The state of the art includes JP-A1 and HP -H09-145677-U H05-84016 documents.

A gas chemiluminescence analyzer generally comprises a reaction chamber configured to be supplied with gas in which a luminescence phenomenon generating photons with photon detecting means issued in said chamber occurs.

In the case where the gas to be analyzed is a sample comprising NOx, that is to say essentially of carbon monoxide, nitrogen dioxide (NO, NO2), the reaction chamber is supplied with the sample and ozone present in excess with a generally internal ozone generator to the device. This produces a redox reaction in the reaction chamber between the ozone (03) and nitrogen monoxide (NO), that generates the emission by chemiluminescence reaction of photons. The reaction chamber is maintained at a pressure below atmospheric pressure to obtain a good yield of the chemiluminescent reaction. With a detector, measuring the quantity of photons emitted in the chamber for determining the NO concentration in the sample,

Ozone reacts with NO but not NO2. The reaction chamber can be supplied with a sample comprising NOx (NO + NO2) where only the NO chemiluminescence react thereby determine the concentration. To determine the fraction of NO2 in the sample comprising NOx, NO2 must first be converted to NO before being measured. The difference between the amount of NOx (NO + NO2) detected and the amount of NO only detected, allows to obtain the sample NO2 concentration.

The photon detecting means comprises, e.g., a photomultiplier which is a commonly used device. In general, it comprises a substantially cylindrical glass wall whose longitudinal ends are closed (vacuum tube) at a first end by a circular glass wall forming a photon input, and at the opposite end by another circular wall glass crossed by electrical terminals. Inside the photomultiplier are mounted a photocathode in the vicinity of the inlet wall, an anode adjacent to the opposite wall, and the dynodes between the photocathode and the anode. The operation of a photomultiplier is also well known in the art.

The photomultiplier of the connecting terminals are generally intended to cooperate by interlocking with complementary terminals of an electrical connection socket mounted coaxially in the extension of the photomultiplier.

The measurement accuracy of a gas by a photomultiplier depends on the surrounding temperature. The more the temperature is high around the photomultiplier and spurious noise disturbs the measurement and causes an inaccuracy in the measurement results. This is called the noise "dark current", which must be minimized. In the absence of light or photons in the reaction chamber, the dark current generated by the photomultiplier should be as low as possible and stability (to reduce the thermionic component). It is well known to cool a photomultiplier in a gas analyzer to optimize the accuracy of the analysis.

In the prior art, the photomultiplier of a gas analyzer is cooled by means of cooling Peltier, also well known to the skilled person. The Peltier effect is a physical phenomenon of heat removal in the presence of an electric current. The effect occurs in conductive materials of different natures linked by connections (contacts). One of the junctions is then cooled slightly, while the other heats. A Peltier device therefore has generally a superposed structure or sandwich having one or more stages, each comprising a cooled plate junctions which cool, and a heated plate junctions that warm.

The photomultiplier having an elongated shape, it is known to mount the coaxially within a tube which is fixed to the aforementioned base. The photomultiplier is thus fixed in cantilever through the base, inside the tube. The tube extends at a distance around the photomultiplier. The photomultiplier tube and are thus separated from each other by an air layer.

The cooling means is configured to cool the tube. The Peltier device is mounted on the tube, for example at one end of the tube and cooled by conduction tube, the tube being formed in a thermally conductive material.

The analyzer includes a housing which defines a housing chamber of the photomultiplier tube and cooling means. This enclosure is isolated from the reaction chamber as the reaction gas must not enter the enclosure to not disturb the analysis. The enclosure contains air.

The photomultiplier is cooled by convection in the current state of the art. The tube, which is cooled by the cooling means cools the air contained in the enclosure and around the photomultiplier, resulting in a cooling of the latter. This type of analyzer has drawbacks. First, the cooling of the photomultiplier is not optimal. So that the photomultiplier is cooled to a low temperature such as 0 ° C, for example, must be supplied with the cooling means with a high electrical power, typically greater than 40W. To avoid consuming much power, today we tend to limit cooling of the photomultiplier to 10 ° C, resulting in a power consumption of about 30-40W. However, the measurement accuracy can be affected by this. Another drawback is that the cooling takes place by convection, which is not optimal in terms of heat exchange. Furthermore, the housing having the photomultiplier is also cooled by convection, which is not useful and consumes power unnecessarily.

