ITPD20100203A1 - EQUIPMENT FOR MEASURING A GAS CONCENTRATION IN A CLOSED CONTAINER - Google Patents

Description

DESCRIPTION

Technical field

The present invention relates to an apparatus for measuring the concentration, and preferably also the pressure, of a gas arranged inside a closed container, such as for example a bottle, according to the characteristics of claim n. 1.

State of the art

The problem of oxidation is widespread in the production of beverages, such as wines and beers, and there is no complete knowledge of the chemical phenomena in this regard. Knowing the concentration of oxygen present in the container can help oenologists in predicting and studying the aging of wine.

The measurement of the concentration of gas, such as oxygen, inside closed containers such as bottles like those for carbonated drinks, beer, juice and wine is generally carried out by contact, or for example by piercing the cap and introducing it into the bottle. of a measuring device such as an oxygen sensor. This type of measurement, although reliable and precise, has some drawbacks, in fact it destroys the possible seal of the bottle that cannot be put on sale and moreover the procedure is slow and invasive. In many fields of the food industry and in particular in the wine and beverage sector there is therefore the need to have a non-contact measuring equipment for oxygen. In fact, a non-contact measurement can be carried out without altering the contents of the bottle and then repeated at will on the same container; it can also be made fast enough to allow its use in an automatic production line, in order to control the entire production. In the international patent application WO 2008/053507 a method is described for the automatic measurement of pressure and concentration inside sealed containers, the device comprising a laser source that emits a laser beam of a predetermined wavelength towards an optically transparent container and detectors that detect the laser beam which is attenuated by the absorption of the gas. The detectors provide a first representative datum of a first gas absorption spectrum which includes distorted absorption lines and noise. The invention implements a calculation method suitable for receiving and processing this first datum. The output of the operation are parameters that represent a second absorption spectrum that is free from noise and distortion in the absorption lines. From these parameters it is possible to obtain the pressure and the concentration of the gas in the container.

The Applicant has noted that this type of method cannot be used for too high gas pressures.

In â € œVCSEL-based oxygen spectroscopy for structural analysis of pharmaceutical solidsâ € written by T. Svensson et al. posted in Appl. Phys. B 90, 345-354 (2008), a single beam instrument based on tunable laser absorption spectroscopy (TDLAS) and its use in a structural analysis of pharmaceutical solids that are highly dispersive is described. Using a vertical cavity laser (VCSEL) to measure molecular oxygen dispersed in pellets, structural properties such as porosity are measured.

The Applicant has verified that this approach can only be used at a gas pressure substantially equal to the atmospheric one.

The apparatus of the invention is suitable for measuring the pressure and concentration of a gas inside sealed containers such as wine and beverage bottles without opening the container itself. A main purpose of this equipment is the measurement of gas concentrations that fall within a range between 0.1% and 100%, more preferably between 0.1% and 5%. This type of equipment can be used, for example, by oenologists as a diagnosis of the bottling and corking processes and the long-term conservation of bottled wine.

These and other objects are achieved according to the invention with a measuring apparatus whose main characteristics are defined in the attached claims.

Brief description of the drawings

Further characteristics and advantages of the invention will appear from the detailed description that follows, carried out purely by way of non-limiting example, with reference to the attached drawings, in which:

- Figure 1 is a block diagram relating to a detail of an operating phase of the apparatus of the invention, in particular the processing of the WMS signal;

- Figure 2 is a block diagram relating to a further detail of an operating phase of the apparatus of the invention, in particular the digital synchronization of the laser signal modulation and the demodulation of the signal downstream of the photodetector; - Figure 3 is a simplified diagram of a detail of the apparatus according to the invention, in particular of a detail to minimize retroreflections (a) or collimated (b);

- Figure 4 represents a perspective view of a different detail of the apparatus made in accordance with the invention; - Figure 5 represents an exemplary diagram of the apparatus according to the invention;

- Figures 6a and 6b are two perspective views, overall and of a detail on an enlarged scale, respectively, of the apparatus according to the invention;

- figure 7 represents an enlarged view of an element of the apparatus according to the invention;

figure 8 represents a different preferred example of the same detail of figure 7;

- figure 9 is a graph representing a calibration curve used in the apparatus according to the invention;

- Figures 10a and 10b represent the signal processed by the apparatus of the invention in two different operating states.

