WO1997010496A1

WO1997010496A1 - A device for measuring the concentration of carbon dioxide in a gas

Info

Publication number: WO1997010496A1
Application number: PCT/SE1996/001129
Authority: WO
Inventors: Andras Gedeon; Paul Krill
Original assignee: Icor Instruments Ab
Priority date: 1995-09-11
Filing date: 1996-09-11
Publication date: 1997-03-20
Also published as: WO1997010497A1; SE9503139L; SE9503139D0

Abstract

A device for quantitatively measuring CO2 in a gas mixture, such as air, comprises an indicator unit (23) including a CO2 indicating element (26) whose ability to return incident light varies with the CO2 concentration of a gas contacting the indicating element, a light source (18, 19) for illuminating the indicating element, and photoelectric light measuring means (21) for measuring the intensity of light returned from the indicating element upon illumination thereof by the light source. The indicator unit (23) includes a cavity (23A) receptive of one end of a light guide (15, 17), one end of which is optically coupled to the indicating element (26) and the opposite end of which is optically coupled to the light source (18, 19) and to the light measuring means (21).

Description

A device for measuring the concentration of carbon dioxide in a gas

This invention relates to a device for measuring the concen- tration of carbon dioxide, C0₂, in a gas mixture, especially air and preferably respiratory air, that is, air containing C0₂ in a concentration of about 7 percent or less.

More particularly, the invention relates to a C0₂ measuring device which comprises an indicator unit supporting a C0₂ indicating element whose ability to return incident light is a function of the C0₂ concentration of a gas being in contact with the indicating element, a light source for illuminating the indicating element, and a photoelectric measuring means for measuring the intensity of light returned from the indi¬ cating element upon illumination of the indicating element by means of the light source.

A prior art device of this kind is disclosed in U.S. Patent No. 3 754 867. In this prior art device the indicator unit, the light source and the photoelectric measuring means with associated electric components are enclosed in a light-tight box into which the gas mixture to be examined in respect of its C0₂ concentration is fed through an intake opening. It is necessary, therefore, to employ a pump for feeding the gas mixture to the indicator unit. As a result, the device is expensive and cumbersome.

Other prior art C0₂ measuring devices of the same kind also suffer from drawbacks which affect their usefulness.

The present invention aims at providing a simple and versa¬ tile C0₂ measuring device.

To this end, the device according to the invention is con¬ structed as set forth in the independent claims . The depen¬ dent claims define advantageous embodiments of the device. An exemplary embodiment of the C0₂ measuring device according to the invention will be described below with reference to the accompanying drawings, in which:

Fig. 1 is a diagrammatic illustration of a C0₂ measuring device embodying the invention;

Fig. 2 is an enlarged longitudinal sectional view of a sensor unit which forms part the measuring device shown in Fig. 1;

Fig. 3 is a diagram showing the relationship of a photo¬ electric detector signal and the C0₂ concentration of a gas mixture being examined;

Fig. 4 is a diagrammatic illustration of a curved portion of an optical-fibre cable forming part of the device according to the invention.

Main components of the C0₂ measuring device shown in Fig. 2 are a detector unit 11 which includes a current supply 12

(may be connected to a battery or to the mains) and a display 13, a sensor unit 14, and a flexible light guide or optical transmission line in the form of an optical-fibre cable 15, one end of which is separably connected to the sensor unit 14 and the other end of which is detachably connected through a connector 16 to a light guide in the form of an optical-fibre cable 17 of the detector unit 11.

The detector unit 11 comprises two light sources in the form of light-emitting diodes 18 and 19. These light-emitting diodes emit pulsed light, the pulse frequency being higher than 10 Hz. Preferably, at least one of the light-emitting diodes emits light in the visible range. The light from the light-emitting diodes is fed to the optical-fibre cable 17 of the detector unit through the limbs of a Y-connector 20. Moreover, the detector unit 11 comprises a photoelectric detector 21 positioned such that it is illuminated by light transmitted through the adjacent end portion 22 of the optical-fibre cable 17 in the direction away from the connector 16 past the Y-connector 20. Detector 21 may be of the wide-band type so as to have substantially the same sensitivity to light over a wide range of wavelengths, but it may also be matched with the wavelengths of the light- emitting diodes.