In JP-A1 145677 -H09, air in the chamber is replaced with a dried gas. However, the dried gas surrounding the photomultiplier which is therefore not cooled effectively by the cooling means. Indeed, although the latter are made of a thermally conductive material, the photomultiplier is spaced from the cooling means and separated from these latter by a layer of dried gas.

Furthermore, although JP-U-84016 H05 provides accommodating the photomultiplier in a chamber evacuated using a vacuum pump, the enclosure pollution may occur when suction is interrupted for example because of a power outage of the pump. External gas may indeed enter the enclosure through its suction port.

Another drawback is the photomultiplier assembly. The mount cantilever is not accurate and can lead to misalignments and microdisplacements photomultiplier inside the tube. The tube is made of glass, these finite ribs are not precise and may vary. This is for example the case of its outer diameter which can vary from tens of millimeters. This does not facilitate an accurate and repeatable assembly of the photomultiplier into the analyzer.

The present invention provides a simple, efficient and economical to at least part of the problems mentioned above.

DISCLOSURE OF INVENTION

The present invention thus provides a gas analyzer including:

- a reaction chamber configured to be supplied with gas and for generating photons by chemiluminescence reaction,

- photon detection means output in said chamber, comprising a photomultiplier elongated

- a supporting said photomultiplier tube, said photomultiplier being coaxially mounted within said tube,

- means for cooling said tube, preferably by the Peltier effect, and

- a housing defining a housing chamber at least a part of the photomultiplier, said tube and said cooling means, said chamber being isolated from said reaction chamber,

characterized in that it further comprises means for evacuating at least part of said enclosure, said vacuum means comprising a first air discharge port contained within said enclosure and a second inlet port air in said enclosure, and in that said tube is tightly mounted on said photomultiplier so that the latter is conduction cooled by said cooling means.

Unlike the prior art, the photomultiplier is cooled by conduction, which is more efficient heat exchange. The chamber is evacuated (at least part), there is no air (or not enough air) for a photomultiplier convection cooling. The fact that the tube encloses the photomultiplier used to allow cooling of the photomultiplier tube by conduction. The partial vacuum acts as thermal insulation around the pipe and avoids undesired heat exchange between the cooling means and analyzer parts.

In a particular embodiment of the invention, it is possible to cool the photomultiplier (supplied with 12v) of an analyzer (supplied with 24v) to 0 ° C with a supply power of the cooling means of order of 10W.

The analyzer according to the invention may comprise one or more of the following characteristics, taken in isolation from each other or in combination with each other:

- said tube is tightly mounted on said photomultiplier by means of a thermally conductive layer interposed between said photo-multiplier and said tube; the tube is thus mounted indirectly on the photomultiplier through the heat conductive layer,

- the heat conductive layer is elastically deformable; this allows to compensate for manufacturing tolerances of the photomultiplier of the glass tube;

- the heat conductive layer is electrically insulating, and is for example made of silicone,

- said tube comprises a longitudinal slot extending throughout its length, said tube having a diameter which can be increased by moving the longitudinal edges of the tube defining said slot; this may help facilitate insertion of the said layer between the tube and the photomultiplier,

- vacuum means are configured to bring said chamber to a partial vacuum between 10 and 500mbar, preferably between 50 and 300mbar, more preferably between 100 and 200 mbar,

- said photo-multiplier comprises a first longitudinal end for receiving photons and a second opposite longitudinal end for connection to an electrical connecting socket,

- said tube extends longitudinally beyond said first end of said photomultiplier and is longitudinal withdrawal of said second end of said photomultiplier; the tube thus extends around the first input of the photomultiplier, which is particularly advantageous for directly cooling the end in which is mounted generally the photocathode,

