GB2114304A

GB2114304A - Electrochemical, membrane sensor

Info

Publication number: GB2114304A
Application number: GB08302595A
Authority: GB
Inventors: Andrew M Dear
Original assignee: Distillers Co Ltd
Current assignee: Distillers Co Ltd
Priority date: 1982-02-01
Filing date: 1983-01-31
Publication date: 1983-08-17
Also published as: GB8302595D0; GB2114304B

Abstract

In an electrochemical sensor having electrodes and electrolyte separated from fluid under investigation by a membrane, the membrane is tensioned by a resilient support. As shown the sensor comprises an electrolyte chamber 4 containing electrodes 8 and 9 and electrolyte 7 and that is enclosed within chamber walls provided by a housing 1 having an open end 2 across which is fitted membrane 5 that separates the electrolyte 7 from the fluid. The resilient support that tensions the membrane may be an electrode support 10 that carries the electrodes and that is resiliently biased by a spring 19 towards the membrane 5. The sensor may measure the amount of a component, such as oxygen, in a fluid, such as a yeast fermentation broth. <IMAGE>

Description

SPECIFICATION Electrolytic device for measuring the content of a fluid It is well known to measure the amount of a component of a fluid using an electrolytic device that comprises an electrolyte chamber that bridges spaced electrodes and that contains or can contain electrolyte and that is enclosed by a chamber wall that has an opening and by a membrane that is across the opening and that can separate the electrolyte from the fluid while being permeable to the component of the fluid. The part of the device including the membrane is immersed in the fluid whereupon the component being studied permeates through the membrane into the electrolyte. The device is calibrated and changes in the current generated within the device are directly proportional to the concentration of the component in the fluid.

It is sometimes necessary to subject the device to guite rigorous environmental conditions. For instance it may occasionally be necessary to steam sterilise it. After each such exposure it is generally necessary to recalibrate the deivce. Also, after a period, it is observed that the performance of the device may deteriorate an that even extensive recalibration may not overcome this satisfactorily.

This is a particular problem in those devices where the electrodes press against the membrane. Accordingly at present such devices which are subjected to rigorous enviromental conditions have to be extensively recalibrated and may give unsatisfactory results even after recalibration.

An electrolytic device according to the invention for measuring the amount of a component of a fluid comprises an electrolyte chamber that bridges spaced electrodes and that contains or can contain electrolyte and that is enclosed by a chamber wall that includes an opening and by a membrane across the opening, and in this device the membrane separates the electrolyte from the fluid, is permeable to the component and is tensioned across the opening by a resilient support.

We have realised that the problems with known devices probably arise from the expansion and contraction that they undergo when exposed to steam sterilisation or other rigorous enviormentai conditions and, in particular, arise from stretching of the membrane. By keeping the membrane tensioned across the opening at all times by a resilient support improved performance is obtained compared with the probes that have been available previously.

In conventional devices an electrode presses against the membrane or against a layer of material over the membrane, for instance a layer of absorbent material that may absorb electrolyte and thus define the electrolyte chamber. In the invention such a device is modified by resiliently biasing the electrode towards the membrane in order that the membrane is tensioned across the opening.

The electrode chamber may be defined by a layer of absorbent material with which the electrodes are in contact, in which event the electrodes will serve as part or all of the chamber wall and the chamber will bridge between the electrodes. Generally however it is preferred that the device comprises a housing that defines part at least of the chamber wall having the opening, with the electrodes being mounted in the housing. The device will include means for securing the membrane across the opening. Generally there are separate means for securing the membrane and for resiliently supporting the membrane under tension across the opening.

It is normally preferred that the electrodes should be mounted on a single electrode support and that an electrode should press against the membrane, or a layer of material over the membrane, and that the support should be resiliently biased towards the membrane. For instance when the device includes a housing, as described above, the electrodes may be carried by a support that may be moved with respect to the housing and the support is resiliently biased with respect to the housing towards the membrane.

One preferred device comprises a substantially cylindrical housing having at one end a removable plug and at the other end the opening across which the membrane is mounted, and the electrodes are mounted on a single electrode support that is movable with respect to the housing and an electrode presses against the membrane or a layer of material over the membrane and there are resilient biasing means between the plug and the support for biasing the support resiliently towards the membrane. The resilient biasing means may be a spring acting between the plug and the electrode support to press the electrode resiliently against the membrane or material over the membrane. Generally the electrode presses direct against the membrane surface.

The plug is preferably adjustable in order that the force exerted by the spring can be adjusted. Conveniently the electrode support comprises an elongated member that extends between one end of a compressed spring, the other end of which presses against the plug, and an electrode that presses against the membrane. This electrode may, for instance, be in the shape of a helix.

The electrode that presses against the membrane, or against material overlying the membrane, preferably has a convex profile.

Although it is often preferred for one of the electrodes to press against the membrane it is possible to construct devices in which both electrodes press against the membrane. It is also possible to construct devices in which neither electrode presses against the membrane provided that the device includes resilient support means for maintaining tension.

