Field of the invention. This invention relates to a gas sensor comprising a thermopile.

Background

Gas sensors with thermopiles are known. These are based on shining IR radiation through a volume where gas is to be detected on to a heat sensor comprising a thermopile. In the presence of gas, radiation is absorbed, and this can be detected as a change in tempera ture by the thermopile.

The use of thermopiles in gas sensors typically involves the use of lenses or mirrors to fo- cus the IR radiation onto the sensor, in order to generate a temperature difference that is large enough to be detected by the thermopile.

Prior art gas sensors have sensor elements with thermopiles with a radial design, that is thermopiles where the hot junctions and the heat sink is placed centrally, and the cold junctions and the cold sink is placed in the periphery as shown in Fig. 2. One example is the thermopile of US20050081905.

Fig. 1 shows a side view of a sensor element for a gas sensor according to prior art, which comprises a housing 100, a thermopile 101 where the optical beam 102 is focused on a spot in the centre area 104 comprising the hot junctions of the thermopile 101. The plane of the surface of the thermopile 101 is in the focal point of the optical beam 102.

GB2391310 is an example of a gas sensor where the thermopile is placed in the focus of the beam. The housing 100 has a window 103 that may comprise a filter. Window 103 forms a part of housing 100. The cold junctions 106 of the thermopile 101 is located in the same plane as the surface of the thermopile 101 and towards the periphery of the sensing area. The leads 107 (Fig. 2) of the thermopile 101 are typically on the surface of the detection area of the sensor, making the sensor vulnerable to physical damage. The housing 100 protects the leads 107 of the thermopile 101 from damage and also shades the cold junctions 106 from incoming radiation. The housing 100 results in a bulky design. Fig. 2 is top view of a thermopile 101 according to prior art with a radial design where the cold junctions 106 of the leads 107 are in the periphery of the thermopile 101 and the hot junctions are in the centre area 104. Optical beam 102 is focused on a spot in the centre area 104.

The gas to be detected is in the space outside housing 100. It is difficult to make the hous ing 100 gas tight. At the same time, housing 100 is often poorly ventilated. This may cause gas to be trapped inside housing 100, which may cause erroneous readings.

Summary of invention.

In a first aspect there is provided a gas sensor comprising an optical source, a sensing ele ment comprising a thermopile, the gas sensor comprising an optical focusing device with a focal point, where the optical source emits infrared radiation through a space where gas is to be detected, and the optical focusing device is arranged to concentrate the radiation from the optical source to an optical beam on to a detection surface of the sensing ele ment, where there optical beam travels along an optical axis onto the detection surface of the sensing element, where the thermopile comprises leads arranged to generate voltage with the use of the thermoelectric effect where the leads of the thermopile are arranged parallel to the optical axis.

Because the leads of the thermopile are parallel to the optical axis, the heat sensing area can be made larger, because the surface of the sensing area is not crowded with the leads of the thermopile. The small hot sink area in centre area 104 of the prior art must be placed in the focal point of the optical beam 102 in order to produce a signal. This has the following results: 1) It is difficult to consistently manufacture the gas sensor with a precision where the hot sink is in the focal point, and 2) the light source may move slightly from its original place when switched on and heated, thereby moving the focal point of the optical beam 102 from the hot sink in centre area 104.

With the inventive design, a larger sensing area is achieved. This makes the gas sensor much less sensitive to relative movement of the radiation source, the focusing device and the sensing element, and solves the problem of having to calibrate each individual gas sensor. Also, the sensing area is scalable.

Previously imaging optics has been used in the design of gas sensors. The inventive gas sensor can be designed using non-imaging optics, which is much easier to use.

Furthermore, because the cold junctions are placed on the reverse side of the thermopile, it is not necessary to have a housing 100 that shades the cold sink 106. The housing 100 is also made unnecessary because the leads of the thermopile can be at least partially em bedded in a matrix that protects them from physical damage. The housing is bulky and may also trap gas, which results in errors as described above. Thus, the use of leads that are parallel to the optical axis and embedded in a matrix provides better accuracy during sensing.

In a preferred embodiment the detection surface of the sensing element is not arranged in the focal point of the optical focusing device.

The detection surface of the sensing element may be arranged between the focusing de vice and the focal point. This results in a more compact sensor. For example, the distance from the focal point to the surface of the sensing element is at least 20% of the distance from the focusing device to the focal point (focal length). The detection surface may also be arranged such that most, or all, of the cross section of the optical beam reaches the detection surface.

