WO2010118749A1

WO2010118749A1 - Gas sensor with filtering sight glass

Info

Publication number: WO2010118749A1
Application number: PCT/DK2010/000046
Authority: WO
Inventors: Jens Møller JENSEN; Arun Krishna; Lars Munch; Rainer Buchner; Thomine Stolberg-Rohr; Henrik Gedde Moos
Original assignee: Danfoss Ixa AS
Current assignee: Danfoss Ixa AS
Priority date: 2009-04-17
Filing date: 2010-04-16
Publication date: 2010-10-21
Anticipated expiration: 2011-10-17

Abstract

The invention relates to 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. However, since particles or substances in general present in the environment where the sensor operates, as 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.

Description

GAS SENSOR WITH FILTERING SIGHT GLASS

BACKGROUND

Such a sensor, which is configured as a gas sensor, is known, for example, from US 5,081 ,998 A. An IR radiation source is provided therein, which acts upon a total of four detectors by way of a filter arrangement. The filter arrangement has two filters having different pass characteristics. A first filter has a pass band for IR radiation that is absorbed by CO2. That filter is therefore also referred to as a "CO₂ filter". The detectors arranged downstream are designated CO₂ detectors. The other filter has a pass band different therefrom which serves for determining a reference quantity. The detectors arranged downstream of that reference filter are referred to as reference detectors. Between the IR source and the two filters there is arranged a third filter which is referred to as a natural density filter and overlaps half of the first filter and half of the second filter. Accordingly, one of the two CO₂ detectors and one of the reference detectors receives only IR radiation that has passed both through the natural density filter and through either the CO₂ filter or the reference filter. In the evaluating device, the difference of the output signals of the two CO₂ detectors and the difference of the two reference detectors is formed. The two differences are then divided by one another. Such a CO₂ sensor is required, for example, for determining CO₂ in a patient's breath so as to be better able to monitor the patient during anaesthesia.

A disadvantage of such sensors is that they have a relatively high power requirement, and another disadvantage is the number of detectors required. The arrangement known from US 5,081 ,998 A requires a source of radiation which, in any case for prolonged use, makes it unsuitable for battery-operated use. Furthermore, such an IR source generally requires a certain heating-up period, so that without a degree of prior preparation it is not always possible to carry out measurements when desired.

The problem underlying the invention is to simplify the use of an IR sensor, which is introduced in the sensor described in US 2008/0283753, wherein the pass band of a first filter is arranged within the pass band of a second filter and the evaluating device forms the difference of the signals of the detectors and normalises it to the signal of a detector.

That configuration makes it possible to evaluate substantially more IR radiation. The IR radiation is therefore not divided into two separate ranges, with each detector detecting only one range. Instead, one detector detects IR radiation having a pre-set spectral range, which also includes, for example, the absorption spectrum of the gas being determined, here CO₂. The other detector detects an IR spectrum from a sub-range thereof, which does not include the absorption spectrum of the gas being determined. The normalisation of the difference to the output signal of a detector enables fluctuations in the intensity of the IR radiation to be compensated. It is also possible to use more than two sensors with a correspondingly greater number of filters, the individual pass ranges then overlapping accordingly. With such a sensor it is also possible to obtain other information, for example relating to temperature, to movement in the room, to the number of persons in the room, etc.. Because it is possible to detect substantially more radiation, the power consumption can be reduced, so that the necessary power can also be supplied by a battery. That in turn gives greater freedom in terms of local mounting and use. The sensor can transmit its signals wirelessly.

The pass band of the first filter is preferably larger than the pass band of the second filter. Accordingly, the first filter, in addition to including the spectral range allowed to pass by the second filter, also includes the spectral range in which IR radiation is absorbed.

The two filters preferably have a common cut-off wavelength. That simplifies evaluation. The difference between the output signals of the detectors can then readily be formed without additional calculation steps being necessary. The cutoff wavelengths are the wavelengths that define, that is to say limit, the pass bands. They are referred to as "lower wavelength" and "upper wavelength".