The device may be designed in known manner, apart from the provision of the resilient support means for the membrane, for galvanic or polarographic operation.

The electrolyte chamber may include free fluid electrolyte, in which event one or both electrodes may be pressed against the membrane as mentioned above. Alternatively the electrode chamber may include or be defined by a layer of absorbent material covering the membrane and that includes the electrolyte, in which event both electrodes will press against this absorbent material. The membrane may be constructed so that its inner surface serves as the absorbent material.

Various semi-permeable materials are well known. Examples include polytetrafluoroethylene and polypropylene. The device generally includes a housing and the materials of the device may be formed of metal, for instance stainless steel, glass, ceramic material or synthetic plastics material, for instance polypropylene, polyamide, polyester or polytetrafluoroethylene.

The electrodes, electrolyte and membrane will be selected having regard to the component that is to be measured and the fluid containing it. For instance the electrodes may be for measuring CO2 or they may be ionselective electrodes, with the result that the device may, for instance, be useful for measuring the amount of carbon dioxide in a fluid or for measuring the content of a specific ion in a fluid. Thus there may be a glass pHelectrode pressing against the membrane and a reference cell elsewhere in the device. Preferably the device is designed for measuring the amount of a component in a liquid and, most preferably, for measuring the amount of oxygen in a liquid.

Such a device is of particular value for measuring the content of oxygen in a yeast or other fermentation broth, especially in a large, commericial scale, fermenter. Prior to the invention it was necessary to recalibrate the electrode from time to time, especially after steam sterilisation. Recalibration was often difficult and costly, since it generally required the elimination of oxygen from the large fermenter by sparging with an inert gas. As a result of the invention it is not necessary to recablibrate so frequently during use, and in particular after each sterilisation in the fermenter. Typically the device is subjected to an initial conditioning process, including at least one steam sterilisation cycle, and is then calibrated whereafter its performance will remain substantially constant for prolonged periods of use, including repeated steam sterilisation cycles.

When the device is constructed for determining oxygen concentration examples of electrode combinations for use as the electrodes include lead-silver, platinum-silver, platinum-gold, platinum-aluminium and aluminium-silver, whilst examples of suitable electrolytes include aqueous solutions of alkalis and chlorides, for instance aqueous solutions of potassium hydroxide or potassium chloride.

Other suitable electrolytes are known and include, for instance, lead acetate. Components may be included in the electrolyte to prevent boiling during steam sterilisation, for instance glycerol or ethane diol.

One device according to the invention is illustrated in the accompanying drawings. It is particularly useful as the oxygen probe in a commercial yeast propagation vessel.

Referring to the accompanying drawings, Figure 1 is a section through a device according to the invention and Figure 2 is an exploded view, partly in section, of the main structural components of the device in use.

The device comprises a cylindrical housing 1 that is open at the end 2 but is plugged at its other end by a taper stud assembly 3. An electrolyte chamber 4 is located at the end 2 and is enclosed within the housing by a semipermeable membrane 5 that is fitted over the end 2 of the housing. This membrane is fitted across the aperture and along the sides of the housing and is secured to the housing by O rings 6 fitted in corresponding grooves in the housing. The electrolyte chamber includes electrolyte 7 and two electrodes 8 and 9.

Electrode 8 is coiled aluminium or other electrode material wound on to a cylindrical electrode support 10 provided with an opening 11 by which an electrode connection 12 can pass into the centre of the support and through the plug. Electrode 9 is a helix of silver wire or other suitable electrode wire having a perpendicular tail 1 3 to which an electrode connection 14 is connected and which leads through the plug. The electrode 9 is secured to the electrode support 10 by cast epoxy or other resin 1 5.

For instance, the helix 9 with the tail 1 3 may first be cast in a small amount of epoxy resin and provided with its connection 14.

The electrode 8 may be wound around the electrode former 10 and provided with its electrode connection 1 2. The tail 1 3 and the surrounding resin on the electrode 9 may then be fitted within the electrode former and the former may then be filled with suitable curable resin 1 5 up to the level of an aperture 1 6 in the support.

The plug 3 carries a spigot 18 that extends into the top of the electrode support 10. A spring 1 9 surrounds the spigot and acts between the lower face of the plug 3 and the upper end of the support 10 so as to urge the electrode support downwardly and to press the electrode 9 into contact with the mem brane 5.

The plug 3 is part of a tapered stud assembly including a tapered connection 20 for receipt of a hose 21, a hose clinch nut 22, a threaded sealing collar 23 and an 0 ring 24.

An opening 1 7 extends through the plug to carry the electrical connections 1 2 and 14 out of the device.

To assemble the probe, the cylindrical housing 1, for instance formed of polyacetal or other synethetic plastics material, open at both ends has the membrane 5 stretched flat across its end 2 and secured using an adhesive and the rings 6. Electrolyte 7 is introduced from the other end. The electrode support 10 carrying the electrodes 8 and 9 in fixed positions, with a predetermined fixed separation between them, is inserted into the cylindrical housing and the electrode 9 allowed to rest on the membrane. To facilitate positioning of the electrode assembly guides, not shown, may be provided to permit the assembly to slide axially without rotation.

