GB1565130A - Cell for a gas analysis apparatus - Google Patents

Description

(54) A CELL FOR A GAS ANALYSIS APPARATUS (71) We, DRAGERWERK AKTIENGESELLSCHAFT. a Germany Company, of Moislinger Allee 53-55. 2400 Lu"beck, Federal Republic of Germany. do hereby delcare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed.

to be particularly described in and by the following statement: The invention relates to a cell for a gas analysis apparatus.

The analysis of gas. the mixture components of which each have an absorption band in the visible or infrared spectrum range, can be carried out by means of electromagnetic radiation which is passed through the gas, the degree of attenuation of such radiation being a measure of concentration of a component. The position and band width of the radiation is adapted as precisely as possible to the position and band width of the absorpiton band of the component being measured.

In that measuring method. the gas sample to be tested is introduced into an optical arrangement constructed as a measuring chamber or irradiatable cell. The irradiated absorpiton path should be given a precisely defined length in the particular gas.

There is known from United State Patent 3792272 an irradiatable measuring cell in an apparatus for determining the proportion of an organic compound, for example alcohol.

in a gas mixture, in which the gas sample to be tested is introduced into the path of radiation from an infrared radiation source to a receiver. In order to provide a long path of radiation with a small overall size.

sperhical hollow mirrors are arranged within the measuring cell. Reflection of the infrared beam at these hollow mirrors produces a radiation path which is folded several times on itself. Quartz windows are provided in the ends of the measuring cell for the passage ofthe entering and issuing beam. At one longitudinal side, the cell comprises an inlet and an outlet for the gas sample. In order to avoid condensation from the gas sample, the measuring cell is provided with electrical resistance heating and heat insulation. A partisuch arcular disadvantage of that measuring cell is the sensitive adjustment of the mirrors, since the radiation source and the receiver must be aligned very precisely to each other. The danger of losing the setting adjusted is considerable. particularly when thermal stresses are caused bv the heating.In addition the construction is complicated by the need for mirrors and the holders which they require. The necessary spacing between the individual sections of the path of radiation results in a relatively considerable measuring cell volume which is disadvatageous as regards overall size, the neces sary quantitv of gas sample. and the time required to change a sample.

According to a known method disclosed in Federal German Published patent Specification 2211835 and for the simultaneous analysis of different components of a mixed gas by means of the absorption of light irradiated into the gas. the light is subjected to successive interference filtering arrangements. In each case a proportion of the irradiated light situated in the absorption range of the gas component being detected is filtered out of the mixed gas after passing along an optical path length specified therefor. The remaining irradiated light is reflected. in a direction deviating from the particular direction in which the filtered-out proportion issues. back into the mixed gas and conducted towards the next following interference filtering arrangement. The direction changes produce a zigzag-shaped light path or one which is in the form of a polvgon.The container in which the gas being measured is conducted on the particular inflected light path in question, has a correspondingly bulky shape with correspondingly large requirements as regards space. In such arrangements it is difficult to set the mirrors since all the mirrors. the radiation source and the receiver must be aligned very precisely relatively to one another. The volume of the cell can be reduced in the case of such multiple mirror arrangements only sacrificing light intensity.

To provide heating, which may be necessary to avoid condensation, for example when testing breathing gases, involves a risk of losing the adjusted setting because of thermal stresses.

According to the present invention there is provided a cell for a gas analysis apparatus, said cell comprising a tube the interior of which, during use of the apparatus, receives a gaseous substance to be analysed and has radiation passed therealong. said tube being of a bent form and of an internal diameter much smaller than its length and having over substantially the whole of its length an internally reflective tubular suface which reflects said radiation, Owing to the invention, it is possible to provide a cell for a gas analysis apparatus with a relatively long path of rays with a relatively small analysis gas volume and with an external shape and size with which the dimensions of the analysis apparatus can be kept small. Furthermore, both the gaseous substance to be analysed and also the radiation can be conducted on the longitudinal axis through the cell tube.Because of the through reflective surface in other words a continuous mirror, the deflection of the radiation is not now dependent on precisely adjusted individual reflects. The bending of the cell tube can be of optional angle and curvature and the path of radiation can be taken from a radiation source at one tube end to a closure element at the other tube end, that is to say a window, a lens. or a radiation detector.

Since the angle and curvature of the bending are optional, it is possible to adapt to existing dimensions within the analvsis apparatus. Even if a long radiation path is necessary, the overall length dimensions can remain small. The internal diameter of the cell tube is kept small so that the gas volume remains correspondingly small also. The adjustment of the radiation source and of the detector is not critical, especially since the active surfaces of these elements can be selected according to the cell tube diameter.