Sometimes it is an advantage to separate the sensor itself from the environments containing the substances or species to be measured. This could be due to the state or form of the media or environment containing the substances or species to be measured, or the conditions of the media or of the environment where the sensor operates, that may be harsh or may just contain substances or particles that over time may get in contact with the more delicate parts of the sensor. It is therefore often for such sensors desired to keep these more delicate parts physically separated from the environment containing the gas,

One such example is described in the document US 6,834,536 disclosing a probe for quantitatively measuring volatile components of a liquid, where the probe includes a measurement gas space separated from the liquid by a permeation membrane. The membrane is permeable for the volatile component, however nonpermeable for liquid. A selective gas detection system is arranged in the measurement gas space for a defined volatile component. The gas detection system has an IR light source, an IR filter and an IR-sensitive light sensor. Light falls through the IR filter on the light sensor. The IR filter comprises a transmission window in the range of an alcohol-specific absorption band.

Another example is the optoacoustic gas sensor disclosed in US 6,006,585 that has a sensor body, a light source, a measurement cell with a gas-permeable membrane, a measurement microphone, and an optical measurement filter between the light source and the measurement cell, a reference cell is included that is separate from the measurement cell. The reference cell has a reference microphone that is shielded against optoacoustic signals from the gas to be detected via the reference cell being substantially free from intensity-modulated optical radiation having an absorption wavelength of the gas to be detected. The measurement signal, which indicates gas concentration, is obtained by subtraction of the signals from the two microphones.

One drawback of these constructions is that the diffusion across the membrane takes time extending the response time of the sensor,, and that the membrane is prone to be clogged.

It is one object of the present invention to introduce methods to solve these problems of the present sensors, and a sensor utilizing the solutions.

SUMMARY

The present invention solves these problems by introducing a sensor equipped with sight glass penetrable to the light at least in the wavelengths of relevance.

However, since the substances or particles in the media or environment (in the following just referred to as the environment) containing the substances or species to be measured may also settle at the sight glass to make it less transparent, or even totally block it for light. Therefore, in order to enhance the operational time of the sensor before maintenance or even an exchange is needed, one or more layers of coating is formed on the surface(s) of the sight glass to be in contact with the environment.

It is therefore an object for the present invention to introduce sight glasses, that in further embodiments are equipped with coating(s) and/or some kind of structures and/or some kind of processing/treatment of at least one of the surface of the sight glass, chosen according to the environment wherein the sight glasses are to be used.

The coating(s) may be for example, PTFE (Polytetrafluorethylene), also known as Teflon©, being chemically resistant, hydrophobic and with non-stick properties. Other examples are Sol-gel derived materials, where the materials derived by this technique cover a wide range of materials, from spin-on glasses to ceramics, metal-chlorides, polymers etc. A further example is DLC (Diamond-Like-Carbon), since DLC has good IR-transmission and can be used as antireflective coating on e.g. silicon or germanium. Furthermore it is mechanically highly robust and shows also good chemical stability. Even a further example is TiO2 containing layers having self-cleaning properties when combining superhydrophilic properties with absorption of UV-radiation, thereby creating activated oxygen that decomposes organic dirt that can be washed off the surface. Superhydrophilic properties might also prevent fogging. These materials are just given as examples, any other materials, processes or surface structures as known by a craftsman will also apply to the present invention.

Alternatively, or additionally, to the coating, the surface(s) of the sight glass may be structured on the micro- and the nanoscale (e.g Lotus-effect), for example to make it superhydrophobic or superhydrophilic in order to achieve a self-cleaning effect.

The base material of the sight glass may be any known within the field of glass making and general making of transparent materials. Among many suitable materials are such as Silicon, Germanium, CaF2, Sapphire etc. The sight glass may further be formed as a pass band filter letting through only selected wavelengths, either preventing the passing of wavelengths above or below some wavelength, or within some band of wavelengths. Such a bandpass filter generally is formed as a transparent base material with some coating letting through wavelengths above or below the desired wavelength, or with both kinds of coatings forming a band of wavelengths allowed to pass. Such secondary coatings preferably would be formed between the base material and the above described primary protective coating, or alternatively, on a surface of the sight glass not to be in contact with the environments.