- a window or a filter (e.g., red) for transmitting photons is mounted at a first longitudinal end of the tube located on the side of said first end of said photomultiplier,

- the window or the tube filter closes one end of the tube; this allows to better control the temperature of the entry of the photomultiplier, which is more uniform between its center and its periphery,

- the window or of the filter tube is held in position at this end by means of a ring, for example made of soft or semirigid plastic material,

- said tube comprises a second longitudinal end fixed by spacers of predetermined length to said base,

- said chamber is defined longitudinally by two substantially parallel walls, respectively front and rear, said front wall being situated on the side of said first end of the tube and spaced from said first end,

- said front wall comprises a filter (for example red) photons or a transmission window from said reaction chamber; folder or window is preferably spaced from the photomultiplier and in particular of its input, so that the latter is not directly in contact with the cold,

- said filter or said window of the front wall closes one end of the reaction chamber,

- said rear wall is on the side of said base and spaced from said base; This allows the above electrical connection terminals to be remote from the cold pipe and thus reduce the risk of condensation at the terminals, the condensation in this region can generate arcing and destruction of the photomultiplier,

- said chamber is defined laterally by side walls, said tube being supported by a single of these walls through said cooling means,

- said cooling means comprises a plate forming a monobloc assembly with said tube, and at least one Peltier device in single or multiple stage mounted on said plate; the Peltier effect device may be a stage (two plates), two floors (three plates), or more,

- said Peltier device is interposed between said plate and said tube holder wall and is clamped together by means of screws or struts extending between the plate and the support wall,

- a thermally conductive layer, for example in the form of paste, is interposed between the Peltier device is said plate and the Peltier device and said support wall,

- said support tube wall is a removable housing cover,

- said cover is formed integrally with the cooling fins, - said housing or said housing has a general shape substantially parallelepiped,

- said housing comprises orifices traversed with play by electric cables, said sets being filled by sealing means, such as for example epoxy resin; the cable can thus be permanently fixed in the orifices,

- said casing is housed in a cavity defined by the foam blocks; This is advantageous because it provides thermal insulation housing; heat diffusion of hot parts, such as the reaction chamber, to the cold parts, such as blades, is so limited,

- said foam blocks define a flow passage of a ventilation air flow over at least a portion of said housing, which is generated by a fan, - said fins are located in said stream,

- the assembly comprising the casing and the foam blocks can be mounted in a chassis of an analysis module intended to be mounted, along with other similar modules in a cabinet,

- the photomultiplier is fully accommodated in the chamber, and preferably also the connection base; this allows to avoid sealing problems of the housing, in particular in an area which is likely to be traversed by the photomultiplier; in this zone, sealing means are mounted around the photomultiplier and would be subject to high stresses due to the

pressure difference between the interior of the housing (vacuum) and the outside (ambient); in the latter case, the photomultiplier may be pushed by "syringe effect" inside the enclosure, and strongly urging the above sealing means,

- the reaction chamber includes heating means, for example beyond 50 ° C and preferably around 60 ° C; This can prevent a condensation on the window or folder of the reaction chamber,

- the reaction chamber may also be connected to said vacuum means, for example to supply the chamber with the reaction gas; the vacuum means may thus be local authority,

- said tube extends substantially around said photomultiplier,

- said photomultiplier comprises a photocathode, and in that said tube extends at least partially around the photocathode,

- said tube extends around at least one longitudinal half of said photomultiplier.

The present invention also relates to a gas analysis method using an analyzer as described above, characterized in that it comprises the steps of placing said chamber under partial vacuum:

- supplying said chamber with an air flow Q1, and

- a suction air flow Q2 of said enclosure, Q2 being at least equal to Q1 so that Q1 air flow is non-zero and the pressure in said chamber less than atmospheric pressure, and e.g. of about 100-200mPa.

thus provided, not only to suck the air in the enclosure, but also supplying the enclosure with air. The suction of the air and the enclosure of the air supply are such that the latter is maintained at a partial vacuum and regulated to a predetermined pressure, for example of the order of 100-200mbar. This allows to always have a positive suction flow and limit the risk of gases such as air or ozone entering the enclosure, in particular in case of power failure. In such a case of power failure, the suction is stopped and a check valve can advantageously maintain the vacuum chamber without the risk of passage of air from outside to inside the analyzer of the enclosure.

