WO2012126470A1

WO2012126470A1 - High temperature gas sensor

Info

Publication number: WO2012126470A1
Application number: PCT/DK2012/000027
Authority: WO
Inventors: Jens Møller JENSEN; Poul Kodahl SØRENSEN; Rainer Buchner; Per Romedahl CHRISTENSEN; Kenneth PIHL; Oluf Sigh OLESEN
Original assignee: Danfoss Ixa A/S; Green Instruments A/S
Priority date: 2011-03-23
Filing date: 2012-03-21
Publication date: 2012-09-27
Also published as: EP2694806A1

Abstract

Gas sensor enable to measure gas based on the absorption bands of radiation characteristic to the gas of interest, where the sensor is able to measure gases of temperatures usually too high for the delicate parts of the sensor to endure. The present invention introduces a cooling device connected to the sensor for ensuring a lowered temperature in the vicinity of the delicate parts.

Description

HIGH TEMPERATURE GAS SENSOR

Gas sensor enable to measure gas based on the absorption bands of radiation characteristic to the gas of interest, where the sensor is adapted to operate in environments with gases having temperatures usually too high for the delicate parts of the sensor to endure. The present invention introduces a cooling device connected to the sensor for ensuring a lowered temperature in the vicinity of the delicate parts.

BACKGROUND

Gas sensors configured to measure a gas based on absorption bands of bands of radiation characteristic to the gas of interest is widely known, for example as disclosed in WO10118748A describing a sensor having a filter arrangement, downstream of which there is arranged a detector arrangement, and an evaluating device which is connected to the detector arrangement, the filter arrangement has at least a first filter, the suspect filter, which is configured as a band pass filter allowing the passage of a first predetermined band, the suspect band, at least one second filter, the reference filter(s), which is configured as a band pass filter allowing the passage of a second predetermined band(s), the reference band(s), and where the detector arrangement has at least one detector associated with at least one of the filters. The band passes reference filters are distributed above and below the band pass of the suspect filter. The sensor with advantage could be utilized within the IR band, and could advantageously be used to detect CO.

A second document WO10118749A discloses a related sensor relating to the fact that particles or substances in general are present in the environment where the sensor operates, and that these over time gets into contact with more delicate parts of the sensor, decreasing the time of operation of the sensor, before maintenances or even exchange is needed. It is therefore often desired to keep the more delicate parts of the sensor physically separated from the media or environment containing the substances or species being measured by the sensor. The present invention solves this problem by introducing sight glasses positioned at least in front of the delicate parts of the sensor, the sight glasses in general being formed with coating(s) chosen according to the environment wherein the sight glasses are to be used.

None of these sensors however describes operating in environments where the gasses are of such high temperatures that the delicate parts of the sensor may be challenged.

The present invention in the preferred, but not exclusive, embodiment relates to sensors of the same kind as described in the documents WO10118749A and WO10118748A, but where the sensor is configured such, that it may operate in conditions with temperatures so high, that they might damage the delicate parts of the sensor.

SUMMARY OF THE INVENTION The object is solved by introducing:

A gas sensor adapted to measure the concentration of a gas in a measurement region by the absorption of spectral band characteristic to the gas, the sensor (1) therefore comprising;

- a light source emitting light in a range at least comprising the absorption band, and

- a detector adapted to measure the light at least in the range of the absorption band,

a first sight tube connected to the detector separating it from the measurement region, and wherein a cooling device is connected in a heat transfer manner to the sight tube. The cooling device thus ensures that the area in front of the detector has a lower temperature than the gas to be measured.

To ensure the detector is sealed from the gas, an embodiment further introduces at least one sight glass positioned in a manner the detector sealed from the measurement region, and it is especially suited that the sight glass is fixed to the first sight tube in a manner where an gas tight internal enclosure of the first sight tube is defined separating the detector from the measurement region.

To ensure no signal is absorbed within the sight glass(es), a second sight glass is fixed to the first sight tube in a manner where a gas tight internal enclosure of the first sight tube is defined separating the detector from the measurement region. Then the internal enclosure is filled with a gas inert to the gas to be measured.