The sensor unit 14 comprises an elongate tubular indicating element holder 23 (indicator unit) which constitutes a female part of a plug-and-socket connector by means of which the optical-fibre cable 15, which comprises a monofilament optical fibre of circular cross-section (such as Super ESKA™, Mitsubishi Rayon Co., Ltd., fibre diameter 1 mm), is separably connected to the detector unit. Extending through the indicating element holder 23 is an axial passage 23A which is widened at the rear end portion of the indicating element holder to form a socket 23B for receiving the male part 24 of the plug-and-socket connector in snap-fit fashion.

The optical-fibre cable 15 is secured in an axial passage of the male connector part 24 such that its end portion protru¬ des forwardly a short distance beyond the male part . When the male part is inserted in the socket 23B in the indicating ele¬ ment holder 23, the protruding portion of the optical-fibre cable 15 is received in the front portion of the passage 23A, which is circular in cross-section.

At the front portion of the indicating element holder 23, the passage 23A is slightly widened in relation to the portion of the passage which receives the forwardly protruding portion of the optical-fibre cable 15. The widened passage portion defines a sensor compartment 25 at the inner end or bottom of which a sheet-like indicating element 26 is positioned which is constructed and arranged such that it forms a substanti- ally gas-tight barrier between the sensor compartment 25 and the portion of the passage 23A which is behind the sensor compartment. The portion of the indicating element holder 23 which protrudes forwardly beyond the indicating element 26 provides mechanical protection for the indicating element.

The indicating element 26 is of the known type which contains a dye which changes its optical properties when it is brought in contact with C0₂. More particularly, the ability of the indicating element to return incident light varies in depend¬ ence on the C0₂ concentration of a gas which contacts the indicating element. Characteristic curves for indicating ele- ments of the above-mentioned kind are shown in Fig. 3.

The indicating element 26 is comprised of two layers, one transparent, substantially gas-impermeable layer 26A, such as a 0.2 mm polyethylene film, and a layer 26B of paper or other porous material, of 0.1 mm thickness, for example, which is impregnated with the dye. The layer 26A is on the side of the indicating element, the rear side, which faces away from the sensor compartment 25, while the layer 26B is on the opposite side, the front side, and thus is exposed to the gas in the sensor compartment.

Suitably, the side of the layer 26A which faces away from the sensor compartment 25 is coated with a layer 27 (indicated by a heavy solid line) of an optical index-matching or coupling fluid, that is, a substance which has approximately the same refractive index as the material of the optical fibre 15 and which in the illustrated interconnected position of the plug- and-socket connector interconnects the layer 26A and the face of the optical fibre. Such an optical coupling fluid may be, for example, castor oil or so-called microscope oil.

Fig. 3 is a diagram showing characteristic curves for two indicating members 26 chosen by way of example, namely curves showing the relationship between the C0₂ concentration of a gas mixture to which the dyes of the indicating members are exposed and the electric signal provided by the photodetector 21 when the indicating members are illuminated by way of the optical-fibre cable 15. Both indicating members are impregna- ted with the same kind of dye (thymol blue) but the dye con¬ centration of the indicating element to which the lower curve A applies is four times that of the indicating element to which the upper curve B applies. As is apparent from Fig. 3, the higher dye concentration provides better linearity.

With each indicating element 26, a C0₂ concentration rising above the range covered by the diagram is associated with a progressively diminishing slope of the curve so that the curve for practical purposes can be regarded as horizontal for C0₂ concentrations above a certain limiting value. Above this limiting value, which is considerably higher for curve A than for curve B, a rising C0₂ concentration will not result in any significant change of the signal. Consequently, the useful measuring range only extends to this limiting C0₂ concentration value. This means no substantial limitation of the application of the measuring device according to the invention which is the main application that is foreseen, namely measuring the C0₂ concentration of respiratory gases, because the C0₂ concentration of such gases rarely exceeds 7 percent. However, in addition to the advantage resulting from the better linearity in comparison with curve B, the indicat¬ ing element to which curve A applies provides a somewhat wider practical measuring range.

Because the relationship of the C0₂ concentration and the signal is such that there is a relatively wide range between the limiting concentration and 100 percent C0₂ in which for practical purposes the signal is independent of the C0₂ con- centration, there is a possibility for a simple and rapid calibration of the measuring device as will be described below.