Preferred examples of embodiment

In figure 6 a total of 100 indicates an apparatus for measuring a concentration of a gas inside a sealed container.

In the attached figures, this container is represented by a bottle 200, however any other type of container can be used, such as cans and the like, in which at least a portion of the material making up the outer casing of the container itself is at least partially optically transparent.

In the example of the bottle, there is generally a liquid in it and between the upper surface of the liquid and the cap there is generally a mixture of gases. In the case of wine bottles it is desirable to calculate the quantity, or the concentration, of oxygen that is above the wine.

Apparatus 100 uses a measurement method based on Tunable Laser Diode Absorption Spectroscopy (TDLAS). The apparatus 100 therefore includes a laser 10, preferably a tunable laser diode, which emits a laser beam with an emission frequency corresponding to the absorption frequency of an atom or a molecule of the target gas (that is, of the gas to be measured the concentration inside the container 200, therefore the emission frequency of the laser beam is adjusted so as to substantially coincide with the absorption frequency of the gas of interest), for example as regards oxygen O2 the emission frequency à ̈ in the region around 760 nm.

In TDLAS technology, since the laser 10 is such that it is possible to modify the wavelength of the beam that is emitted, the laser beam that passes through the container 200 has a wavelength that varies within a determined interval (for example in the case of oxygen this interval is equal to 0.2 nm around 763 nm): the absorption by the gas included in the container is then measured for each distinct wavelength .

The apparatus 100 also includes a seat 90 for housing the container 200 which contains the gas mixture, including the target gas, of which the absorption is measured.

Therefore, the laser 10 according to this preferred example can be for example a vertical cavity laser emitting from a surface (VCSEL), or a laser of the distributed feedback type (DFB). These are single-mode emission lasers with a width of less than 1 GHz that can be tuned by varying the temperature or the junction current, the temperature or the junction current.

In addition to the laser 10, the apparatus 100 includes a detector 30, preferably a photodetector, which receives the laser beam emitted by the laser 10 after it has passed through the container 200. To collect most of the light exiting the sample à In the preferred example shown in Figure 4, a silicon photodiode with a large sensitive surface has been used as detector 30, however other detectors can be used as known to those skilled in the art.

Laser 10 and detector 30 are mounted on a frame 20, for example both supported at opposite ends of a beam 55, in which the positioning of laser 10 and detector 30 is adjustable, for example by means of knobs or other means known per se. , so as to axially move the laser / detector as well as to adjust the distance between them, as well as perform a vertical positioning movement. In fact, depending on the container 200, the positioning of the first measurement channel identified by the laser 10 and the detector 30 so as to correspond to the area in which the gas mixture is present can be different.

A support 95 is also preferably provided on which the container 200 is positioned so as to adjust the height of the same (i.e. the support 95 can be moved axially so as to vary the position along the Z axis of the container 200 ; see figure 6). This support 95 is also preferably rotatable about its own axis to allow as many degrees of freedom as possible in positioning the container 200, for example the support 95 comprises a mandrel. Furthermore, the support 95 and frame 20 are preferably independent of each other, ie there is no mechanical connection between the two elements, so as to be mechanically decoupled.

As shown in figure 3b, the apparatus 100 also includes a control electronics that performs a plurality of functions for the correct operation of the apparatus 100. These functions, some of a known nature in the technical sector of reference and others specific to the reference technical sector. ™ invention, will be better detailed below. By way of example, from the electronics 40 are carried out the scanning, the modulation of the signal emitted by the laser 10, the gain control, the normalization and the demodulation of the optical signal reaching the detector 30, all functions which are preferably totally controlled. via software. Therefore, all the means of calculation for the elaborations detailed below are included in the electronics 40.