It is also preferred that the cell tube consists of a main body of electrically insulating material and the internally reflective surface is that of a metal film on the inner periphery of the main body. The film is taken outwards round the ends of the tube. The film ends form contacts for an electrical connection.

With this solution it is possible in an advantageously simple manner to use the reflective layer at the same time as a heating element with which condensation from the analysis gas within the cell tube can be avoided. With the continuous reflective layer there is also no risk of loss of adjustment because of thermal stresses when heating is provided, especially since the heat is now supplied directly to the internal surface of the tube, so that the tube has to be heated to a lesser extent than when heat is applied from the outside.

Advantageously, the metal film is a gold film. With this noble metal, which is applied from a gold solution in a known reaction, very good and lasting reflection ability is ensured.

The cell tube ends are closed by windows, lenses or optical filters. The closure element can be a radiant lamp or a detector. In this wav the other necessarv elements are fitted directly to the cell tube without disturbing space conditions.

Making advantageous use of the underlying ideas of the invention it is also possible to use the cell also for the analysis of gas mixtures. For this purpose there are arranged in bends in the tube interference filters whose transmission ranges correspond to the respective absorption bands of the gas components being measured. The radiation which is not allowed through at one filter is reflected to the next interference filter.

According to a further feature, partially translucent mirrors are arranged in the bends. Thus possibilities are afforded in an advantageous manner of operating with absorption distances adapted in an optimum manner to the particular absorption coefficients in the case of gas mixtures.

When the path of radiation is divided into individual sections, the security against loss of adjustment is significant. The gas analysis apparatus thus remains reliable in operation and easy to handle. whilst being of simple construction.

In order that the invention may be clearly understood and readilv carried into effect, reference will now be made, by way of example. to the accompanying drawings, in which: Figure I shows a side elevation of a cell for a gas analysis apparatus.

Fizo e 2 shows a side elevation of a modified version of the cell, Fgtire 3 shows an axial section through a cell end.

Figures 4 to 7 show axial sections through respective closure elements of the cell end, and Figllre 8 shows an axial section through a radiation outlet window at a cell bend.

Referring to Figures 1 and 2, the gas cell 1 includes a tube 2 made of quartz and having an internal diameter of about 6 mm, and a wall thickness of about 1 mm. The length and therefore the path of radiation therethrough amounts to about 80 cm. in this example, Thus an internal volume of about 23 cm3. is obtained. Thus the volume of the gas sample required for analysis is small.

The tube 2 is curved suitably for the dimensions of the apparatus in which it has to be fitted. Figures 1 and 2 show alternative forms, and other forms are also possible.

The internal peripheral surface of the tube 2 is that of a reflective layer 3 arranged on the main body 12 of the tube. For this purpose it is a gold film 3 with a mirror finish. The gold film has been applied from a known gold solution by means of a redox reaction. The tube has been brought to a temperaure of about 550"C to evaporate the solvent and to effect shaping.

The layer 3 is taken outwards from the inside at both cell ends 4. and the coatings 5 which are thus formed and which are about 1 cm. in length constitute external contacts for electrical connection. which is effected for example by means of clips. The electrical resistance of the gold layer 3 is about 10 Q which is very suitable for electrical heating.

Each cell end 4 is closed by an adhesivelv secured window 6 (see Figure 4) made of a material which is pervious in the range of the spectrum in question. such as quartz. a lens 7 (see Figure 5) made of such material, a radiation detector 8 (see Figure 6) or a radiant lamp 9 (see Figure 7). The window 6 may also be an interference filter. Spaced inwardly from the ends of the cell. the tube 2 is provided with gas apertures 10 through which the gas being analysed is introduced and discharged respectively.

As shown in Figure 5, the tube can also be provided in the curves thereof with interference filters or partially pervious mirrors 11.

WHAT WE CLAIM IS: 1. A cell for a gas analysis apparatus.

said cell comprising a tube the interior of which, during use of the apparatus receives a gaseous substance to be analysed and has radiation passed therealong. said tube being of a bent form and of an internal diameter much smaller than its length and having over substantially the whole of its length an internally reflective tubular surface which reflects said radiation.

2. A cell as claimed in claim 1. wherein said surface is the internal peripheral surface of said tube.

3. A cell as claimed in claim I or 2.

wherein said tube comprises a main body of electrically insulating material and a tbular layer providing said surface and in the form of a metal film.

4. A cell as claimed in claim 3 as appended to claim 2, wherein said metal film extends outwardly of the ends of said tube, thereby to provide electrical contacts.

