cell, a graphical LCD to show the measure and interact, key+rotating knob, embedded GPS and

(38) BlueTooth and electronic board to manage all the above.

(39) The laser sensor can be divided into an optical part and the measuring electronic.

(40) The principle behind the selective measure of methane is TDLAS, used with both direct absorption and “second derivative—2F” techniques. Laser used is a VCSEL at appropriate wavelength of approximately 1.654 nm.

(41) The optical cell consists in a tube (cell tube) which contains the gaseous mixture to be measured and also provide mechanical support function. The cell is constantly injected with the gas sampled by the pump. At both ends of the tube there are two concave custom built mirror (mirror A, mirror B) which forms a multipass cell that provide an optical length of approximately 3.3 meters.

(42) On one end is located the laser emitter (laser) with specific lens to give optimal focus to the laser beam. On the opposite end are located two photodiodes, one is the “reference” and one is the “measure”.

(43) Reference photodiode (first optical detector 3a) is placed after the first pass of the laser beam, so it's optical path length is the mechanical length of the cell (approximately 13 cm). The mirror on this end (mirror B) is manufactured to have a small quantity of transmission of approximately 1%, so it is possible to have some light hitting the photodiode. Also before the photodiode and after the mirror is present a closed portion of dedicated metallic pipe which forms a cell in which is present a mixture of gas with a relevant amount of CH4 to have a reference absorption line to keep the system locked to the absorption peak.

(44) Measure photodiode (second optical detector) is placed at the end of the multipass optical path.

(45) This configuration determines that is possible to measure with two photodiodes at the same time, one at the long path length and one at the short.

(46) The electronic that manage the laser sensor incorporates what is needed to have the regulation of the laser (thermal stability, laser driving) and to measure in direct absorption and in second derivative. It generates DC bias, ramp and primary RF frequency generation, and acquire photodiode signal in DC (direct absorption) and also through lock-in amplifier for the second derivative measurement.

(47) Due to the optical path length and the required detection limit and full range of measure it's impossible to produce a measure from a single channel that can cover the whole range of measure (0.1 ppm to 1,000,000 ppm) so a strategy to manage those signal was implemented, so it is possible to identify four stage of measure:

(48) LOW RANGE:

(49) up to approximately 1,000 ppm, uses the full optical path and works in second derivative

(50) MID LOW RANGE:

(51) from approximately 300 ppm to approximately 30,000 ppm, uses the full optical path and works in direct absorption

(52) MID HIGH RANGE:

(53) from approximately 20,000 ppm to approximately 200,000 ppm, uses the single pass and works in direct absorption; in this mode is present a constant absorption due to the presence of the reference cell 7, so the calibration need to take into account this aspect

(54) HIGH RANGE:

(55) from approximately 150,000 ppm to approximately 1,000,000 ppm, uses the full optical path and works in direct absorption, but the laser emission is switched to a nearby weaker absorption line of methane

(56) All the switch procedure is done automatically and is seamless for the user.

(57) Although the invention has been described in detail with particular reference to these preferred embodiments, other embodiments can achieve the same results. Variations and modifications of the present invention will be obvious to those skilled in the art and it is intended to cover all such modifications and equivalents.