As known in the art, when a monochromatic light beam of intensity I (Î », z) enters an absorbing medium, the radiation transmitted through a thickness Z has an intensity equal to

I (Î », z) = I <(> λ <)> â ‹… exp [âˆ'k <(> λ, T <)> â ‹… z] (1) where <k (λ, T)> is a positive constant called the absorption coefficient. According to Beer's law, the absorption coefficient is proportional to the concentration of the absorbent material. In particular:

k (Î », T) = Si <(> T <)> â‹… Ï † <(> Î », T, P <)> â‹… n <(> p, T <)> (2) where <S> i <(T)> is the total absorption per molecule for the i-th transition, <Ï † (Î », T, P)> is the function of the shape of the spectral line and n (p, T) is the density of the absorbent molecules. T and P are respectively the temperature and the pressure, p the partial pressure of the gas of interest.

The density of the gas molecules, which is related to the concentration, is the value to be obtained, given the total absorption per molecule (for example from tables) and the optical path (z).

The spectral line is not a zero-dimensional line, but is a distribution called the "spectrum line". Commonly this distribution is represented as a Lorentzian distribution, that is

<(Î “> / 2 <)>

<Ï †> (<Î », T, P>) <=> (3) Ï € (λ 2 (Î “/ 2) 2)

where Ð “is the amplitude at half height of the maximum, or FWHM, in Hertz.

In a regime of widening of the absorption line mainly due to shocks only (approximation valid at room temperature at pressures above 0.5 bar) the width in Hertz (HWHM) can be written as:

<4N> Ï <2 P> Î “= (4) RTÎ1⁄4Ï €

with Ï <2> cross section, N Avogadro number, P total gas pressure, R universal gas constant, Î1⁄4 mass of the particle and T temperature.

Equation (4) is valid in the approximation of line widening due to shocks alone, valid in the operating regime of the apparatus 100. The widening is here expressed in frequency.

The apparatus 100 also operates in a wavelength modulation regime (WMS) to monitor the weak absorption lines in the spectrum region indicated above, in fact in a possible application of this apparatus the gas absorption signal, for example of oxygen, it is very weak.

The WMS is used to meet the traditional problems of absorption spectroscopy, problems that are due to the need to measure a small variation superimposed on a signal of high intensity such as the signal coming from the absorption of the gas of interest which is particularly weak. compared to background noise. This WMS technique can be implemented in such a way as to be extremely immune to the power and efficiency fluctuations always present in the tunability range provided by laser diodes.

Figure 2 illustrates a block diagram of the circuitry 50 which allows the implementation of the WMS, this circuitry 50 is part of the electronics 40. The measurement in modulation of wavelength is carried out by modulating the current of driving the laser by means of a sinusoidal signal of frequency Î © (for example a few tens of kHz) by means of a modulation buffer 70 with an amplitude such as to cause a maximum shift in the emission wavelength of the laser comparable to the width of the absorption line of interest. A further modulation is superimposed on this modulation with a triangular wave of variable amplitude and frequency of the order of tens of Hz in order to obtain more "brushed" of the same spectral region.

To this end, again with reference to figure 2:

- a conversion card 60 operating in digital to analog D / A mode converts the modulation signal;

- the signal generated by the D / A conversion board 60 is sent to the laser 10 to obtain the aforementioned triangular wave modulation superimposed on a radiofrequency sinusoid;

- the signal received by the photo-detector 20 is digitized by means of a further card 60â € ™ operating in analogue to digital A / D mode synchronously to the previous card by means of a clock 80; cards 60 and 60â € ™ can coincide;

- the digitized signal is averaged, normalized in amplitude and subsequently processed with filters in the manner described below. However, the double modulation of the signal sent to the laser 10 causes the non-linearities of the measurement channel due to absorption to cause a distortion of the modulating signal, increasing its harmonic content. On the receiver side, the signal of the photodetector 30 is filtered around one of the harmonics of the modulation frequency and then detected (Figure 1).