As shown in Figs. 1 and 2, the front portion of the indicat- ing element holder 23 is provided with a removable protective cap 28 the front portion of which together with the indicat¬ ing element holder forms a compartment 28A which is in open communication with the sensor compartment 25. When the pro- tective cap 28 is applied to the indicating element holder 23 during manufacture of the sensor unit 14, the compartments 25 and 28A are filled with pure C0₂ or a gas mixture of a very high C0₂ concentration, far above the limiting concentration. Then the sensor unit 14 formed by the indicating element holder 23 and the protective cap 28 is enclosed in a gas¬ tight envelope 29, indicated by a dash-dot line in Fig. 2, which is preferably filled with pure C0₂ or a gas mixture likewise of a C0₂ concentration far above the limiting con- centration. The envelope may be made from any suitable mate¬ rial, such as plastic or metal or metallized foil, that is substantially diffusion-proof so that the envelope will be capable of holding the calibration gas without substantial reduction of the C0₂ content thereof for extended storage periods.

Advantageously, the sensor unit 14 formed by the indicating element holder 23 and the protective cap 28 may be used as a disposable item which is thus discarded after a single use. The sensor unit can be manufactured and packaged at low cost and the advantages in respect of hygiene, for example, re¬ sulting from one-way use, coupled with the low production cost, means that one-way use is justifiable. When made as a disposable item, the sensor unit may also include the optical-fibre cable 15, and possibly even the optical-fibre cable 17, that is, the entire optical-fibre cable assembly between the indicating element 26 and the detector 21. In the latter case, both optical-fibre cables 15 and 17 may be com¬ prised of a single continuous optical fibre.

When a measurement is carried out using the above-described measuring device, the procedure is as follows.

The sensor unit 14 is taken out of the gas-tight package and connected to the flexible optical-fibre cable 15 without re¬ moving the protective cap 28. The C0₂ concentration value then obtained is stored by the electronic system of the mea¬ suring device. Because of the very high C0₂ concentration in the sensor compartment 25, this measured value corresponds to maximum colour change of the indicating element 26, that is, a maximum photodetector signal .

Then the protective cap 28 is removed so that the indicating element 26 is exposed to ambient air. This means that the indicating element is exposed to air of a very low C0₂ con¬ centration, about 0.03 percent. The measured value obtained with this concentration is also stored. Using these two stored measured values and previously stored values repre¬ senting the signal-versus-concentration curve of the indi¬ cating element being used, the electronic system automati¬ cally carries out a calibration of the measuring device.

The sensor unit 14, the protective cap 28 having been remo¬ ved, can then be connected to a device, such as a hose coupling or other connector, which contains or transports the gas to be examined. For example, such connection can be brought about by inserting the narrow front portion of the indicating element holder 23 in a suitably shaped socket in the gas-containing or gas-transporting device. The front portion of the indicating element holder 23 advantageously can be shaped as a standard connector part (Luer fitting) so that it will mate with the fittings which are common in medical apparatus or instruments.

Whenever the sensor unit 14 is replaced, the above-described calibration is carried out. As the sensor unit is enclosed in a C0₂-filled gas-tight envelope until it is used, the protec- tive cap need not be perfectly tight-fitting. It suffices for it to be able to retain most of the gas in the compartments 25 and 28A for the normally short period from the opening of the envelope to the completion of the first calibration step.

Naturally, the calibration gas in the form of ambient gas which is caused to contact the indicating element 26 in the second calibration step may be supplied otherwise than by removing the protective cap 28 or a similar component from the sensor unit. As an exemplary alternative, a seal can be ruptured or a valve can be opened to permit a rapid gas exchange in the space adjoining the indicating element.

The above-described calibration procedure, in which one first uses a calibrating gas supplied together with and constantly being in contact with the indicating element until a measure¬ ment is to be carried out, and whose concentration of the relevant substance is well above the limiting concentration so that the gas provides the same signal regardless of the actual concentration of the substance, and then uses an ambient gas whose concentration of the relevant substance is always the same, is useful also in measuring devices of kinds other than that described above and for substances other than C0₂, provided that the relationship between the concentration of the substance and the signal representing the concentra¬ tion includes a wide range in which the signal may be regar¬ ded as independent of the concentration.

A substantial improvement of the reproducibility of the mea¬ surements carried out using the measuring device according to the invention can be achieved by imposing on at least one section of the optical-fibre cable assembly 15/17 a bend of a permanent or fixed radius curvature which is preferably shorter than the shortest radius of curvature that the flex¬ ible portion of the optical-fibre cable is expected to have in use of the measuring device.