The radiation beam may have a cross section with a size and shape such that the entire surface of the sensing element is illuminated by the radiation from the focusing device. This makes it less important to calibrate the sensor, since it is enough only to check that the entire surface is illuminated. Also, because the beam is not focused on the detection surface, a certain loss of beam does not affect the signal strength so much. The detection surface may be arranged such that not all of the cross section of the optical beam reaches the detection surface.

The sensing element comprises a heat sink arranged between the thermopile and the in coming radiation. The heat sink may comprise a heat absorbing layer arranged perpendic ular to the direction of the conductors of the thermopile. The heat sink may cover the hot junctions of the thermopile.

The gas sensor may have a cold sink on the cold side of the sensing element.

The optical source, the focusing device and the sensing element, in particular the detec- tion surface of the sensing element, may be comprised in a housing that comprises the gas to be analysed. This provides a simple and cost-efficient way to build the gas sensor and minimizes the problem with gas trapped in a separate housing.

The leads of the thermopile are preferably at least partially or completely embedded, for example at least partially or completely embedded in a matrix. This has the advantage that the thermopile is protected against physical damage.

In a second aspect of the invention there is provided a method of assembly of a gas sensor according to the first aspect of the invention comprising the step of mounting the focusing device and the sensing element relative each other such that most of the cross section of the optical beam reaches the detection surface. Brief description of drawings

The accompanying drawings form a part of the specification and schematically illustrate preferred embodiments of the invention and serve to illustrate the principles of the inven- tion.

Fig. 1 and 2 shows sensing elements according to the prior art, where the thermopile has a radial design.

Fig. 3 is a schematic drawing of a gas sensor.

Fig. 4 is a schematic side view of a sensing element.

Fig. 5 is a front view of a cut through along line A-A in Fig. 4

Fig. 6 is a schematic representation of a how a sensing element is arranged in relationship to an optical beam.

Fig. 7 is a schematic drawing of a gas sensor comprising a gas container.

Detailed description

Fig. 3 schematically show an example of a gas sensor according to one embodiment of the invention. Optical source 1 typically emits visible light or IR radiation, preferably IR radia- tion. The optical source 1 may be any suitable radiation source, such as an IR source, for example an IR LED. The optical source 1 may be powered by a battery or by mains power (optionally via a transformer). Gas inside housing 2 is detected in the space covered by the optical beam 3 emitted from optical source 1. Focusing device 5 focuses the optical beam 3 towards a focal point 6. In Fig. 3 the focusing device 5 is a lens but any suitable arrange- ments of lenses, such as Fresnel lenses, mirrors, or the like may be used. For example, the optical source 1 and the sensing element 7 may be placed opposite a mirror that focuses the radiation from the optical source 1 towards the sensing element as in GB2391310.

Housing 2 is preferably common for the optical source 1, focusing device 5 and sensing el- ement 7. Hence, sensing element 7 is preferably naked in housing 2. The housing 2 is pref- erably well ventilated, preferably having several openings 21 for allowing gas to be ana lysed into housing 2. The space that comprises the gas may thus comprise the sensing ele ment 7, as shown in Fig. 3, in particular the detection surface 4 of the sensing element 7. The housing 2 may be made in for example sheet metal or a polymer material. Preferably there is no housing that separates the sensing element 7 from the optical source 1 and the focusing device 5 (like the housing 100 of the prior art).

Sensing element 7 comprises a thermopile 8, i.e. a number of thermocouple pairs as dis cussed below with reference to Figs. 4-5. Sensing element 7 may comprise a heat sink 9 arranged in contact with thermopile 8 such that heat from beam 3 can be absorbed by the heat sink 9 and transferred to leads 11, 12 of thermopile 8. The heat sink 9 may be a heat absorbing layer. The heat absorbing layer is preferably arranged perpendicular to the di rection of the leads 11, 12 of the thermopile 8. The heat sink 9 should be able to absorb heat and conduct heat to the thermopile 8. The heat sink 9 may cover the entire detec- tion surface 4. Suitable materials for the heat sink includes a thermally conducting poly mer or a metal, for example copper. The heat sink 9 preferably covers the hot side 23 of the thermopile 8, thus being arranged between the focusing device 5 and the thermopile 8. The cold side 24 side of the thermopile 8 may be arranged in contact with a cold sink 20

There may be a band pass filter 10 arranged between the sensing element 7 and the opti cal source 1. The filter 10 may be a band pass filter chosen depending on the gas to be de tected, as every gas has its own specific absorption spectrum. The choice of filter 10 thus can make the gas sensor specific for one gas, as is known in the art. There are suitable fil- ters 10 for C0 ₂ , methane, nitrogen, etc.