The invention also relates to a sensor device, especially a gas sensor, equipped with such a sight glass.

A further problem is, since tt is known that many ambient factors such as changes in temperature, humidity, strain, vibration, stress etc. affects constructions in such a manner, that the alignment of two elements may fluctuate in relation to fluctuations of such ambient conditions.

Therefore, a further aspect of the present invention, especially relevant, but not excluded to, where a non-natural light source is introduced as a part of the sensor. Since it is critical that the light source and the detection parts of the sensor are correctly aligned at any time, because, if the position of source and reflector relative to each other is changed, the sensor properties are affected.

Further, misalignment and inclination of the sight glasses relative to the light source might also affect the properties of the system.

Therefore, in order to ensure a correct alignment of the light source as well as the sight glasses relative to the detector, a fixing structure is introduced having two ends, such that the sensor device is fixed at the first end, and the light source/emitter is fixed at the second end. The fixing structure preferably is hollow and equipped with a sight glass at each end in order to ensure optic communication from the light source to the detector device. It is further a known situation, that the amount as well as the spectral distribution of radiation of a emitter has a dependence of the temperature of the emitter. This is given by the well known Planck's distribution of radiation. Given a temperature of the emitter, a Planck curve then gives the dependence of the radiation to the wavelength, where the Planck curves has a maximum radiation at some wavelength, where maximum radiation value as well as the wavelength of the maximum radiation depends on the temperature.

Using a natural source in sensor systems such like the one described in for example US 2008/0283753, would make the pass bands of the filters change in energy (or in other words, the radiation intensity density) over the band of wavelengths.

This construction is able to compensate for changes in the intensity of radiation of the (IR) source, however, is not robust to for example temperature changes of IR source.

It is therefore a further aspect of the present invention to introduce a sensor introducing methods to solve these problems.

This is solved by ensuring that the suspect filter and the reference filter(s) have different cut-off wavelengths, in that both the both lower wavelengths and upper wavelengths differ. The "lower wavelength" is the lowest wavelength from which the filters allow passage of radiation, and the "upper wavelength" is the highest wavelength higher that the lower wavelength, from which the filters shuts off passage of radiation.

The ranges of allowed wavelengths of the suspect filter are in the following being referred to as the "suspect band", and the allowed wavelengths of the reference filter(s) are in the following being referred to as the "reference band(s)". As written, the suspect lower wavelength in the present invention is different to the reference lower length(s), and the suspect upper wavelength is different to the reference upper wavelength(s). This has the advantage that changes, such like the spectral distribution of the intensity of the incoming radiation, for example caused by temperature fluctuations of the source, can be compensated by distributing the reference band(s) above and below the suspect band. In one preferred embodiment of the present invention, this distribution is so that by a change in temperature, the increase in radiation intensity (or intensity density or energy) over the reference band roughly equals the increase in radiation intensity (or intensity density or energy) over the suspect band.

In one alternative or additional embodiment, the mean value, or average, of the radiation intensity density (or energy) over the suspect band roughly equals the mean value, or average, of the radiation intensity density (or energy) over each of the reference bands.

In one alternative or additional embodiment, the radiation intensity density (or energy) over the suspect band roughly equals the mean value, or average, of the radiation intensity density (or energy) over the whole of the combined reference bands, (the 'reference filter system band' is the combined reference bands of all the reference filters).

In another alternative or additional embodiment, the radiation intensity density (or energy) over the suspect band roughly equals the mean value, or average, of the radiation intensity density (or energy) of one of or each of the reference bands.

The filters of the present invention may be formed by filter elements in series, or by one single filter element. When two or more filters are arranged as filter elements in series, they are arranged one after the other in the radiation direction, that is to say between the radiation source(s) and the detectors. The sensor with advantage may operate within any radiation wavelength, and the source may be any radiation source.

The example in the following describes a sensor for determining the CO₂ content in an environment where a IR source would be preferred as light source, however, any other substances than CO₂ would also apply to the present invention, just as any other light source than within the IR band would apply.