Preferably, said reaction chamber is heated to a temperature, for example beyond 50 ° C and preferably around 60 ° C, and said cooling means is supplied to a power less than 15 W to maintain said tube at a temperature of about 0 ° C.

The reaction chamber can be supplied with nitrogen oxide (NOx) and ozone. DESCRIPTION OF FIGURES

The invention will be better understood and other details, features and advantages of the invention will become apparent from reading the following description given by way of example and with reference to the accompanying drawings, wherein:

- Figure 1 is a highly diagrammatic view of a gas analysis device;

- Figure 2 is a schematic perspective view of a gas analyzer according to one embodiment of the invention;

- Figure 3 is a schematic longitudinal sectional view of the gas analyzer of Figure 2;

- Figure 4 is a schematic cross-sectional view of the gas analyzer of Figure 2;

- Figure 5 is a schematic exploded perspective view of the gas analyzer of Figure 2;

- Figure 6 is another schematic longitudinal sectional view of the gas analyzer of Figure 2;

- Figures 7 to 9 are diagrammatic perspective views of the gas analyzer of Figure 2, including protection of foam blocks;

- Figures 10a-10c are very schematic views of alternative embodiments of the gas analyzer of the invention.

DETAILED DESCRIPTION

Referring first to Figure 1 which shows very schematically the modules of a device 10 for gas analysis, which is a NOx analysis device in the example shown.

The complete device 10 may be in the form of a cabinet in which are stacked and stored modules. The device 10 comprises a first analysis module 12 schematically represented by a rectangle in dotted lines and which will be described in more detail with reference to Figures 2 et seq.

The analysis module 12 includes a not shown frame in which is mounted an analyzer 13 (Figure 2) which essentially comprises a reaction chamber 14 configured to be supplied with gas and to generate photons 18 by chemiluminescence, and means 16 for detecting the photons emitted in the chamber 14.

The device 10 further comprises means 20 for gas discharge, such as a pump, means 22 for generating ozone (O3), and feed means 24 in particular in sample gas and standard gas.

The means 22 ozone production are to produce ozone from oxygen in the air. They include 22a of the air drying means and an ozone generator 22b whose output is connected directly or indirectly (e.g., via a filter pump 22c), to a gas inlet of the reaction chamber 14 analyzer.

The supply means 24 are associated with 26 three-way valves. The device 10 comprises means 28 for converting NO2 contained in a gas, for example sample, NO. The circuit, known to those skilled in the art, is designed so that only the NO contained in the sample feeds the reaction chamber 14 and is analyzed, or NOx (NO and NO2 converted into NO) feeding the reaction chamber

14 and are analyzed. The supply means 24 comprise a gas outlet connected to another gas inlet to the reaction chamber 14.

Figures 2 to 9 show an embodiment of an analyzer 13 according to the invention, that is shown without its blocks of insulating foam 15 in Figures 2 to 6 and with foam blocks 15 in Figures 7 to 9. The analyzer 13 and the foam blocks

15 are intended to be housed in said frame, which is generally formed of a metal transport cage and assembly of the analyzer in the cabinet mentioned in the foregoing.

Referring first to Figures 2 to 6. The analyzer 13 has a general shape substantially parallelepipedal. It generally comprises two parts, as discussed in the foregoing, namely a reaction chamber 14 and means 16 for detecting the photons emitted in the chamber 14.

The chamber 14 is here formed in a block 30 of material, for example aluminum coated with a protective layer of gold. The chamber 14 (Figure 3) has a generally cylindrical shape and opens at one longitudinal end with a circular section on a mounting face 32 of the block. A window 34 transparent to photons or a filter, such as red, is applied to this face 32 and seals the chamber 14. In the position of use and as seen in the drawings, the face 32 and window 34 are substantially vertical (Figure 3).