In the same manner a second sight tube constructed as the first sight tube may be connected to the light source sealing it from the measurement region. In one embodiment the cooling device is adapted to transfer a heat transfer fluid the cooling based on vapour-compression.

In an alternative embodiment the cooling device is a heat sink. In order to lower the temperature even more than is achieved by the cooling device, the sight tube comprises a section of a material with a heat transfer characteristic substantially lower than the rest of the sight tube.

In order to ensure a substantial even distribution of the light on the detector, at least a section of the internal surface(s) of the sight tube(s) is adapted to spread the light emitted by the light, such as by forming a substantially rough surface of a highly reflective material.

To ensure alignment of the detector to the light source despite influences from the surroundings, the measurement region is the volume within the internal of a porous structure comprising at least one opening forming communication from the measurement region to the environment comprising the gas. It is especially advantagous that the porous structure is fixed to the sight tube(s). To avoid shears and stresses in the sensor, several of the different are made of materials with substantially similar heat transfer characteristics, however it is an advantage that the cooling device is made of materials with substantially higher heat transfer characteristics than the sight tubes.

FIGURES

Fig. 1 Illustration of a setup of a gas sensor according to the present invention.

Fig. 2 Illustration of the sensor of the present invention positioned in a sensor container.

DETAILED DESCRIPTION

Fig. 1 shows a diagrammatic view of a gas sensor (1) for determining for example the CO2 content (carbon dioxide content) in a measurement region (3), where the sensor (1) comprises a detection part (2) and an light source (4) emitting light within a desired span of wavelengths defined by the specific gas, or gasses, to be measured, frequently within the such span of IR frequencies.

In the example, a large number of CO2 molecules are present in the measurement region (2), the CO2 molecules being represented herein by small circles. The gas molecules (5) absorb IR rays in a specific spectral range, as represented by the arrows. The greater the concentration of CO₂ the lower the energy in a specific spectral range that can be detected in the gas sensor (1).

The measurement region (3) may be defined as the internal a porous structure (6), such that the sensor device (2) is fixed at first end of the porous structure (6) and the light source (4) is fixed at a second end. The porous structure (6) may be formed such that gas (5) may enter and leave its internal hollow, the measurement region (3), through openings (21) and (22)) in its wall. However, in an alternative version, it is gas tight the gas to be measured being filled into the internal. In one preferred embodiment of the present invention, sight glasses (8) are introduced to form a gas tight separation of the light source (4) and the detector (2) from the measurement region (2), but still ensuring optic communication from the light source (4) to the detector device (2). Further it is preferred that the porous structure (6) is such that it forms a stable alignment of the detector (2) to the light source (4), despite changes in the ambient conditions such as the humidity, temperature, vibrations etc.

A first (9) and / or second (10) sight tube may be connected to the porous structure (6) and detector (2) and / or light source (4). The sight tube is hollow and may be forming an internal being gas tight sealed from the externals and the measurement region (3), still maintaining the optic communication from the light source (4) to the detector device (2). The internal(s) of the sight tube(s) may be filled with a gas inert to the wavelengths of the system. This helps to ensure no absorption occurs in the sight tubes (9) and (10) affecting the signal. In an embodiment, one or both of the sight tubes (9) and (10) comprises sight glasses (8) attached either at one or both ends of the sight tube(s), where they may be attached in a manner, where they form the gas tight internal(s) of the sight tube(s).

For the sensor (1) to be able to operate in environments where the gasses may be of substantially high temperatures, where this in the present context is to be understood as temperatures too for the delicate parts of the sensor (1 ) to endure or being able to operate, where this especially is related to the detector (2), a cooling device (1 1 ) is connected to at least one of the sight tubes (9) and / or (10), being adapted to cool down the gasses inside the sight tube(s) (9) and / or (10). It has been found that even for conditions in the measurement region (2) with a temperature higher than 300 degrees Celcius, just three such cooling ribs (1 1 a) are sufficient to ensure temperatures close to the detector (2) and /or light source (4) in the area of 40-50 degrees Celcius. The sight tubes (9) and (10), are made of material(s) able to endure the high temperature, but is also internally equipped with surfaces with at least some reflectance in order to ensure that sufficient of the light emitted by the light source (4) reaches the detector (2), rather than being absorbed by the surfaces.