However, by using a conversion of the signal that reaches the photodetector 30 from analog to digital, an advantage is obtained which lies in the possibility of obtaining particularly narrow comb filters thanks to a coherent sampling in the A / D and D / A sections of the instrument (figure 2). The signal acquired by the photodetector 30 and sent to the A / D conversion board is in fact subjected to the following steps, carried out via software:

- a coherent average is carried out (step A in figure 1) between multiple scans (one scan is the input signal in figure 1), which allows a considerable improvement in the signal-to-noise ratio; - a narrow band (supergaussian) filtering is performed (see block B of figure 1) around the second harmonic of the modulation signal, in order to obtain the signal to be demodulated without introducing phase distortions and without altering the shape of the track ; - a demodulation of the signal thus obtained is carried out by translating it into the base band (block C in figure 3), with the application of an adequate phase rotation to compensate for the modulation delays introduced by the laser and current driver assembly;

- a fit of the signal thus obtained is performed (block D), for example by means of a least squares fit or matched filter with reference absorption profiles, depending on the range of operating pressures required by the instrument; the fit performed is typical of the invention and will be better described below;

- and the gas concentration is then measured by analyzing the signal obtained after steps A-D as described below.

This type of signal treatment, i.e. the modulation and demodulation method of the signal, is also known by T. Svensson, M. Andersson, L. Rippe, S. Svanberg, S. Andersson-Engels, J. Johansson, S Folestad, VCSEL-based oxygen spectroscopy for structural analysis of pharmaceutical solids, Appl. Phys. B 90, 345â € “354 (2008) which is incorporated herein. In this article, however, different filters are used than those of the present invention.

In addition, the synchronous averaging operation between several scans as described in step A of the above description carries out a comb filtering of the signal detected by the detector 30, attenuating any disturbing component which is not synchronized with the scanning signal. The type of signal obtained after each step of figure 1 is schematized in figure 1 itself under each corresponding block.

The main problem of TDLAS-WMS methods lies in the interpretation of the acquired data, that is in the interpretation of the demodulated signal, such as for example of the demodulated signal in the way just described in steps A-C of figure 3, an interpretation that generally requires sophisticated algorithms to the simultaneous detection of total pressure and concentration of the species of interest starting from the shape and intensity of the absorption signal. Therefore, having obtained the absorption spectrum through the first channel of the gas whose concentration you want to know, it is extremely complex to obtain both the pressure value and the concentration value from this spectrum at the same time. As known in the art, in the WMS-TDLAS the demodulated waveforms are (in the hypothesis of small modulation depth) an approximation of the n-th derivatives of the absorption profile, where n is the order of the demodulated harmonic ; from measurements of the width of the peaks present in the demodulated waveforms it is possible, after calibration, to trace the pressure of the sample under examination; the concentration is instead obtained from the amplitude of the demodulated signal, known the pressure.

While in direct absorption spectroscopy it is possible to use a simple fit algorithm to relate the acquired signal to a theoretical model of the absorption lines, in the case of WMS the situation is complicated due to the interaction between the line width and the depth of wavelength modulation applied to the laser.

According to the invention, to solve the above problem, two measurement channels are used to obtain the total pressure and concentration values of the gas of interest. The first channel is given by the diode 10 and detector 30, and the second is described below. The peak intensity of the absorption signal of the gas of interest can be obtained from the analysis of the demodulated waveforms described above obtained in the first channel by measuring the peak on the signal or on a version obtained by means of a fit or a matched filter to increase sensitivity in the case of low S / N ratios. The concentration value will then be obtained starting from the peak intensity by applying a calibration curve, phase analyzed below, knowing the pressure (information that is obtained through a second channel and a second absorption spectrum, as will be better described in the sequel). In fact, the calibration curve shows the peak intensity value with which the instrument responds for a known concentration, as the pressure varies. Therefore, given the pressure (calculated as described below), and calculated the peak intensity of the spectrum of the gas whose concentration is to be obtained (through a measurement of the signal or its processing as explained below), from aforementioned calibration curve, the desired concentration value is obtained.

For example, figure 9 shows a calibration curve for a plurality of pressures and peak-to-peak amplitudes of the signal, for an oxygen concentration of 20%. This curve belongs to a family of curves obtained for different concentrations of the gas of interest whose concentration value is to be known (a curve = a specific gas concentration), curves that are saved in a memory of the equipment. Therefore, having obtained the peak value of the signal given by the first channel and the value of the total pressure of the gas mixture, from the two points a (single) curve belonging to the family of aforementioned calibration curves is identified and from this identification the value of concentration of the gas of interest (since a curve corresponds to a specific gas concentration).