The gas inlets, namely ozone and sample are in fluid communication with the internal passages 35 of the block 30, which open into the chamber 14, preferably coaxially. The reaction chamber 14 is further connected to a gas outlet, which allows to evacuate the gas contained in the chamber after the reaction, for example using the means 20 of Figure 1.

The block 30 may be equipped with several devices such as sensors, for example for measuring the temperature in the chamber 14. It can also be equipped with heating means 36. The reaction chamber 14 is preferably heated to a temperature 60 ° C in use.

The detection means 16 comprise a substantially rectangular housing 38 which houses a photomultiplier 40. The housing 38 includes a front wall 38a on the side of the reaction chamber 14, a rear wall 38b on the opposite side, and side walls 38c, four in number, respectively top, bottom, right lateral and left lateral. The front and rear walls 38a, 38b are substantially parallel and here are removable. They are fixed to the rest of the housing, namely the side walls 38c by screws 42.

The front wall 38a has an opening 41 substantially at its middle, passage of the photons from the reaction chamber 14 and the window 34 to a chamber 43 defined by the housing walls 38. The front wall 38a is pressed against the mounting face of the block 14, either directly or via a washer and / or window 34, and holds the window 34 against this face. Wall 38a and the window 34 are sandwiched and clamped between the block 30 and the front peripheral edge of the side walls 38c of the housing, by means of screws 42. The opening 41 has a substantially circular shape whose diameter is substantially equal to that of the chamber 14.

Sealing means, such as seals, are advantageously mounted between the front wall 38a and rear 38b and the peripheral front and rear edges of the side walls 38c.

As best seen in Figure 3, the side walls 38c of the housing 38 are formed integrally, for example from an extruded tubular profile. The top wall is recessed and receives on its outer surface a removable cover 46 (Figure 4). The lid 46 is attached to the housing 38 by means of screws 48 and sealing means 44, such as a gasket, are advantageously mounted between the cover 46 and the upper wall of housing 38.

The cover 46 supports cooling means or heat exchange, which are here formed by a series of fins 52 projecting from an outer surface of the cover. The fins 52 may be formed in one piece with the cover 46. The fins 52 are substantially disposed on a front half of the cover. The remainder of the cover comprises apertures 50, 54 and an opening 56.

The orifices 50 are two in number and include a hole 50a with a first discharge port or suction of air contained in the enclosure, and a hole 50b with a second port of intake air in the enclosure. These ports are shown schematically in Figure 1. As will be explained in more detail in the following, air, preferably pre-dried by a dryer 58, supplies the chamber 50b via the second port, and the air from the enclosure is drawn for example by pump discharge means 20 via the first 50a port. Alternatively, a dedicated pump can be connected to 50a port. A check valve is advantageously provided between the port and the pump 50a to prevent the introduction of air into the enclosure via the port 50a in the event of pump failure or disconnection of its power supply. Indeed, the presence of air cooled at the areas in the enclosure might cause condensation.

The orifices 54 are two in number and are tapped to receive the tubular screw 60 intended to be traversed axially by electric cables or conductors, for example for the supply of temperature sensors.

Opening 56 is here closed by a removable closure plate 61, fixed by screws 62 on the cover 46. Sealing means, such as a gasket, are advantageously mounted between the cover 46 and the plate 61. This plate comprises a passage 63 intended to be traversed by a bundle of cables or electrical conductors, for example for the power supply of the photomultiplier 40. As shown in Figure 9, when mounting the cables and bushings of the passage 63, the latter is blocked by means of a sealing resin 65, for example epoxy based.

The photomultiplier 40 has an elongated shape. As mentioned in the above, it is usually formed by a glass tube and is therefore relatively fragile. The photomultiplier 40 is here mounted within the enclosure 43 so as to be at a distance from all walls of the housing 38 (Figures 3 and 4).