With reference to Figs. 4 and 5, the sensing element 7 comprises thermopile 8. A thermo pile 8 comprises at least two thermocouples. Each thermocouple consists of a first lead 11 of a first metal and a second lead 12 of a second metal, where the first lead 11 and the second lead 12 have different Seebeck coefficients. Thus, there is at least a first lead 11 that has a first Seebeck coefficient and a second lead 12 that has a second Seebeck coeffi cient. Examples of suitable pairs of metals include chromel-constantan (type E thermno- couple), iron-constantan (type J), chromel-alumel (type K). The leads 11, 12 of the ther mopile is preferably embedded in matrix 17. When there is a temperature difference be- tween the hot side 23 and the cold side 24 a voltage potential will be generated. Matrix 17 is made from a non-conductive material such as, for example, an epoxy polymer. Matrix 17 is preferably a poor conductor of heat and electricity. The material of the matrix 17 can be selected by a person skilled in the art. The leads 11 and 12 may be connected with con nectors 19 on the hot side 23 and the cold side 24 of the thermopile 8. The individual ther- mocouples pairs 11,12 and 11', 12' of the thermopile 8 are coupled to provide a voltage potential that is sufficient to be detected. Matrix 17 preferably has uniform thickness (the thickness of matrix 17 is indicated with T in Fig 4). Matrix 17 is preferably made from a ma terial of even thickness, such as a film or a board. The thermopile can be manufactured using methods known in the art, it is referred to M. Lindeberg, K. Hjort Interconnected nanowire clusters in polyimide for flexible circuits and magnetic sensing applications, Sen sors and Actuators A 105 (2003) 150-161, for details.

Any suitable thickness of the matrix 17 may be used. The thickness of matrix 17 is prefera bly from 50 pm to 500 pm, where 75 pm to 200 pm is preferred, and where from 100 pm to 150 pm is even more preferred. The length of the leads 11, 12 matches the thickness of the matrix 17, and is thus preferably from 50 pm to 500 pm, where 75 pm to 200 pm is preferred, and where from 100 pm to 150 pm is even more preferred.

The lead 11, 12 may comprise a plurality of separate metal nanowires starting from the first side or the second side of the matrix 17. The thickness of the nanowires may be from 200 to 1000 nanometres, even more preferred from 300 to 800 nanometres. A suitable number of nanowires may be from 30 to 300 in each lead 11,12.

The detection surface 4 of the sensing element 7 may have any suitable shape. A quad- ratic detection surface 4 or circular or essentially circular detection surface 4 is preferred. Fig. 5 shows a square configuration of the thermopile 8, but a circular detection surface 4 may be preferred.

The area of detection surface 4 is preferably from 2 mm ² to 20 mm ² where 3 mm ² to 16 mm ² is preferred and from 4 mm ² to 12 mm ² is even more preferred and where from 5 mm ² to 10 mm ² is most preferred. Hence the area of detection surface 4 is preferably at least 2 mm ² where at least 3 mm ² is more preferred, and at least 4 mm ² is even more pre ferred, where at least 5 mm ² is most preferred. This ensures an easy calibration of the po sition of the detection surface 4 in relation to focusing device 5. The distance from the fo- cusing device to the thermopile may be approximately 5 mm to 50 mm.

The leads 11,12 are arranged parallel to the optical axis 18 of the radiation 3 that is incom ing towards the sensing element 7 from the focusing device 5. "Parallel" as used in this context comprises an angle of up to 10°, more preferably 5°, between the optical axis 18 and the leads 11, 12 of the thermopile 8. This arrangement makes it possible to provide a large and scalable detection surface 4. Hence, a larger detection surface 4 may be de signed by increasing the number of leads 11, 12 and the size of the matrix 17. The matrix 17 moreover protects leads 11, 1 12, 12' from physical damage and shades junctions 19 on the cold side 24 from radiation 3.

The sensing element 7 and the focusing device 5 are placed in relation to each other so that a strong signal from thermopile is obtained.