According to a preferred feature of the invention, a matched filter is used on the signal arriving at the photodetector 30 (see again block D of Figure 3), which is a linear time invariant filter with the characteristic of maximizing the signal / signal ratio. output noise in the case of an input consisting of a pulse of a known shape. The impulse response that verifies this condition is given by the conjugate complex inverted in the time of the model impulse; in our case the model is represented by the second derivative of the Lorentzian function from the equation (3), this function being the shape of the spectrum line (and it is known that this second derivative is a good approximation of the signal as revealed and filtered by equipment 100). Simplifying equation (3):

Ï † (Î », T, P) 1

to

Î »<2> + 1

5

whereby the second derivative is given by

(Î »4

P) 3 <+> 2Î »2 <−> 1

Ï † Î », T, '' ∠2â‹…. (5)

(Î »2+ 1) 4

This curve is also called the â € œsecond harmonicâ €.

It is also possible to implement a more accurate model that takes into account the non-ideality due to the finite depth of modulation. This model will not consist of the nth analytical derivative of the Lorenzian function, but will be obtained by filtering the absorption profile with an impulse response suitable for modeling the effect of the interaction between the modulated source and the absorption line.

This last type of matched filter is described in detail in

R. Arndt, Analytical lineshapes for lorentzian signals broadened by modulation, J. Appl. Phys., 36, 2522-2524 (1965)

And

J. Reid, D. Labrie, Second harmonic detection with tunable diode lasers - comparison of experiments and theory, Appl. Phys. B, 26, 203-210 (1981), which are incorporated herein.

In fig. 10a shows the superposition of the signals received by the detector 30 of the first channel and demodulated as described by thirty-eight different measures (in the abscissa are indicated arbitrary units proportional to the wavelength, in ordinate arbitrary units proportional to the intensity of the second harmonic signal , normalized with respect to the RMS amplitude of the input signal): in fig. 10b shows the signals of the same measurements processed with the matched filter in which there is a considerable reduction in noise.

Thanks to the digital modulation-demodulation techniques to improve the signal-to-noise ratio described above, the measurement is also possible in the presence of absorbances lower than 3 x10 <-6>.

By means of the preferred characteristics of the invention, ie a digital management of the signal obtained from the detector 30, as well as the presence of the matched filter, the â € œnoiseâ € components in the detected signal are minimized. However, as aforesaid in the measurements using TDLAS, the measurement of total pressure and concentration starting from the `` similar-derivative '' form of the absorption profile (see equation (4)) of a gas (in this case the gas of which you want to know the concentration) can be very difficult in the case of small concentrations (eg below 5%, typical of the oxygen concentration in wine bottles), due to the shortness of the optical path. To improve the operation at these regimes, in the apparatus of the invention 100 a second measurement channel has been added (see figure 6 and figure 5) based on direct absorption TDLAS with a second laser source 110, for example a VCSEL laser, and a second detector 130, also preferably a photodiode.

The second channel, for example, can consist of a VCSEL operating around 2 Î1⁄4m for the detection of carbon dioxide with an InGaAs extended photodiode.

This second measurement channel can be configured in an orthogonal direction with respect to the first, ie laser 110 and photodiode 130 are arranged substantially at 90 ° with respect to the direction defined by the first laser 10 and photodiode 30. However, any other configuration is included in the present invention. Obviously, the laser 110 and the photodiode 130 are arranged in correspondence with the frame 20 and the container in such a way as to cause the laser beam emitted by the laser 110 to cross the gas mixture in the container 200 containing the gas whose concentration is to be known.

A channel thus defined is used for measuring the total pressure of the gaseous mixture present in the container 200 which must be used for calculating the concentration of the gas of interest.

This measurement of the total pressure, to be used together with the calibration curve and the intensity of the peak of the first spectrum, is obtained starting from the absorption profile of another gas (e.g. CO2 or H2O) contained in the gas mixture present. in container 200. For example, a measurement method for calculating the pressure starting from the absorption profile of a second gas is described in the international patent application WO 2008/053507 (i.e. the measurement method for obtaining the value of the total pressure of the gas is that described in this patent application). This method can be used for example in the present invention, however any other TDLAS spectroscopy method can be used.

