GB2306658A

GB2306658A - Device for measuring gas concentration electrically

Info

Publication number: GB2306658A
Application number: GB9522237A
Authority: GB
Inventors: Christopher Arthur Sta Gillard; Manuel Robert Alecio
Original assignee: CO ORDINATED ENVIRONMENTAL SER
Current assignee: CO ORDINATED ENVIRONMENTAL SER
Priority date: 1995-10-31
Filing date: 1995-10-31
Publication date: 1997-05-07
Also published as: GB9522237D0

Abstract

A device for measuring a concentration of a selected gas such as carbon dioxide comprises a container 30 for a liquid, preferably an electrolyte such as an aqueous amine solution, capable of dissolving or reacting with the gas. Means 16 are provided in the container for measuring an electrical property of the liquid such as conductivity or capacitance. For bringing the gas into contact with the liquid there is provided a wall or membrane which is water-impermeable but permeable to the gas whose concentration is to be measured. The membrane may comprise a wall of the container 30. The gas diffuses through the wall into the container at a rate which depends on its concentration outside the container. Gas diffusing into the container dissolves in and/or reacts with the liquid, altering its electrical properties to give a reading indicative of the gas concentration to be measured.

Description

DEVICE FOR MEASURING GAS CONCENTRATION This invention relates to a device for measuring concentrations of a gas such as carbon dioxide, for example in a closed pipe or container.

There are various application in which it is necessary to monitor carbon dioxide concentration, for example in closed cabins such as submarines, in experiments on the respiration of plants and in chemical engineering processes. Carbon dioxide sensors are typically designed to cause little or no perturbation of the sample during operation, but for some applications it is desirable to remove the carbon dioxide from the sample or to integrate carbon dioxide exposure over an extended period. This is normally achieved by drawing a sample of the carbon dioxide containing gas through an alkaline solution such as barium hydroxide and back-titrating the resulting solution containing barium carbonate. This method however requires complex external equipment and is not directly applicable to liquid samples.

The present invention aims to provide a simple device capable of directly monitoring the partial pressure of a selected gas such as carbon dioxide, in liquid as well as gaseous samples.

According to the present invention there is provided a device for measuring a concentration of a selected gas, comprising a container for a liquid which is capable of dissolving or reacting with said gas and means in said container for measuring an electrical property of the liquid, for example conductivity or capacitance. Means are also provided for bringing the gas into contact with the electrolyte, typically through a wall or membrane which is water impermeable but permeable to the gas whose concentration is to be measured. When the electrolyte and the fluid containing the gas to be measured are in contact with respective opposite sides of the wall or membrane, the gas diffuses through the said wall into the container at a rate which depends on its concentration outside the container.Gas diffusing into the container dissolves in and/or reacts with the liquid therein altering its electrical properties to give a reading indicative of the gas concentration to be measured.

The invention also provides a method of measuring a concentration of a selected gas, wherein the gas is caused to diffuse into a liquid capable of dissolving or reacting with the gas, and an electrical property of the liquid is monitored to measure changes in the said property resulting from the diffusion of the gas therein.

In one embodiment of the invention, the liquid container has at least one wall in the form of a membrane of a gas permeable material, which may be mounted on an internal support. Alternatively, the membrane or other permeable wall may form part of a container for the gas.

For monitoring carbon dioxide concentrations, the liquid container may suitably be made from a silicone rubber, which is water impermeable but permeable to carbon dioxide. The liquid will preferably be an aqueous alkaline solution, preferably of an amine, which has initially low electrical conductivity but which reacts with carbon dioxide to form carbonate, bicarbonate and organo ammonium ions, thereby increasing its conductivity. The conductivity can be measured by a pair of inert electrodes which make contact with the solution and to which an alternating electrical potential is applied.

various organic amines in aqueous solution can react with carbon dioxide as follows: RNH2 + CO2 + H20 - > RNH3 + HC03 RNH2 + HCO3 - > RNH + C03 In these reactions R may be any suitable organic radical, for example an alkyl, substituted alkyl or aryl group. Because the reaction yields ionised products, the conductivity of the solution increases. Electrolysis of the solution and degradation of the electrodes can be avoided by using electrodes of a non-reactive metal such as platinum and applying an alternating current.

The conductivity can be measured by conventional electronic means and processed to give a reading indicative of the measured concentration of carbon dioxide.

A preferred base for use in measuring carbon dioxide concentrations is a hydroxy amine such a ethanolamine, in aqueous solution typically at a concentration of 10 to 50% v/v, more preferably 20 to 30% v/v and more especially about 25% v/v.

A preferred embodiment of the invention will now be described with reference to the accompanying drawings wherein: Figure 1 is a perspective view of a gas measuring device in accordance with a first embodiment of the present invention, with its three main parts separated, Figure 2 is an elevation of the device of Figure 1 from the side opposite that shown in Figure 1, with the electrolyte container assembled.

Figure 3 is an elevation, partly in cross section, of a second embodiment; Figure 4 is a perspective view of the device of Figure 3, showing the two parts separated and Figure 5 is a top plan view of the electrolyte container of Figures 3 and 4.

Referring to Figs. 1 and 2, a device in accordance with the present invention for monitoring CO2 concentration comprises a main housing 10, which may contain the electronic circuitry needed for conductivity measurement.

The housing is preferably made of a suitable plastics material and is of generally cylindrical shape. At one end of the housing is a threaded portion 12 of reduced diameter onto which a membrane support 20 can be screwed.

The membrane support 20 may suitably be made of the same material as the housing 10 and is of essentially cylindrical shape with large openings 24 on both sides. It also has open ends 22, 26, the end 26 being provided with an internal screw thread to enable the support to be screwed on to the threaded end 12 of the housing.