In a further embodiment of the present invention, at least one reference filter (to be called the first reference filter) has a reference band, called the first reference band, with a wider span of wavelengths than the suspect band, where the first reference lower wavelength of this first reference filter is at a lower wavelength than the suspect lower wavelength, and the first reference upper wavelength of this first reference filter has a higher wavelength than the suspect upper wavelength. In this manner, the suspect band overlaps the first reference band.

In this embodiment, the centre wavelength of the first reference band (the first centre reference wavelength) and the centre wavelength of the suspect band may be the same, or may be different.

For a change in temperature, the relative change in intensity in the suspect and reference band must be equal in order for the temperature dependency to cancel out.

When using radiation sources, actively powered or natural the relative change in intensity depends unlinearly on the wavelengths spanned by the bands. Therefore the unmatching centre wavelength can be introduced to improve stability to temperature drift.

In this example, the reference filter(s) advantageously has a pass band that is from 0.2 to 1 μm greater than the pass band of the suspect filter. It is desirable for the suspect filter to cover basically only a relatively narrow wavelength range or spectral range of the radiation spectrum, for example the range in which IR radiation is absorbed by CO₂. The range indicated is sufficient for this. The risk that absorption by other gases will have an adverse effect on the measurement result and falsify that result is kept small.

It is preferable here for the first reference filter to have a pass band in the range from 4 to 4.5 μm and the suspect filter to have a pass band in the range from 4.1 to 4.4 μm. In dependence upon the gases or other quantities being detected, those spectral ranges can of course also be shifted.

In another preferred embodiment of the present invention, the system comprises a first and a second reference filter with a first and second reference band respectively (together constituting the combined reference bands), where the first and second reference bands are non-overlapping, meaning they span no common wavelengths. This may be an advantage if there are other gasses etc. in the environment than the gas(ses) of interest, with absorption bands in the vicinity of the suspect band, that could influence the measurements, in that it is difficult to avoid overlapping a reference band with such 'pollution' bands . By ensuring that at most one references band is affected by such a 'polluting' absorption band, it will be known that at least the other is unaffected.

In one preferred version of this embodiment, at least one of the first or second reference bands overlaps the suspect band, meaning that the first reference upper wavelength is at a higher wavelength than the suspect lower wavelength, and/or the second reference lower wavelength is at a lower wavelength than the suspect upper wavelength, but at a higher than the first reference upper wavelength, thus leading to the first and second reference bands extending at each side of the suspect band, but without overlapping.

In another preferred version of this embodiment, the first reference upper wavelength is at a lower wavelength than the suspect lower wavelength, and the second reference lower wavelength is at a higher wavelength than the suspect upper wavelength, thus leading to the first and second reference bands extending at each side of the suspect band.

In an alternative embodiment, the first and second reference bands are overlapping having at least one common wavelength.

In an especially preferred configuration, the sensor uses the natural radiation, such as IR radiation, from the environment. There is therefore no need for a source of radiation that needs a separate power supply and accordingly has a certain power requirement. IR radiation is generally present everywhere, even when there is no incident sunlight. In principle every body emits a certain amount of thermal radiation. Because it is then possible to do without an IR radiation source, the "measurement range" is also broadened, that is to say it is possible to monitor relatively large areas of a room for the content of the gas in question. This facilitates the monitoring and establishment of a "personal room climate" or the indoor air quality. It is unnecessary first to conduct the air in the room to a sensor where it is passed between the source of IR radiation and the detectors with upstream filters. It is sufficient for the sensor to be arranged at a point in the room where it can, as it were, "survey" the volume of air to be monitored. In that case, the gas sensor can, as it were, detect the averaged gas concentration in a simple way. The sensor therefore determines an average value, which, particularly for the personal room climate, constitutes a substantially better measurement result. Of course, it is also possible to use the sensor to improve the technology of sensors that operate with lamps or other means of lighting. When natural or ambient IR radiation is used, the energy of the light means can be reduced. That results in longer maintenance intervals and a longer service life.