The cooling device may be active or in-active. Active in the present context is to be understood as making an active cooling e.g. using a technology similar to the one in refrigerators and freezers based on the principle of vapor- compression where a heat transfer fluid circulated in the cooling ribs (1 1 a). Inactive in the present context is to be understood as 'passive' transfer of the heat e.g. by a heat sink.

The cooling device (1 1 ), preferable is made of materials with substantially higher heat transfer characteristics than other parts of the gas sensor (1 ), especially the sight tubes (9) and (10), such as in a not limiting example, the sight tubes (9) and (10) being made of titanium and the cooling device (1 1 ) of aluminium

The materials of the sight tubes (9) and (10) preferable is such that they at least has the same heat transfer characteristics as the other parts of the gas sensor (1 ) (but not the cooling device (1 1 )), since this would ensure the different connected parts react to the temperature in the same manner.

Actual measurements with sight tubes (9) and (10) each 100 mm. long and made of ordinary steel, and a cooling device (1 1 ) in aluminium, the latter in the form of 3 discs each 40 mm. in diameter, yields a temperature drop of more than 200 Kelvin without forced convection. Additional temperature drop can easily be achieved using other materials, e.g. titanium and aluminium, and even further temperature drop can be achieved using either forced convection or by arranging a structured natural convection It is preferred that at least a substantially part of the sight tubes (9) and (10), especially the section(s) in contact to the measurement region (3) is made of metal, this however having a significant heat conductance that makes a conflict to the desire not to have heat transferred to the delicate parts. Therefore in a further embodiment one or both of the sight tubes (9) and (10) is equipped with a section (12) (or even a plural of sections (12) in succession) of a low heat conductance material, such as plastic, or other suitable material. The separate sections (12', 12) of the sight tubes (9) and (10) may be connected and fixed together in any manner as known in the art.

In a further embodiment of the present invention the internal surfaces (13) of the sight tubes is made such that that it scatters the light as it reflects from the internal surfaces (13) in its path from the light source (4) to the detector (2). Hereby it is ensured that 'dark spots' does not influence the measurements in that the incoming light reaching the detector (2) in that the light will be scattered to 'blur out' any such 'dark spots'. In the present context 'dark spots' is to be understood such that there is areas of the detector (2) not being enlightened, this affecting the measurements in that the detector (2) will be calibrated as if all the detector area are 'active' in the measurement. Therefore, if lesser part of the detector is enlightened it will appear as if a smaller concentration of the gas is detected. Since such 'dark spots' may be due to dynamic factors internal to the sensor, such as impurities or moist in the gas settling at the sight glasses (8) making lesser transparent spots.

Making such an internal surface helping to scatter the light may be performed in any manner as known in the art, such as forming a substantial rough surface scattering the light sufficient randomly. The present invention is not limited to one or all of the described embodiments, but any number and combination of the embodiments apply.

The sensor (1 ) of the present system preferable is made in a manner where each of the parts, sensor (2), light source (4), separation structures (9) and (10), and porous structure (6) are individually easily exchangeable. Each of the parts may be connected to other parts in any manner as known in the art, such as by equipping the parts with windings so that they may be screwed together, or by e.g. using feasible standard fittings such as those known for hydraulic tubing.

The sensor (1 ) in one embodiment of the present invention is positioned within the internal of a sensor container (200) as seen in Fig. 2, where the sensor container (200) is equipped with a gas inlet (210) externally connected to the environment comprising the gas to be measured, thus forming a gas communication from this environment to the internal of the sensor container (200). The communication may be by a tube connected to the gas inlet (210). Further the sensor container (200 has a gas outlet (211) forming a connection from its inside to any environment where the gas that has been measured are to be fed. This could be to an exchangeable container, to the externals, or more preferable back into the environment containing the gas to be measured.