Such bend should subtend a certain minimum angle, at least 90°, for example, and advantageously subtends a much larger angle, such as a full 360° turn. Preferably, the radius of curvature and the angle are chosen such that at least 3 and preferably 10 percent of the intensity of a signal of diffuse light fed into the optical-fibre cable will be lost in the bend.

For practical reasons it is normally preferable to position the bend in the detector unit 11 or in a housing which enclo- ses the detector unit and the current supply 12 and other "stationary" components of the measuring device. However, it is also within the scope of the invention to position the bend anywhere between the ends of the optical-fibre cable assembly. For example, the illustrated embodiment may be modified such that the bend is integrated in the part of the optical-fibre cable assembly which comprises the flexible optical-fibre cable section 15.

A permanent bend comprising a full 360° loop is shown in Fig. 1 and embodied in the optical-fibre cable portion 17 provided in the current supply 12. Fig. 4 is a heavily en¬ larged view of a bend subtending only a quarter of a full turn, corresponding to a change of direction of the optical- fibre cable of 90°, and serves to illustrate the function of the bend of the optical-fibre cable section as a kind of fil¬ ter which eliminates the disturbances of the light transmis¬ sion which would otherwise result from varying bending of the optical-fibre cable during measurements.

In practical use of the device, also during the actual mea¬ surement, and thus while light is being transmitted, the flexible optical-fibre cable section 15, which may be several metres long and very flexible, may change its configuration in an indeterminate or random way. In other words, a certain portion of the optical-fibre cable may move in a random way relative to other portions.

If an optical fibre is subjected to randomly varying bending along its length, the light transmission characteristics of the fibre will be affected in the fibre region where the bending varies so that the light transmission characteristic of the entire fibre cable may vary in an unpredictable way. This is true at least in respect of diffuse light, that is, light which is uncollimated and thus enters the optical fibre at angles distributed over a wide range, as is the case with light which is returned from the indicating element 26 into the optical-fibre cable portion 15. When light is propagated along an optical fibre, the light is reflected at the interface between the fibre core and the cladding surrounding the core; the refractive index of this cladding is different from that of the core . Most of the light has such a large angle of incidence to the interface that total reflection occurs at the interface. However, light having an angle of incidence (as measured from a normal to the interface) which is smaller than the critical angle, here designated as __G, for total reflection, will be radiated out through the cladding and will thus be lost from the optical fibre.

Light having an angle of incidence close to the critical angle α_G for total reflection is sensitive to a reduction of the curvature of the optical fibre. If the radius of curva¬ ture of the optical fibre is reduced locally, light which is totally reflected before it reaches the location where the curvature is reduced may have an angle of incidence at that location which is smaller than the critical angle a_G so that the light will be radiated out of the fibre.

The above-mentioned intentional and fixed or permanent bend which is provided in the optical-fibre cable and has a radius of curvature smaller than the smallest radius of curvature likely to result from varying bending during use of the measuring device, ensures that substantially all light which enters the optical-fibre cable at an angle which is, so to speak, in the dangerous zone will be caused to leave the fibre through the cladding because the intentional sharp bend makes the angle of incidence of such light fall below the critical angle α_G. Accordingly, only light which enters the optical-fibre cable at an angle sufficiently close to the longitudinal axis of the optical fibre to avoid the dangerous zone will be transmitted the full length of the cable.

This filtering out of such light is illustrated in Fig. 4, which shows a 90° bend the ends of which are marked by two orthogonal lines K. Two light rays symbolized by full lines h. represent light having an angle of incidence sufficiently larger than the critical angle α_G so that it will be above the critical angle even in the bend, while a broken-line light ray L₂ represents light whose angle of incidence up- stream of the bend is only slightly larger than the critical angle __G and in the bend will therefore become smaller than the critical angle __G so that the light will escape through the cladding surrounding the fibre core.

The intentional bend in the optical-fibre cable may be per¬ manent in the sense that it cannot be changed or removed without subjecting the optical-fibre cable to substantial external forces. It need not be permanent in that sense, however; within the scope of the invention it may be produced by means of suitable retaining means which clamps or other¬ wise stabilizes the optical-fibre cable in a certain curved configuration which may, however, be removed or changed when the clamping or stabilizing effect of the retaining means is eliminated. What is essential is that the intentionally cur- ved optical-fibre cable section does not change its curvature while the measurement is carried out.