In figure 5 the two channels are schematized, the first channel emitting a laser beam F and the second channel emitting the second laser beam Fâ € ™ with detectors 30,130 for the acquisition of the two absorption spectra.

The total pressure is then detected by the analysis of the CO2 gaseous absorption spectrum present in the mixture, or other gas as mentioned (another gas of interest can be for example water vapor), measurement obtained through the second channel (the measurement of the absorption spectrum of the second gas is preferably carried out as described in patent application WO 2008/053507).

This second part of the instrument may constitute a separate device and not connected with the first, however the incorporation in the same frame 20 is the preferred arrangement. In this operating mode, the total pressure is obtained by measuring the width of some absorption lines of the second gas contained in the mixture (e.g. CO2) and the value is used to measure the concentration of the gas of interest (e.g. O2) by means of the application of the calibration curve to the intensity value of the absorption characteristic of the gas of interest.

The calibration curve (example in fig. 7), obtained starting from a series of measurements carried out in the presence of fixed concentrations and known pressures, is useful to compensate for the variation in the response of the instrument as a function of pressure. This curve is obtained during the calibration phase, with various measurements carried out as the pressure varies by injecting the gas of interest (for example oxygen) at a constant concentration into a test container equipped with a traditional pressure gauge. A curve for each concentration is subsequently recorded permanently in apparatus 100, so as to obtain a family of recorded curves.

It is important to note that the peak absorbance of the most intense lines in the 763 nm band for 0.2% of O2 contained in a typical bottle neck (typical container used in the present invention) is about 1.7x10 <-6>, limit of detection of the equipment.

According to a particular preferred feature of the invention, in order to further lower the noise of the signal obtained through the first channel, the apparatus 100 comprises at least one gasket 80 to be positioned as better described below so as to be crossed by the laser beam which is emitted and propagates between the laser 10 and the container 200. Preferably, a further gasket 80a is provided (or alternatively only this second gasket is present) positioned in such a way as to intercept the laser beam after it has passed through the container 200 until it reaches the detector 30. This configuration, that is the introduction of at least one gasket 80 / 80a, is made to avoid the introduction of air coming from the outside of the container and to reduce parasitic reflections and phenomena of resonant cavity (etalon effect) which cause noise in the signal reaching the photodetector 30. In particular, according to a preferred configuration, airtight optical contacts are made between the apparatus 100 and the container 200 thanks to two deformable gaskets 80,80a in transparent silicone rubber (Figure 4).

Figure 4 shows the elastomer seals 80,80a on an enlarged scale. In particular, figure 7 shows the gaskets for laser 10 and photodiode 30 suitable for an optical geometry of free expansion in contact with Bordeaux-type bottles (they are the same gaskets used in figure 4 assembled in the apparatus 100). The squares visible in the background have a side equal to 5 mm.

In fact, the geometry of each gasket 80 is modified in such a way as to allow a complete coupling of shape between the external surface of the laser 10 and the external surface of the container 200 on which it is placed. The purpose of the gasket is precisely to minimize the optical disturbance caused by gases external to the gas mixture present inside the container 200 along the optical path of the laser beam. Therefore there must be the best possible contact, and therefore also a complementarity between the surface of the laser / container and the corresponding surface of the gasket.

Similarly, this correspondence is present in the gasket 80a interposed between the photodiode 30 and the container 200, a gasket suitable for intercepting the laser beam exiting the container along its entire optical path.

The materials used for making the gaskets 80, 80a are preferably optically transparent rubbers, for example:

Prochima, Cristal Rubber (two-component transparent silicone rubber with polyaddition, vulcanization at room temperature, refractive index about 1.4);

Dow Corning, JCR6122 (two-component clear silicone rubber, high transparency, refractive index 1.4, vulcanization at 150 ° C).

According to a further preferred example of Figure 8, the gasket 80 comprises a spherical collimation lens (not visible), for collimating the laser beam coming out of the laser 10, for example an 8 mm diameter sapphire glass lens suspended in the elastomer. The squares visible in the background have a side equal to 5 mm.