The filters preferably contain CaF₂, germanium or silicon. The filter and any other parts of the sensor device where it would make sense, preferably has an anti-reflective coating in order to improve transmission. The invention will be described herein below with reference to a preferred exemplary embodiment in conjunction with the drawings.

FIGURES

Fig. 1 is a diagrammatic view for explaining the operating principle of the present invention;

Fig. 2 is a schematically illustrates a sight glass with surface modifications such as coatings, being a first aspect of the present invention.

Fig. 3 is a schematically illustration of an aligning structure with fixing a detecting part and light source together, this being a further optional aspect of the present invention.

Fig. 4A-E shows, in diagrammatic form, pass bands of two or three filters;

Fig. 5 illustrates a Planck curve and a pass band.

Fig. 6 shows, in diagrammatic form, the amount of energy that can be detected by detectors;

Fig. 7A-D are block circuit diagrams for explaining different embodiments of the structure of the gas sensor;

DETAILED DESCRIPTION OF THE SYSTEM

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). The measurement region may be, for example, a room or the portion of a room in which the personal room climate is to be regulated. A sun symbol (4) represents a radiation source, such as for example a natural IR source, passive sources, or any imaginable active source (sunlight, laser, light diodes, controlled hated sources etc.) The sun symbol (4) serves here merely for explanation purposes. The gas sensor (1) also operates in the absence of sunlight, because in principle virtually any body radiates heat and thus generates IR rays.

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 figure shows a first aspect of the present invention, where a sight glass (100) is positioned in front of the detecting part (2) of the sensor (1).

Fig. 2 shows a further aspect of the present invention, being a side view of such a sight glass having a base substrate (102) and having a coating (101) and/or some kind of structures and/or some kind of processing / treatment of at least a part of one of the surface of the sight glass (100).

The coating(s) (101) may be for example, PTFE (Polytetrafluorethylene), also known as Teflon©, being chemically resistant, hydrophobic and with non-stick properties and being. Other examples are Sol-gel derived materials, where the materials derived by this technique cover a wide range of materials, from spin- on glasses to ceramics, metal-chlorides, polymers etc. A further example is DLC (Diamond-Like-Carbon), since DLC has good IR-transmission and can be used as antireflective coating on e.g. silicon or germanium. Furthermore it is mechanically highly robust and shows also good chemical stability. Even a further example is TiO2 containing layers having self-cleaning properties when combining superhydrophilic properties with absorption of UV-radiation, thereby creating activated oxygen that decomposes organic dirt that can be washed off the surface. Superhydrophilic properties might also prevent fogging. These materials are just given as examples, any other materials, processes or surface structures as known by a craftsman will also apply to the present invention.

The base material (102) of the sight glass may be any known within the field of glass making and general making of transparent materials. Among many suitable materials are such as Silicon, Germanium, CaF2, Sapphire etc.

The sight glass(es) (100) may further be formed as a pass band filter letting through only selected wavelengths, either preventing the passing of wavelengths above or below some wavelength, or within some band of wavelengths. Such a bandpass filter generally is formed as a transparent base material with some coating letting through wavelengths above or below the desired wavelength, or with both kinds of coatings forming a band of wavelengths allowed to pass. Such secondary coatings preferably would be formed between the base material and the above described primary protective coating, or alternatively, on a surface of the sight glass not to be in contact with the environments.

Fig. 3 illustrates a further embodiment of the present invention, showing the detecting part (2) fixed in a first structure (103) and a constructed light source (4) fixed in a second structure (104). Light illustrated by the arrow (107) is emitted by the light source (4) and as it passes the measuring region (3) some wavelengths are absorbed by the suspect species (5). A sight glass (100a) is positioned in front of the detecting part (2) protecting it from any dirt, particles etc. present in the measuring region (3). Further an aligning structure (105) is illustrated fixing the first (103) and second (104) structures, and thereby the detecting part (2) and light source (4), together in a manner that ensures a correct alignment despite any ambient influences. The aligning structure (105) is preferably hollow and solid enough to fix the parts (103) and (104) solid to each other, but comprises pores, openings, holes etc. (106) that allows the suspect species (5) to easily flow into and out of the internal hollow, being the measuring region (3), of the of aligning structure (105).