The photomultiplier 40 and the opening 41 of the front wall 38a are aligned. The front end of the photomultiplier 40 comprising the photocathode is located at an axial distance from the plate 38a, for example a few millimeters or a few centimeters. The rear end of the photomultiplier 40 comprises pins or electrical connection terminals to a base 64 for electrical connection. The base 64 has a generally cylindrical shape and extends rearwards in the extension of the photomultiplier 40. The rear end of the base 64 is located at an axial distance from the rear wall 38b, for example a few millimeters or even few centimeters.

The side walls 38c of the housing 38 are located away from the photomultiplier, for example a few millimeters or a few centimeters.

The photomultiplier 40 is held in place in the housing 38 by means of a support tube 66. The tube 66 comprises a cylindrical wall and is coaxially traversed by the photomultiplier 40. The tube wall has for example a thickness of between 0, 5-1 mm. The tube 66 is preferably made of copper.

In the example shown, the tube 66 has a length similar to that of the photomultiplier. However, the front end of the tube extends forward beyond the front end of the photomultiplier 40. The free end carries a filter 82, for example red, or a window transparent to photons, which is example held in place on the tube by means of a ring 84, for example flexible material such as elastomer (figures 5 and 6). The front end of tube 66 and the filter (or window) are located at an axial distance from the front wall 38a and the front end of the photomultiplier 40, for example a few millimeters.

The rear end of tube 66 does not extend to the rear end of the photomultiplier 40. In other words, the rear end of tube 66 is recessed from the rear end of the photomultiplier 40 (figure 3).

The tube 66 includes a longitudinal slot 66a (Figure 4) which extends over its entire length and enables, by spacing the longitudinal edges of the tube defining said slot, to increase the internal diameter of the tube 66. This can be useful for the mounting of the photomultiplier 40 in the tube 66. the tube may be directly mounted on the photomultiplier and thus be in direct contact with the photomultiplier of the glass wall.

Alternatively and as shown in Figure 4, a thermally conductive layer 72 is interposed between the photomultiplier 40 and the tube 66. The layer 72 preferably extends over the entire cylindrical surface of the photomultiplier 40 surrounded by the tube. It is preferably made of silicone and has for example a thickness of between 0.1 to 0.5 mm thick.

72a an electromagnetic shielding layer may further be interposed between the layer 72 and the tube 66. The layer 72a may be made of mu-metal. It can facilitate assembly by inserting the photomultiplier 40 equipped with the layer 72 within the tube 66. The layer 72a has for example a thickness of about 0.8mm. It can be obtained by winding a metal foil of generally rectangular shape, the longitudinal facing edges of the wound foil is not necessarily joined. The layer 72a may thus have in cross section a substantially C

In addition to the tube 66, the cooling means comprise a plate 73 which is integral with the tube and preferably formed integrally with the tube. The plate 73 serves as a part of cooling means for the fastening means as it comprises orifices traversed by mounting screws 74 on the lid 46. The plate 73 also serves as a mounting means of a Peltier device 76 which comprises conventional manner one or more superimposed levels. The Peltier device 76 here comprises a low cold end in contact with the plate 73, for example via a thermally conductive resin, and a high hot end here in contact with the lid 46 and located substantially in the fins 52, as can be seen in Figure 3.

The Peltier device 76 is thus sandwiched between the cover 46 and the plate 73. The plate 73 and the cover 46 are connected together by screws 74 or by struts the length of which is predetermined and used to define precisely the level of tightening of the Peltier device 76 between the cover 46 and the plate 73. a minimum tightening is needed to ensure good heat conduction, but should not be too important not to damage the device 76, which is relatively fragile.

The plate 73 also serves as the tube support means 66 and connecting the tube 66 to the lid 46. The plate 73 is here located in the vicinity of the front end of the tube and the photomultiplier 40. The photomultiplier tube 66 and 40 are thus fixed in cantilever in the housing.

Figure 3 shows that the plate 73 includes a mounting hole 78 of a temperature sensor 75. The connecting cable of this sensor passes through a screw 60, as mentioned in the above (Figure 5). The other screw 60 is crossed by the Peltier device power cables 76. The connection cables of the base 64 pass them through the passage 63 of the plate 61.