Alternatively, a collimation lens is included inside the packaging of the laser 10 itself, so as to emit a collimated laser beam. The use of these materials combined with glass optics with a high refractive index (eg sapphire) allows to obtain collimated measurement beams and elastic and low retro-reflection instrument-sample couplings. In fact, through the gaskets 80,80a the retro-reflections introduced by the container are reduced as an adaptation of the refractive index to the equipment-container interfaces is achieved, ie avoiding strong index jumps. For example, if the bottle is made of glass, a refractive index approximately equal to 1.4 is selected, therefore close to that of the glass of the bottle, the laser exit window and the protective cover of the sensor. Preferably, given the refractive index of the container 200 inside which the gas to be examined is present, the refractive index of the gasket is equal to that of the container ± 0.1. Avoiding retro reflections implies reducing the etalon effect which strongly limits the sensitivity of the equipment. In fact, it is known that in a collimated laser beam the reflections and scattering are present in the device substantially at each interface. The various rays belonging to the same starting beam but which have been reflected differently have different phases from each other due to their different optical path and therefore involve interference phenomena causing therefore a variation in intensity of the signal detected by the photodiode .

Alternatively, again to minimize the retro-reflections, it is possible to avoid collimating the laser beam: the difference between the beams of the beam in a device 100 in which the laser beam F is not collimated and one in which the beam F is collimated It is represented in figures 3a and 3b respectively. In the first case 3a the optical emission of the laser and the retro reflections of the first surface of the container and of those of the photo-detector are illustrated. In the second case (b) the collimated configuration is illustrated which, if applied without the precautions described above, would give rise to external resonant cavities and therefore disturbances of the etalon type.

Furthermore, again by means of the gaskets 80,80a, a substantial exclusion of the external air from the optical path of measurement is achieved, which is very useful in the case of application when there are very low concentrations of gas to be measured at the inside the container 200. For example, the oxygen concentrations commonly present inside the bottles are generally included in the range from 1% (or lower) up to 20%, have absorbance values comparable with the uncertainty introduced by the irregularities of shape of the external surface of the container under examination. It should also be remembered that in the case of low concentrations and high pressures inside the container 200, the separation of the two contributions of the internal-external gas would be particularly difficult due to the superimposition of two absorption profiles characterized by a different line width.

A possible alternative to eliminate the contribution due to gas, such as oxygen, outside the container is to insert the container and optoelectronics in a sealed chamber filled with nitrogen, instead of using the 80 / 80a gaskets.

With the aim of further reducing the residual interferential effects present in the optical path (which are generally of an intensity even greater than the weak useful absorption signal) a small vibrating motor has been fixed to the laser-detector mounting frame of the first channel ( not shown) for example an engine equipped with an eccentric flywheel which has proved useful for the destruction of the geometric coherence on successive averages of the optical paths due to parasitic reflections. The motor is fixed, for example, to the upper beam 55 which supports the laser 10 and the detector 30, the beam which is therefore subject to vibration and which induces a vibration in the first channel, while the container 200 is stationary in the support 95 and is contact with the frame 20 exclusively through the seals 80,80a. In this way, in fact, the difference in optical path is mechanically modulated and when the spectrum is mediated, the etalon effect is reduced to almost zero.

In this preferred example in which such a motor which gives a vibration is present in the apparatus 200, the gaskets 80,80a additionally have the function of efficiently coupling the vibration of the vibrating motor to the first channel, or of the motor to the optical scheme at the cavities formed by the different optical surfaces. The gaskets are also easily replaceable in case of need.

The variable attenuation of the optical signal introduced by the container 200 does not affect the TDLAS measurement in the first channel since the intensity of the amplitude modulation imposed on the optical signal is used as a normalization reference. The apparatus 100 in fact performs a normalization of the measurement using the root mean square value of the amplitude (RMS) of the signal acquired by the A / D section (signal of the detector 30 transformed into digital, as previously mentioned, see figure 2) . There is also a software programmable gain control to exploit the entire dynamics of the A / D converter (fig. 2) regardless of the type of container 200 to be measured. This is an automatic gain control carried out by a digital switch (not displayed) which varies the transimpedance of the amplifier stage of the photodiode 30 in order to keep the amplitude of the acquired signal within predetermined limits to avoid saturation (in the case of excessive gain) o low signal to noise ratio (in case of insufficient gain). The coupling of the signal received by the photodiode 30 to the subsequent amplification and acquisition stages is carried out in AC since the strong base band component of the signal is not of interest for the measurement (which uses only the first and second components harmonic 1f and 2f) and would limit the resolution in digitization. The optical signal at the level of the photodiode 30 is in fact constituted by a strong DC component due to the power emitted by the laser 10 and by a triangular fluctuation at the scanning frequency due to the waveform that scans in wavelength by modulation of the laser current. These two components, of high intensity with respect to the sinusoidal modulation, are not of interest for the WMS measurement.