The illustration further shows a second sight glass (100b) covering the light source (4) protecting it's delicate parts.

Fig. 7A shows, in diagrammatic form, a block circuit diagram for explaining the structure of the simple detecting part (2) of a gas sensor (1). The detecting part (2) has a filter arrangement (6), a detector arrangement (7) and an evaluating device (8). Further details, such as the housing, fixing means or the like, are not shown herein.

The shown filter arrangement has a first reference filter (10) and a suspect filter (9), where the two filters (9) and (10) have different pass characteristics, where one embodiment is shown in Fig. 4A. The first reference filter (10) allows passing of wavelengths within the firsts reference band RB1 , and the suspect filter (10) allows the passing of wavelengths within the suspect band SB. In the following figures the radiation dependence of wavelength is not seen. The embodiment in Fig. 4B shows the first reference band RB1 spanning wider than the suspect band SB, but where the suspect band SB overlaps the first reference band RB1 in such a manner, that the first reference band RB1 comprises the same wavelengths as the suspect band SB. The first reference lower wavelength RLW1 therefore is at a lower wavelength than the suspect lower wavelength SLW, and the first reference upper wavelength RUW1 has a higher wavelength than the suspect upper wavelength SUW. The first reference band RB1 has a first centre wavelength RCW1, and the suspect band has a suspect centre wavelength SCW. The figure shows the two bands having a common centre wavelength RCW1 and SCW.

Fig. 4B shows a related embodiment to that shown in Fig. 4A, only where they dissimilar centre wavelengths RCW and RCW1. For a change in temperature, the relative change in intensity in the suspect and reference band must be equal in order for the temperature dependency to cancel out. When using radiation sources, actively powered or natural the relative change in intensity depends unlinearly on the wavelengths spanned by the bands. Therefore the unmatching centre wavelength can be introduced to improve stability to temperature drift.

Fig. 5 illustrates the situation of a source emitting with a spectral distribution represented by a general Planck curve having a maximum radiation at the wavelength λmax, and having a continuously decreasing radiation for increasing wavelengths above λmax, so using a band Δλ between two such wavelengths λ1 and λ2. The radiation R1 at the lower wavelength λ1 being larger than the radiation R2 at the upper wavelength λ2.

Therefore, to ensure the same mean values (or averages) of radiation, unmatching centre wavelengths are introduced giving different spans of wavelengths of the reference band(s) below and above the suspect band respectively, where these different spans then compensates the changing radiation intensity.

Fig. 4C shows another embodiment where a second reference filter (20) has been introduced into the system spanning over a second reference band RB2 extending from a second reference lower wavelength RLW2 to a second reference upper wavelength RUW2. The shown embodiment further has the suspect band SB only partly overlapping both the first and second reference bands RB1 and RB2 in such a manner, that the suspect lower wavelength SLW is between the first reference lower wavelength RLW1 and the first reference upper wavelength RUW1. The suspect upper wavelength SUW is between the second reference lower wavelength RLW2 and the second reference upper wavelength RUW2. The shown embodiment has the first reference upper wavelength RUW1 being higher than the second reference lower wavelength RLW2, but in other embodiments the first and second reference bands RB1 and RB2 might not overlap, meaning that the first reference upper wavelength RUW1 would be lower or equal to the second reference lower wavelength RLW2.

Fig. 4D shows an alternative embodiment with two reference filters (10) and (20), where none of the reference bands RB1 and RB2 at least substantially overlaps the suspect band SB, at least, but extends at each side of it, here meaning, that the first reference upper wavelength RUW1 is not higher than the suspect lower wavelength SLW₁ but could optionally be the same, and the second reference lower wavelength RLW2 is not lower than the suspect upper wavelength SUW, but could optionally be the same. The figure shows the two reference bands RB1 and RB2 having substantially the same pass range of wavelengths, but as seen in Fig. 2E this may not be the case, the two reference bands RB1 and RB2 might have very different pass ranges of wavelengths.