A membrane 30, in the form of cylindrical container of silicone rubber or the like which is water impermeable but permeable to CO2 gas, fits over the support 20 as shown in Figure 2. The membrane is closed at one end and open at the other, an annular constriction 32 being provided at the open end to allow the membrane to snap fit into a corresponding circumferential channel 28 on the membrane support. The membrane can then be held in place by an Oring in the constriction 32 to seal the end of the membrane around the support.

Projecting longitudinally from the threaded portion 12 of the housing 10 is a pair of electrodes 16, comprising wires of an inert metal such as platinum. The electrodes are partly enclosed by insulating sheaths 14.

Between the electrodes 16 is a paddle 18 for stirring the solution in the container 30. This paddle comprises an elongate flat strip of resilient material, suitably plastics, secured at one end to the threaded portion of the housing by a bearing 17. Adjacent the housing the paddle 18 has a radially projecting armature 13 of steel or the like. Within the housing, shown in dotted line in Figure 2, is an electromagnetic coil 15 aligned with the armature 13. When an alternating current is applied to the coil, the armature is caused to oscillate and, since it is integral with the paddle 18, the paddle in turn vibrates to stir the solution in the container 30.

In use the container 30 contains a measured amount, suitably about 10 ml, of an alkaline solution such as the 25% v/v ethanolamine solution described above. The container 30 is inserted into the region in which the carbon dioxide concentration is to be measured, and the electromagnetic coil 15 is switched on to cause the paddle 18 to stir the solution. Carbon dioxide diffuses through the membrane 30 at a rate reflecting the partial pressure of the gas in the atmosphere surrounding the membrane, and enters the ethanolamine solution. Ions are generated both by the reaction of carbon dioxide referred to above and by carbon dioxide simply dissolving in the aqueous solution to yield carbonate and bicarbonate ions. The consequent increase in the electrical conductivity of the solution is measured via the electrodes 16.A suitable microprocessor can be provided to convert the increase in conductivity into a signal indicative of the CO2 uptake.

In the embodiment of Figures 3 to 5, the gas permeable membrane 40 forms the bottom wall of a container 35 for the gas. This container may be made from any suitable gas-impermeable material such as metal, glass or plastics. A magnetic stirrer bar 38 is provided in the gas container, mounted on a support bar 36.

To measure the content of a particular gas in container 35, the container is stood in a vessel 50 which form a base for the device and includes a chamber 42 for a measured amount of an electrolyte. An O-ring seal 46 ensures leak proof assembly. A porous grid 9, extending across the chamber 42, supports the membrane 40 against the weight of the sample.

Also provided in the chamber 42 is a magnetic stirrer bar 52 aligned with the stirrer bar 38. The whole device can then be stood on a magnetic stirrer which can rotate both stirrer bars together to stir the electrolyte and the gas sample.

The chamber 42 can be filled with a measured amount of electrolyte through a port 47 which is then sealed by means of a screw 48. The electrolyte contacts the whole of that part of the membrane 40 which extends across the open bottom end of the gas container 35, and overflows into an air space 49 above the grid 45. Electrodes 44 are provided for monitoring changes in the electrical properties of the electrolyte in the chamber as gas diffuses into it through the membrane.

Further embodiments are envisaged. For example, the gas-permeable wall could be dispensed with altogether and the gas sample brought into direct contact with the electrolyte.

Claims

CLAIMS:

1. A device for measuring a concentration of a selected gas, comprising a container for a liquid which is capable of dissolving or reacting with said gas, means in said container for measuring an electrical property of the liquid and means for bringing said gas into contact with the liquid.

2. A device as claimed in claim 1 wherein said liquid is an electrolyte and the means for measuring an electrical property of said liquid comprises conductivity measuring means.

3. A device as claimed in claim 2 wherein said conductivity measuring means comprises a pair of inert electrodes which make contact with the electrolyte and means for applying an alternating electrical potential to said electrodes.

4. A device as claimed in claim 1 wherein said means for measuring an electrical property of the liquid comprises capacitance measuring means.

5. A device as claimed in any preceding claim wherein said means for bringing the gas into contact with the liquid comprises a layer of material which is waterimpermeable but permeable to the gas whose concentration is to be measured.

6. A device as claimed in claim 5 wherein the liquid container has at least one wall in the form of a membrane of said gas permeable material.

7. A device as claimed in claim 6 wherein the membrane is mounted on an internal support.

8. A device as claimed in claim 5 wherein said layer of water-impermeable, gas-permeable material forms part of a container for the gas.

9. A device as claimed in any one of claims 5 to 8 wherein said water-impermeable, gas-permeable material comprises a silicone rubber.

10. A device as claimed in any preceding claim wherein said liquid comprises an aqueous alkaline solution.

11. A device as claimed in claim 10 wherein said liquid comprises an aqueous solution of an amine.

12. A device as claimed in claim 11 wherein said amine is a hydroxy amine.

13. A device as claimed in claim 12 wherein said hydroxy amine is ethanolamine.

14. A device as claimed in any one of claims 11 to 13 wherein said amine is present at a concentration of 10 to 50 percent v/v.

15. A device as claimed in claim 14 wherein said concentration is 20 to 30 percent v/v.

16. A method of measuring a concentration of a selected gas, comprising the steps of causing said gas to diffuse into a liquid capable of dissolving or reacting with said gas and monitoring an electrical property of said liquid to measure changes in said property resulting from the diffusion of the gas therein.

17. A method as claimed in claim 16 wherein said monitoring step comprises monitoring the conductivity of said liquid.

18. A method as claimed in claim 16 wherein said monitoring step comprises monitoring the capacitance of said liquid.

19. A method as claimed in claim 16 wherein said selected gas is carbon dioxide and wherein said liquid comprises an aqueous alkaline solution of an amine.