Naturally, the radius of curvature and the length of the in¬ tentionally curved portion should be chosen so as to elimi- nate a substantial influence on the measuring resulting from such effects as are caused by light the angle of incidence of which is in the sensitive or dangerous zone as explained above. Generally, the size of the radius of curvature and the extent of the curved optical-fibre cable portion, as expres- sed in terms of the change of direction of the optical-fibre cable portion between the ends thereof, should be so chosen that a light signal being transmitted through the optical- fibre cable loses at least 3 and preferably 10 percent of its intensity in the bend.

Claims

1. A device for quantitatively measuring C0₂ in a gas mixture, such as air, comprising an indicator unit (23) including a C0₂ indicating ele¬ ment (26) whose ability to return incident light varies, at least within a concentration range, with the C0₂ concentra¬ tion of a gas contacting the indicating element, a light source (18, 19) for illuminating the indicating element, a photoelectric light measuring means (21) for measuring the intensity of light returned from the indicating element upon illumination thereof by the light source, characterised in that the indicator unit (21) includes a cavity (23A) receptive of one end of a light guide (15, 17) and in that one end of the light guide is optically coupled to the indicating element (26) and the opposite end of the light guide is optically coupled to the light source (18, 19) and to the light measuring means (21) .

2. A device according to claim 1, characterised in that the indicator unit (23) and the light guide (15, 17) are interconnected by means of a separable connector (24) , prefe- rably a plug-and-socket connector.

3. A device according to claim 1, characterised in that the indicator unit (23) and at least a section of the optical fibre (15, 17) are inseparably interconnected and separable as a unit from the light source (18, 19) and the light measuring means (21) .

4. A device according to any one of claim 1-3, characterised in that the indicating element (26) comprises a transparent layer (26A) , which is substantially impermeable to gas and one side of which faces the cavity (23A) , and a dye-bearing indicating layer (26B) on the opposite side, the transparent layer (26A) forming a substantially gas-imperme- able barrier between the cavity (23A) and the dye-bearing layer.

5. A device according to claim 4, characterised in that the side of the transparent layer (26A) which faces the cavity (23A) supports a layer (27) of an optical coupling fluid.

6. A device according to claim 4 or 5, characterised in that the indicator unit (23) on said one side of the dye- bearing layer (26B) is formed with a compartment (25) for receiving a gas the C0₂ concentration of which is to be mea¬ sured, and in that the indicator unit projects beyond the indicating element (26) in the direction away from the cavity (23A) .

7. A device according to claim 6, characterised in that the compartment (25) is sealed in substantially gas¬ tight fashion by means of a removable or rupturable seal (28) and prior to the sealing thereof has been filled with C0₂ or a gas containing C0₂ in a concentration substantially higher than a concentration limit above which the ability of the indicating element (26) to return incident light is inde¬ pendent of the C0₂ concentration.

8. A device according to claim 7, characterised in that the indicator unit (23) prior to the first use thereof is enclosed in a gas-tight enclosure (29) filled with C0₂ or a gas containing C0₂ in a concentration substantially higher than said concentration limit.

9. A device according to any one of claims 1-8, characterised in that the indicator unit (23) is elongate and defines a lumen extending from one end of the indicator unit to the other, in that said socket receptive of said one end of the light guide is defined by a first section of the lumen extending from said one end of the indicator unit, and in that the indicating element (26) is disposed at the inner end of a second section (25) of the lumen which extends from the other end of the indicator unit to the vicinity of the first section.

10. A device according to any one of claims 1-9, characterised in that the light guide (15, 17) is an optical fibre, preferably a monofilament optical fibre.

11. A device according to claim 10, characterised in that the optical fibre (15, 17) comprises a flexible fibre section and over at least a section of the length thereof exhibits a constant radius of curvature, and in that the radius of curvature and the angle subtended by said section exhibiting a constant radius of curvature, are chosen such that at least 3 percent of the intensity of diffuse light signals which are transmitted through the light guide will be lost in said section.

12. A device according to claim 11, characterised in that the radius of curvature of said section preferably is smaller than the smallest expected radius of curvature of the remaining portion of the light guide.