The relative positions and sizes of the bands depends on a number of factors, such as the tolerances of the edges of the filters, the width of the suspect bandpass, the distribution of the absorption lines of the suspect band, and of any other gasses that might cause cross sensitivities.

In the example of the sensor (1) operating as a CO₂ sensor, there is a spectral range λ (CO₂) in which IR radiation is absorbed by CO₂. That spectral range is located at about from 4.2 to 4.3 μm. Accordingly, the suspect band SB could with advantage have a suspect lower wavelength SLW at about 4.0 μm and a suspect upper wavelength SUW at about 4.5 μm, or with an even more narrow range of the suspect band from 4.1 μm - 4.4 μm, or any other band covering the spectral range of CO₂ The reference start and upper wavelengths then with advantage could extend about 0.5 μm above and below the suspect lower wavelength SLW and suspect upper wavelength SUW respectively.

Fig. 6 illustrates a first reference band RB1 and the suspect band SB of the first embodiment of the invention as seen in Fig. 5, where the suspect band has a unreduced energy indicated by reference letter A. That energy is reduced by an amount C which is absorbed by for example CO₂. The two sections of the first reference band RB1 extending at each side of the suspect band each has an energy indicated by reference letters B. That energy is virtually constant, because it is not affected by for example CO₂.

Fig. 7A shows one simple embodiment of a construction of a filter arrangement (6), where the suspect filter (9) comprises two filter elements (11) and (12), the first suspect filter element (11) defining the suspect upper wavelength SUW and having a lower wavelength lower than the suspect lower wavelength SLW. The second suspect filter element (12) defines the suspect lower wavelength SLW and has an upper wavelength substantially higher than the suspect upper wavelength SUW. In the same manner the first reference filter (10) comprises two filter elements (13) and (14) defining the first reference upper wavelength RUW1 and the first reference lower wavelength RLW1 respectively. Depending on the number of filters like (9) and (10) introduced into the system, any number of such constructions of filter elements (11), (12), (13) and (14) may be introduced into the filter arrangement (6). Some filter elements in this and any other embodiment may be common to two or more of the filters when the filters have the same end and/or lower wavelength, this being illustrated in Fig. 7B, where the two 'upper' filter elements (11) and (13) is one common filter element.

Fig. 7C shows a similar sensor having a extra reference filter, the second reference filter (20), and where each filter only has a single filter element (21 , 22, 23) comprising the desired band pass characteristic both for the upper and lower wavelengths, the suspect filter (21) thus both defining both the suspect lower wavelength SLW and upper suspect wavelength SUW. The first reference filter (22) defining both the first reference upper and lower wavelengths RUW1 and RLW1 , and the second reference filter (23) defining both the second reference upper and lower wavelengths RUW2 and RLW2. The two filter elopements (22, 23) are in this illustrated embodiment connected to the same detector (16) though in reality what would be none, is to add their signals mathematically after they have been acquired by for example two separated Thermopiles. Fig. 7D shows an embodiment related to that of Fig. 7C, only where a third detector (24) is connected to the second reference filter (20).

It shall be noted that any combination, permutation, number and positioning of filter elements (11 , 12, 13, 14) as for example disclosed in Figs. 5A-D would apply to the present invention.

In general the sensor could also be used to measure more than one gas, then just including the needed number of sensors, detectors etc., as it will be known to a craftsman.

Because, in a thermopile sensor, usually a temperature measurement is carried out (because the output signal varies with temperature), measurement of the temperature around the sensor has already been incorporated. As it is conceivable that the radiation temperature of the room is also obtainable by means of the sensor, it is possible on the basis of those two measurements simultaneously to obtain directly an operating temperature which can then be used for controlling the room temperature or something quite different.

In connection with IR it is also conceivable that measurement of a movement in the room is directly possible with the sensor, which can then be used, for example, for controlling a ventilating system, which, for example, is activated only in the event of a movement indicating that there is someone in the room. On the basis of various movement measurements it is also conceivable that it would be possible to estimate the number of people in the room, such an estimate also being usable for control purposes, so that the room temperature or the ventilation is controlled / modified in dependence upon the number of people in the room.