5. A cell as claimed in claim 3 or 4, wherein said metal film is of gold.

6. A cell as claimed in any preceding claim, wherein each end of said tube is closed by a window, a lens, an optical filter, a lamp for emitting said radiation, or a detector for detecting said radiation.

7. A cell as claimed in any preceding claim, wherein a bend of said tube has an interference filter which is arranged to transmit from said interior radiation of frequencies absorbed by a gaseous component of said substance and to reflect back into said interior radiation not of said frequencies.

8. A cell as claimed in claim 7, wherein a second bend of said tube has a second interference filter which is arranged to transmit from said interior radiation of second frequencies absorbed by a second gaseous component of said substance and to reflect back into said interior radiation not of said second frequencies.

9. A cell as claimed in any one of claims 1 to 6. wherein a bend of said tube includes a partially translucent mirror which reflects back into said interior a part of said radiation and trasmits from said interior another part thereof.

10. A cell for a gas analysis apparatus, substantially as hereinbefore described with reference to any Figure of the accompany ing drawings.

I I. In a gas analysis apparatus. a cell as claimed in any preceding claim.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (10)

**WARNING** start of CLMS field may overlap end of DESC **. includes a tube 2 made of quartz and having an internal diameter of about 6 mm, and a wall thickness of about 1 mm. The length and therefore the path of radiation therethrough amounts to about 80 cm. in this example, Thus an internal volume of about 23 cm3. is obtained. Thus the volume of the gas sample required for analysis is small. The tube 2 is curved suitably for the dimensions of the apparatus in which it has to be fitted. Figures 1 and 2 show alternative forms, and other forms are also possible. The internal peripheral surface of the tube 2 is that of a reflective layer 3 arranged on the main body 12 of the tube. For this purpose it is a gold film 3 with a mirror finish. The gold film has been applied from a known gold solution by means of a redox reaction. The tube has been brought to a temperaure of about 550"C to evaporate the solvent and to effect shaping. The layer 3 is taken outwards from the inside at both cell ends 4. and the coatings 5 which are thus formed and which are about 1 cm. in length constitute external contacts for electrical connection. which is effected for example by means of clips. The electrical resistance of the gold layer 3 is about 10 Q which is very suitable for electrical heating. Each cell end 4 is closed by an adhesivelv secured window 6 (see Figure 4) made of a material which is pervious in the range of the spectrum in question. such as quartz. a lens 7 (see Figure 5) made of such material, a radiation detector 8 (see Figure 6) or a radiant lamp 9 (see Figure 7). The window 6 may also be an interference filter. Spaced inwardly from the ends of the cell. the tube 2 is provided with gas apertures 10 through which the gas being analysed is introduced and discharged respectively. As shown in Figure 5, the tube can also be provided in the curves thereof with interference filters or partially pervious mirrors 11. WHAT WE CLAIM IS:

1. A cell for a gas analysis apparatus.

said cell comprising a tube the interior of which, during use of the apparatus receives a gaseous substance to be analysed and has radiation passed therealong. said tube being of a bent form and of an internal diameter much smaller than its length and having over substantially the whole of its length an internally reflective tubular surface which reflects said radiation.

2. A cell as claimed in claim 1. wherein said surface is the internal peripheral surface of said tube.

3. A cell as claimed in claim I or 2.

wherein said tube comprises a main body of electrically insulating material and a tbular layer providing said surface and in the form of a metal film.

4. A cell as claimed in claim 3 as appended to claim 2, wherein said metal film extends outwardly of the ends of said tube, thereby to provide electrical contacts.

5. A cell as claimed in claim 3 or 4, wherein said metal film is of gold.

6. A cell as claimed in any preceding claim, wherein each end of said tube is closed by a window, a lens, an optical filter, a lamp for emitting said radiation, or a detector for detecting said radiation.

7. A cell as claimed in any preceding claim, wherein a bend of said tube has an interference filter which is arranged to transmit from said interior radiation of frequencies absorbed by a gaseous component of said substance and to reflect back into said interior radiation not of said frequencies.

8. A cell as claimed in claim 7, wherein a second bend of said tube has a second interference filter which is arranged to transmit from said interior radiation of second frequencies absorbed by a second gaseous component of said substance and to reflect back into said interior radiation not of said second frequencies.

9. A cell as claimed in any one of claims 1 to 6. wherein a bend of said tube includes a partially translucent mirror which reflects back into said interior a part of said radiation and trasmits from said interior another part thereof.

10. A cell for a gas analysis apparatus, substantially as hereinbefore described with reference to any Figure of the accompany ing drawings.

I I. In a gas analysis apparatus. a cell as claimed in any preceding claim.