The developed equipment proved to be capable of operating even in situations characterized by an extremely low signal-to-noise ratio, typical of dark glass bottles; A high level of immunity was also obtained with respect to the disturbances introduced by ambient lighting.

Claims (10)

CLAIMS 1. An apparatus (100) for measuring the concentration of a gas inside an optically transparent sealed container (200) for at least a portion thereof, said container (200) containing a gas mixture including said gas to be measured, said apparatus comprising: - an adjustable laser source (10) capable of emitting a laser beam (F) at a given wavelength, said source being positioned in such a way that said laser beam passes through the optically transparent portion of the container (200) which contains the mixture including the gas whose concentration is to be measured; - a detector (30) suitable for detecting the laser beam (F) which is attenuated by the absorption of the gas whose concentration is to be measured; said detector (30) emitting as output a datum representing an absorption spectrum of the gas whose concentration is to be measured; said laser source (10) and said detector (30) forming a first measurement channel; characterized by understanding further - a second laser source (110) able to emit a second laser beam (Fâ € ™) at a given wavelength, said second source being positioned in such a way that said second laser beam crosses the optically transparent portion of the container ( 200) which contains said gas mixture; - a second detector (130) adapted to detect the second laser beam (Fâ € ™) which is attenuated by the absorption of a second gas in said mixture; said second detector (130) emitting as an output a datum representing a second absorption spectrum of said second gas; said second laser source (110) and said second detector (130) forming a second measuring channel; - a calculation device (40) adapted to use parameters of said first and said second absorption spectrum for calculating the concentration of said gas to be measured. The apparatus (100) according to claim 1, wherein said laser source (10) is an adjustable laser source for the implementation of a TDLAS tunable laser diode absorption spectroscopy measurement and a modulation signal of the WMS wavelength is sent to said laser source (10) so as to obtain a modulated laser beam. The apparatus (100) according to claim 2, wherein said modulation signal to said laser source (10) is analog and the corresponding signal coming from said detector (30) is transformed into a digital signal. 4. The apparatus (100) according to one or more of the preceding claims, including a gasket (80; 80a) arranged between said laser source (10) and said container (200) and / or between said container (200) and said detector (30) so as to intercept said laser beam (F) in its entire optical path between said laser source (10) and said container (200) and / or between said container (200) and said detector (30). The apparatus (100) according to claim 4, wherein said gasket (80) includes a converging lens for converging the laser beam (F) before entering said container (200). 6. The apparatus according to claim 4 or 5, in which the refractive index of the gasket (80.80a) is equal to the refractive index of the material in which said container (200) is made ± 0.1. 7. The apparatus (100) according to one or more of the preceding claims, in which said calculation device (40) is able to calculate the pressure of said gas mixture from the parameters of the second spectrum acquired by said second measurement channel present in said container (200), and, using said pressure value, obtain said concentration of the gas whose concentration is to be measured, from the parameters of the spectrum of said gas whose concentration is to be measured. 8. The apparatus (100) according to claim 7, wherein said calculation device (409 obtains the value of said concentration by obtaining said total pressure of the mixture from the parameters of the second absorption spectrum and calculating the intensity of the peak of the absorption line of the spectrum of said gas of which the concentration carried out by said first channel is to be measured. 9. The apparatus according to claim 7 or 8, wherein said calculation device (40) includes a memory in which a plurality of calibration curves are stored, each curve matches for a specific gas concentration value the concentration of which is to be measured the values of the peak of the absorption line of said first absorption spectrum with the values of the total pressure of said gas mixture contained in said container (200). 10. Apparatus (100) according to one or more of the preceding claims, including a motor for vibrating said apparatus (100).