The basic sensor of this invention such as the one seen in fig. 6A operates by the two signals S1 , S2 being supplied to the evaluating device (8). Accordingly, this gives 51 = a (IC0₂n)

52 = a (U)₁

where IcO₂ '^s *^ne electrical quantity, for example the current or the voltage, containing the information relating to the IR absorption, while l_ref is the reference quantity that is not affected by the IR absorption. When the difference between S1 and S2 is formed (the "effective reference" being the part of the reference band which does not include the suspect band), for which purpose a difference former (17) is shown diagrammatically, the following quantity is obtained:

S^ - S2 = a (IcO₂ - Iref )

That difference S1 - S2 is normalised to the output signal S1 of the first detector (15), so that a signal S3 is obtained.

_{S3 =} si _ ^afW effective Reference = (S2 - S 1 ) a(l_Ref ;

The sensor of this invention may be used to measure any kinds of gas such like for example nitrogen, nitric oxides, oxygen or CO, and is not even limited to measure gasses, but may also be used to measure the suspect in other forms like liquids and solids. When changing suspect from CO₂, the pass bands would have to be shifted accordingly, for example the absorption band of H2O is around 2.7 μm

The sensor of the present invention may further comprise any possible other optical components, for example a sapphire window, that acts as additional band pass filter, reflectors, a collecting device, being a device that gathers or focuses for example IR radiation, for example a collimator, positioned upstream of the sensor, etc. It is also possible to use such a sensor directly for waste gas monitoring. For that purpose, it is installed in the chimney or exhaust. Particularly in the case of heating systems, combustion can then be controlled with the aid of the output signals of the sensor (or of a plurality of sensors).

This invention is not excluded to the above descriptions and drawings, any permutation of the above descriptions and drawings, including any number and permutations of filters such as suspect filters (9) and reference filters (10, 20), filter elements (21 , 22, 23), detectors (15, 16, 24) etc. would also apply to the present invention.

Further, this invention is not excluded to measuring gasses, the sensor may as well be implemented in measuring substances in general being a part of a media, where the media is not excluded to be a gas it self, but could for example be a liquid.

Claims

1. Sensor for measuring substances in an environment, the sensor having a detection part at least partly being separated from the environment by a sight glass being a material transparent to at least some specified wavelength of light.

2. Sight glass according to claim 1 , wherein at least the surface of the sight glass exposed to the environment comprises one of or any combination of coating(s), surface processing modifying the surface structure, and/or surface structures.

3. Sensor according to clam 2, wherein the detection part comprises 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 having a suspect filter passing through radiation with wavelengths within a suspect band, and a reference filter system with at least one reference filter, each reference filter passing through radiation with wavelengths at least within a reference band, and where the reference filter system band is the combined reference bands of all the reference filters; the detector arrangement has at least two detectors, each of which is associated with a filter.

4. Sensor according to claim 3, where the reference system band is distributed on both sides of the suspect band.

5. Sensor according to one of claims 1-4, wherein the sensor further includes a light source, and where an aligning structure fixes the light source to the detecting part in such a manner, that their relative position are substantially unaffected by changes in ambient conditions such as temperature, humidity, strain, vibration, stress, the aligning structure further forming an path for the light emitted by the light source to the detecting part.

6 Sensor according to claim 5, wherein the aligning structure is a hollow body, the internal hollow forming an path for the light emitted by the light source to the detecting part.

7. Sensor according to claim 6, wherein the aligning structure has holes in the wall making open passes for gasses.

8. Sensor according to claim 5, wherein the aligning structure is a hollow tube, one or more rods, or a frame.

9. Sensor according to any of the preceding claims, wherein the sight glass further operates as a band pass filter letting through only radiation within a band of wavelengths.

10. Sensor according to any of the preceding claims, wherein the coating(s) of the sight glass comprises PTFE (Polytetrafluorethylene), Sol-gel derived materials, DLC (Diamond-Like-Carbon) and/or TiO2 containing layers

11. Sensor according to claim 9, wherein at least one surface of the sight glass has superhydrophilic or superhydrophobic properties, for example formed by structures on the micro- and/or the nano-scale.