The gas inlet (210) of the sensor container (200) is connected to the inlet opening (7, 21) of the gas sensor (1)) and the gas outlet (211) connected to the outlet opening (22) of the gas sensor (1).

A stream of gas in the internal of the sensor container (200) may then be induced by vacuum or pumping means of any kind as known in the arts. When the sensor (1) is inserted into the internal of the sensor container (200), the gas will then flow in and out through the openings (21) and (22) of the porous structure (6) and thus in and out of the measurement region (2), preferable at a constant flow rate.

The first (9) and second (10) sight tubes may optionally be mounted in a way such that they defines hermetically sealed volumes, where one or both of these in a preferred embodiment is filled with and gas inactive to the radiation frequency span of the light source (4). This may help to get rid of any cross- correlations, and the volume(s) may additionally be filled with a gas of high concentration in order to filter the specific wavelengths out. The volume may be filled with any gas, as long as the concentrations are stable over time meaning that the volume is hermetically filled.

Claims

1. A gas sensor (1 ) adapted to measure the concentration of a gas in a measurement region (3) by the absorption of spectral band characteristic to the gas, the sensor (1 ) therefore comprising;

- a light source (4) emitting light in a range at least comprising the absorption band, and

- a detector (2) adapted to measure the light at least in the range of the absorption band,

a first sight tube (9) connected to the detector (2) separating it from the measurement region (3), and wherein a cooling device (1 1 ) is connected in a heat transfer manner to the sight tube (9).

2. A gas sensor (1 ) according to claim 1 , where at least one sight glass (8) is positioned in a manner the detector (2) sealed from the measurement region

(3).

3. A gas sensor (1 ) according to claim 2, where the sight glass (8) is fixed to the first sight tube (9) in a manner where an gas tight internal enclosure of the first sight tube (9) is defined separating the detector (2) from the measurement region (3).

4. A gas sensor (1 ) according to claim 2, where second sight glass (8) is fixed to the first sight tube (9) in a manner where a gas tight internal enclosure of the first sight tube (9) is defined separating the detector (2) from the measurement region (3).

5. A gas sensor (1 ) as in one of claims 3 or 4, wherein the internal enclosure is filled with a gas inert to the gas to be measured.

6. A gas sensor (1 ) according to one of claims 2-5, wherein a second sight tube (10) constructed as the first sight tube (9) is connected to the light source (4) sealing it from the measurement region (3).

7. A gas sensor (1 ) according to any of the previous claims, where the cooling device (1 1 ) is adapted to transfer a heat transfer fluid the cooling based on vapour-compression.

8. A gas sensor (1 ) according to any of the previous claims, wherein the cooling device (1 1 ) is a heat sink.

9. A gas sensor (1 ) according to any of the previous claims, wherein the sight tube (9) comprises a section (12) of a material with a heat transfer characteristic substantially lower than the rest of the sight tube (9).

10. A gas sensor (1 ) according to any of the previous claims, wherein a least a section of the internal surface(s) of the sight tube(s) (9) and (10) is adapted to spread the light emitted by the light, such as by forming a substantially rough surface of a highly reflective material.

1 1. A gas sensor (1) according any of the preceding claims, where the measurement region (3) is within the internal of a porous structure (6) comprising at least one opening (7) forming communication from the measurement region (3) to the environment comprising the gas.

12. A gas sensor (1 ) according to claim 1 1 , where the porous structure (6) is fixed to the sight tube(s) (9) and / or (10).

13. A gas sensor (1 ) according to any of the preceding claims, where the different parts (9), (10), (6), and of the sensor are made of materials with substantially similar heat transfer characteristics.

14. A gas sensor (1 ) according to claim 13, wherein the cooling device (1 1 ) is made of materials with substantially higher heat transfer characteristics than the sight tubes (9) and (10).