The base 64 is fixed to the tube 66 by means of spacers 80 whose length is predetermined and can be positioned axially with precision the tube 66 vis-à-vis the base 64 and therefore of the photomultiplier 40 on which the base is mounted. The struts 80 extend from the rear end of the tube 66 to the front end of the base. The spacers thus possible to maintain the electrical connection pins of the photomultiplier 40 and the base 64, away from the tube 66.

Figures 7 to 9 show the analyzer 13 and the foam blocks 15 which surround it to assure a particular shock protection and thermal. They can also absorb vibrations.

The foam blocks 15 define a cavity 86 housing the analyzer 13, not cooperating complementary shapes with the analyzer so as to keep it stationary in the cavity. The foam blocks are secured in the aforesaid frame.

The foam blocks 15 define a seam 88 extending along the analyzer 13. The longitudinal end of the vein 88 on the side of the rear wall

38b comprises a fan 90 and the opposite end opens out of the side of the block 30. The vein 88 passes through the vanes 52.

The foam blocks 15 comprise at least two elements nested one inside the other by means of pegs of one of the members engaged in holes 91 of the other of the members (Figure 9).

Figures 5-9 are used to display the electronic card 92 for controlling the base 64, the sensors, the Peltier device 76, etc. The cards 92 are mounted slidably in grooves 93 of the blocks of foam 15 (Figure 9). Figure 9 further shows the sealing means 65 between the cables and the passage 63 of plate 61.

The operation of the analyzer 13 has been discussed in the foregoing. Regarding the cooling of the photomultiplier 40, it is provided by the cooling means. The Peltier device 76 is cooled by the conduction plate 73 and the tube 66. The tube 66 is cooled by conduction the photomultiplier 40 through the layer 72. The chamber 43 is partially evacuated by suction through the port 50a . The absence of air in the enclosure limit convective cooling of the housing walls 30. The heat generated by the Peltier device 76 is discharged by the fins 52 which are traversed by the air flow generated by the fan 90 and flowing in the duct 88. air is injected through the port 50b into the chamber 43 via the desiccant. The aspiration and injection of air in the chamber 43 are controlled so that the suction is always active, which prevents any contamination of the chamber with external gas which might enter the enclosure 50a via the harbor in the absence of suction. The photomultiplier 40 can thus be cooled to a temperature of 0 ° C, thereby reducing the dark current at a minimum.

In the embodiment described above, all of the photomultiplier 10 and the base 64 are housed within the enclosure 43. This allows many advantages. The photomultiplier 40 is protected from stray light as entirely housed within the enclosure 43. There is no alignment constraint or pressure stress on the tube 66.

Figures 10a to 10c schematically illustrate alternative embodiments of the invention.

In the figures of the embodiments 10a and 10b, the base 64 is located outside of the chamber 43. The rear wall 38b extending substantially in the rear end of the photomultiplier 40 or connecting pins electric photomultiplier 40 to the base. The rear wall 38b is traversed by the photomultiplier or pins and sealing means are provided at the crossing.

In the embodiment of Figure 10c, the base 64 is located outside of the enclosure 43. The photomultiplier 40 through the rear wall 38b and the sealing means are provided at the crossing.

In the variants of Figures 10a to 10c, the tube 66 on only part of the length of the photomultiplier 40. In Figure 10a, it extends over substantially half of the photomultiplier. In Figures 10b and 10c, the tube extends only

on a front end portion of the photomultiplier, that is to say at its photocathode only.

CLAIMS

1. gas analyzer (13), comprising:

- a reaction chamber (14) configured to be supplied with gas and for generating photons by chemiluminescence reaction,

- photon detection means output in said chamber, having a photomultiplier (40) of elongate shape,

- a tube (66) supporting said photomultiplier, said photomultiplier being coaxially mounted within said tube,

- means (73, 76) for cooling said tube, preferably by the Peltier effect, and

- a housing (38) defining an enclosure (43) for receiving at least a portion of the photomultiplier, said tube and said cooling means, said chamber being isolated from said reaction chamber,

characterized in that it further comprises means (50a, 50b) for evacuating at least part of said enclosure, said vacuum means comprising a first port (50a) to exhaust air contained in said enclosure and a second port (50b) for admitting air into said enclosure, and in that said tube is tightly mounted on said photomultiplier so that the latter is conduction cooled by said cooling means.