The electrode connections 1 2 and 14 are then threaded through the bore 1 7 of the plug 3 which is then screwed into the housing 1. The tension in the spring, and thus the force by which the electrode 9 presses onto the membrane, is adjustable by altering the depth to which the plug 3 is screwed into the housing. The sealing collar is then tightened, trapping the ring 24 between the oblique faces of the collar 23 and the housing 1, thus compressing the ring inwards to form a three point seal and to lock the plug in its selected position.

The hose 21 is clamped onto the taper 20 by clinch nut 22 and serves to carry the electrical connections out of the fermenter and also to allow the electrolyte chamber to communicate with the outside atmosphere. The aperture 1 6 facilitates free quilibration of pressure between the electrolyte chamber 4 and the atmosphere outside the fermenter.

The device is installed in a sleeve or carrier 25 carrying metal collars 26, the sleeve 25 and the collars 26 serving to locate the device in the desired position in the fermenter, with the membrane 5 immersed in the liquor. An O ring 27 cooperates with the rings 6 to provide a firm fit between the device and the cylinder 25.

Claims

1. An electrolytic device for measuring the amount of a component of a fluid, the device comprising an electrolyte chamber that bridges spaced electrodes and that contains or can contain electrolyte and that is enclosed by a chamber wall that includes an opening and by a membrane across the opening and in which the membrane can separate the electrolyte from the fluid, is permeable to the component, and is tensioned across the opening by a resilient support.

2. A device according to claim 1 in which the membrane is tensioned across the opening by an electrode that is resiliently mounted and biased towards the membrane and that presses against it or against material over the membrane.

3. A device according to either preceding claim that includes a housing that defines the chamber wall and that has the opening and in which the electrodes are mounted in the housing.

4. A device according to any preceding claim in which the electrodes are mounted on a single electrode support and an electrode presses against the membrane or against the layer of material over the membrane and the support is resiliently biased towards the membrane.

5. A device according to any preceding claim comprising a substantially cylindrical housing having at one end a removable plug and at the other end the opening across which the membrane is mounted, and in which the electrodes are mounted on a single electrode support that is movable with respect to the housing and an electrode presses against the membrane or against the layer of material over the membrane and the support is resiliently biased towards the membrane.

6. A device according to claim 5 in which the support is resiliently biased towards the membrane by a spring and the plug is adjustable in order that the force exerted by the spring towards the membrane can be adjusted.

7. A device according to any preceding claim in which an electrode presses against the membrane or against material over the membrane and has a convex profile.

8. A device according to any preceding claim in which an electrode presses against the membrane.

9. A method in which the amount of a component of a fluid is recorded using a device according to any preceding claim and in which the device is occasionally subjected to steam sterilisation.

1 0. A method in which the oxygen content of a yeast fermentation broth is measured using a device according to any of claims 1 to 8 immersed in the broth.

9. A device according to claim 1 substantially as herein described with reference to either of the accompanying drawings.

10. A method in which the amount of a component of a fluid is recorded using a device according to any preceding claim and in which the device is occasionally subjected to steam sterilisation.

11. A method in which the oxygen content of a yeast fermentation broth is measured by immersing a device according to any of claims 1 to 9 in the broth.

CLAIMS (15 Apr 1983)

1. An electrolytic device for measuring the amount of a component of a fluid and which comprises a housing, first and second electrodes mounted in the housing, an electrolyte chamber that bridges the spaced electrodes and that is partially filled with electrolyte and that is enclosed by the housing and a membrane that is across an opening in the housing and that can separate the electrolyte from the fluid and that is permeable to the component, and in which the first electrode is resiliently mounted and biased towards the membrane and presses against the membrane or material over the membrane.

2. A device according to claim 1 in which there is an aperture in the unfilled part of the electrolyte chamber leading to the outside atmosphere for equilibration of pressure between the chamber and the atmopshere.

3. A device according to any preceding claim in which the electrodes are mounted on a single electrode support and an electrode presses against the membrane or against the layer of material over the membrane and the support is resiliently biased towards the membrane.

4. A device according to any preceding claim comprising a substantially cylindrical housing having at one end a removable plug and at the other end the opening across which the membrane is mounted, and in which the electrodes are mounted on a single electrode support that is movable with respect to the housing and an electrode presses against the membrane or against the layer of material over the member and the support is resiliently biased towards the membrane.

5. A device according to claim 4 in which the support is resiliently biased towards the membrane by a spring and the plug is adjustable in order that the force exerted by the spring towards the membrane can be adjusted.

6. A device according to any preceding claim in which the first electrode has a convex profile that presses against the membrane or against material over the membrane.

7. A device according to any preceding claim in which an electrode presses against the membrane.

8. A device according to claim 1 substantially as herein described with reference to either of the accompanying drawings.