In a preferred embodiment, the detection surface 4 of the sensing element 7 is not in the focal point 6 but placed at a distance along the optical axis 18 from the focal point 6, pref erably between the focal point 6 and the focusing device 5. Preferably the detection sur face 4 of the sensing element 7 is placed between the focal point 6 and the focusing de vice 5. Preferably the distance D between the detection surface 4 of the sensing element 7 and the focusing point 6 is at least 5%, more preferably at least 10 %, more preferably at least 20% and most preferably at least 30% of the distance F from the focusing device 5 to the focal point 6 (the focal length). It is to be noted that detection surface 4 may be placed between focusing device 5 and focal point 6, or on the other side of focal point 6, such that optical beam 3 passes focal point 6 before reaching detection surface 4.

In one embodiment, the sensing element 7 and the focusing device 5 are placed relative each other such that most of cross section of beam 3 reaches detection surface 4. In a pre ferred embodiment the entire optical beam 3 is caught by the detection surface 4, such that no radiation from beam 3 falls beside the detection surface 4 (Fig 3). This ensures that the signal from thermopile 8 is strong enough to be detected. Hence, preferably more than 50% of the area of the cross section of beam 3 reaches detection surface, but more preferably at least 70%, even more preferably at least 90%, even more preferably at least 95% and most preferably at least 99% of the area of the cross section of beam 3 reaches the detection surface 4. At assembly of the gas sensor, the gas sensor 1 may be calibrated so that this condition is fulfilled. Fig. 3 shows an embodiment where all of the cross sec tion of optical beam reaches detection surface 4. Figs. 6 and 7 show embodiments where most of the cross section of optical beam 3 reaches the detection surface 4. Again, beam 22 in Fig 6 indicates a part of optical beam 3 that does not reach detection surface 4.

The optical beam 3 from the focusing device 5 that illuminates the sensing element 7 pref erably has a cross section with a size and shape, and the sensing element may be arranged in relation to the focusing device 5, such that the entire detection surface 4 of the sensing element is illuminated by the radiation 3 from the focusing device 5. The entire heat ab sorbing layer 9 or, when a heat absorbing layer 9 is not present, the entire surface of the thermopile 8 may be illuminated. Some radiation, indicated with 22 in Fig. 6, from the ra diation source may fall outside detection surface 4 of the sensing element 7. In one em- bodiment at least some part of optical beam 3 falls outside the detection surface 4.

Fig. 7 shows an embodiment where the gas to be analysed is provided in a gas container 13 which lets in gas from a space where gas is to be detected, trough inlet 14. The arrows indicate flow of gas. Gas may leave the gas container trough outlet 15. Inlet 14 and outlet 15 can be combined in one channel or opening. The gas inside gas container 13 is being sensed by the gas sensor. The use of a gas container 13 may be useful because it isolates the gas being detected, which may be corrosive, from the electronics and other sensitive parts of the of the sensor. Windows 16, 25 in gas container 13 enables the radiation from optical source 1 to enter and leave the gas container 13. In the embodiment shown in Fig. 7, the optical source 1 and the sensing element 7 is provided in the same housing 2, and the gas container 13 is arranged between the optical source 1 and the sensing element 7 in the housing 2.

Generally, the thermopile 8 is typically electrically connected to one or more of the follow ing: amplifier, analog to digital converter, processor, memory, output means for transmit- ting a signal to another device (by wire or wireless means), alarm, as is known in the art. Thus, the leads of the thermopile 8 may be electrically connected to an amplifier that am plifies the signal from the thermophile 8. The signal from the amplifier may be digitalized by an analog to digital converter and then further processed by the processor. The signal may be provided to other devices, systems, such as alarms, displays and communication devices.

The gas sensor may comprise a reference cell that contains a known gas that is also al lowed to absorb IR, as is known in the art. Preferably the same optical source 2 is used for the reference cell. The reference cell and the gas in housing 2 or gas container 13 may be alternatively illuminated and analysed using a radiation chopper.

A method for assembly of a gas sensor as described herein may comprising the step of mounting the focusing device 5 and the sensing element 7 relative each other such that most, or preferably all, of the cross section of the optical beam 3 reaches the detection surface 4. This may be done by ensuring that the detection surface 4 is within a distance span from focusing device 5.

While the invention has been described with reference to specific exemplary embodiments, the description is in general only intended to illustrate the inventive concept and should not be taken as limiting the scope of the invention. The invention is generally defined by the claims.