2. Analyzer (13) according to the preceding claim, wherein said tube (66) is fitted tightly onto said photomultiplier (40) by means of a heat conductive layer (72) interposed between said photomultiplier and said tube.

3. Analyzer (13) according to claim 1 or 2, wherein said tube (66) comprises a longitudinal slot (66a) extending over its entire length, said tube having a diameter which can be increased by moving the longitudinal edges the tube defining said slot.

4. Analyzer (13) according to one of the preceding claims, wherein said photomultiplier (40) comprises a first longitudinal end for receiving photons and a second opposite longitudinal end for connection to a base (64) of electrical connection, said tube (66) extending longitudinally beyond said first end of said photomultiplier (40) and being in longitudinal withdrawal of said second end of said photomultiplier.

5. Analyzer (13) according to the preceding claim, wherein a window or a filter (82) for transmission of photons is mounted at a first longitudinal end of the tube (66) located towards said first end of said photomultiplier (40).

6. Analyzer (13) according to claim 4 or 5, wherein said tube (66) comprises a second longitudinal end fixed by spacers (80) of predetermined length to said base (66).

7. Analyzer (13) according to one of claims 4 to 6, wherein said enclosure (43) is defined longitudinally by two substantially parallel walls, respectively front (38a) and rear (38b), said front wall being situated on the side said first end of the tube (66) and spaced from said first end, said front wall comprising a filter or a window (34) for transmission of photons from said reaction chamber (14) and said rear wall (38b) being located the side of said base (64) and spaced from said base.

8. Analyzer (13) according to one of claims 4 to 7, wherein said enclosure (43) is defined laterally by side walls (38c), said tube (66) being supported by a single of these walls by through said cooling means (73, 76).

9. Analyzer (13) according to one of the preceding claims, wherein said cooling means comprises a plate (73) forming a monobloc assembly with said tube (66) and at least one Peltier device (76) with single or multiple stage mounted on said plate.

10. Analyzer (13) according to the preceding claim, when dependent on claim 9, wherein said Peltier device (76) is interposed between said plate (73) and said tube support wall and is clamped between them by means of screws (74) or struts extending between the plate and the support wall.

January 1. Analyzer (13) according to one of Claim 8 to 10, wherein said tube support wall (66) is a cover (46) removable from the housing (38).

12. Analyzer (13) according to the preceding claim, wherein said cover (46) is formed integrally with the cooling fins (52).

13. Analyzer (13) according to one of the preceding claims, wherein said housing (38) or said housing (43) has a general shape substantially parallelepipedal.

14. Analyzer (13) according to one of the preceding claims, wherein said housing (38) comprises orifices traversed with play by electric cables, said sets being filled by the sealing means.

15. Analyzer (13) according to one of the preceding claims, wherein said housing (38) is housed in a cavity defined by the foam blocks (15), said foam blocks defines a vein (88) of flow a ventilation air flow over at least a portion of said housing, which is generated by a fan (90).

16. Analyzer (13) according to the preceding claim, when dependent on claim 13, wherein said fins (52) are located in said duct (88). 17. A method of gas analysis using an analyzer (13) according to one of the preceding claims, characterized in that it comprises the steps of said enclosure (43) under partial vacuum:

- supplying said chamber with an air flow Q1, and

- a suction air flow Q2 of said enclosure, Q2 being at least equal to Q1 so that Q1 air flow is non-zero and the pressure in said chamber less than atmospheric pressure, and e.g. of about 100-200mPa.

18. Method according to the preceding claim, wherein said reaction chamber (14) is heated to a temperature of about 60 ° C, and said means (73, 76) for cooling are supplied to a power less than 15 W to maintain said tube (66) at a temperature of around 0 ° C.

19. The method of claim 17 or 18, wherein the reaction chamber (14) is supplied with a nitrogen oxide (NOx) and ozone.