GB2162320A - Oxygen sensor - Google Patents

Abstract

An oxygen sensor in which a generally cylindrical anode 14 surrounds a concentric radially spaced cathode 16 to form at one end a sensitive annular region the improvement wherein the end of the anode and the end of the cathode defining the said sensitive annular region occupy substantially the same radial plane and a semi-permeable membrane 20 is stretched across the said sensitive region in a second radial plane axially spaced apart from the first with a film of electrolytic gel sandwiched between the ends of the electrodes and the said membrane. The gel charge is held in a sleeve 22 fitted in a cap 24 which is separable from the body of the sensor to permit a new cap containing a fresh charge of gel to be fitted. Excess gel can escape through furrows in the thread profile of screw thread 26 which secures cap 24. <IMAGE>

Description

SPECIFICATION Improved oxygen sensor Field of invention This invention concerns oxygen sensors which by means of a change in the electrical conductivity between a pair of electrodes by the introduction of oxygen molecules through a membrane can give a measure of the quantity of oxygen present on the other side of the membrane.

Background to the invention Oxygen sensors formed from a pair of electrodes surrounded by an electrolytic gel and separated from a fluid containing dissolved oxygen by semi-permeable membranes are known. One such type of sensor is described in U.S. Patent 366650 although it is to be understood that this is merely exemplary of the many different types of sensor currently available.

Where such sensors are used in a clinical or surgical application so as for example to monitor the proportion of oxygen in the air supply to an anaesthetised patient, it is imperative that the sensor will work reliably and predictably and after being introduced into an airline or the like will rapidly settle to its steady state condition.

One of objects of the present invention is therefore to improve the response tupe of oxygen sensor of the type described hitherto.

A fall of in the response of a sensor of the type described is normally associated with a degradation of the electrolyte.

Accordingly it is another object of the present invention to provide a design of sensor which will allow the electrolyte to be changed more readily than hitherto by making it easier to replace the gel and associated membrane.

A long term degradation of response is associated with an electrolytic attack on at least the anode assembly and accordingly it is a further object of the present invention that any new design will make it no less difficult to keep the electrodes clear as by mild abrasion whenever the electrolyte is to be changed, thus prolonging the useful life of the sensor.

It is known to push-fit sensors of the type described into an appropriately dimensioned sleeve in a T-piece or elbo joint associated with an airline and it is another object of the present invention to provide a design of oxygen sensor which can be more reliably fitted into such an airline.

Summary of the invention According to one aspect of the invention in a sensor in which a generally cylindrical anode surrounds a concentric radially spaced cathode, to form at one end a sensitive annular region, the end of the anode and the end of the cathode defining the said sensitive annular region occupy the same or substantially the same radial plane and a semi-permeable membrane is stretched across the said sensitive region in a second axially spaced apart radial plane with a film of electrolytic gel sandwiched between the ends of the electrodes and the said membrane This structure brings both the anode and the cathode into close proximity with the membrane and reduced the stabilisation time required after switching on the sensor.The design is quite distinct from the previous designs of sensor such as illustrated in United States Patent 3666650 in which the anode terminates at a considerable distance from the plane containing the stretched membrane leaving the central cathode to protrude axially beyond the end of the cylindrical anode.

According to a preferred feature of this aspect of the invention, the cylindrical anode is formed from a relatively thin walled cylindrical member of silver or sliver-plated material as compared with the relatively thick walled anode structure of the prior art designs typically exemplified in the United States Patent 3666650.

As is conventional, the annular space between the anode and cathode is preferably filled with a plastics material or other electrical insulating material which serves as a dielectric material between the two electrodes.

Also as is well known in previous designs, a small temperature sensitive resistor such as a thermistor is located in relatively close proximity to the anode and cathode assembly so as to produce varying electrical current depending on the temperature of the electrode assembly and the environment thereof to permit temperature compenstion to be introduced in the electric current amplifying and measuring circuits associated with the sensor.

In the previous designs of sensor, the electrode assembly as hitherto been protected by a cylindrical end cap the cylindrical walls of which have extended beyond the end of the protruding cathode and the remote end wall of which has been provided with a central normally circular aperture and a membrane has been located within the cap so as to cover the window formed in the remote end thereof by the said aperture and to trap a charge of electrolytic gel in contact with the electrode assembly.

It is known to fit the electrode assembly into a generally cylindrical pencil-like probe and to provide for screw-threaded engagement between the end cap and the end of the probe adjacent the exposed electrodes for fitting the cap to the probe body.

United States Patent 3666650 is concerned with the charging of the cap with a predetermined quantity of gel and locating the membrane within the cap. The fitting of the cap to the probe body necessitates the introduction of the electrode assembly into the charge of gel and the penetration of the gel by at least the cathode. Displacement of the exces gel is shown in the aforementioned United States Patent Specification as simply a transference of the gel from one annular region to another within the end cap and the penetration of the gel into the screw-threaded engagement between the end cap and the probe body.

Subsequently radial ports have been provided in the cylindrical wall of the end cap to allow excess gel to flow to the outside of the cap as the latter is screwed onto the probe body and the gel is displaced from the annular region by the relative movement of the cap and protruding electrode assembly. The excess gel must bye cleaned from the exterior of the end cap before the probe assembly can be used and since the port will always communicate with the internal charge of gel, any local heating or rise in temperature will tend to cause expansion of the gel charge with the result that gel once again is found on the surface of the end cap.

Where the latter is inserted into a cylindrical sleeve as for example in a T-piece in an airline, this can result in the gel contaminating the airline and/or serving as a lubricant and causing the probe assembly to slip in the sleeve and to impair the friction fit of the probe assembly therewith in.

According therefore to another aspect of the present invention, in an oxygen sensor probe assembly comprising a probe having electrodes at one end and a replaceable end cap housing a membrane which is adapted to be secured to the electrode end of the probe, the pressure relieving gel exit or exits from the interior of the end cap communicates with the end of the cap remote from the end containing the membrane so that any gel which is expelled from the end cap during the fitting thereof to the electrode assembly end of the probe is directed rearwards towards the probe body thereby reducing the risk of the gel penetrating into an airline or the like and causing contamination and further ensuring that the gel does not form a film between the cylindrical external face of the end cap thereby reducing the component of static friction between the end cap and a sleeve into which the latter is fitted and thereby causing failure of the friction fit on which the junction would normally rely.

According to a further aspect of the invention, in an oxygen sensor assembly formed from a probe supporting an anode and cathode assembly at one end and having a replaceable end cap which can be readily secured and removed from the probe wherein the end cap includes a semi-permeable membrane and a charge of electrolytic gel which together with the electrodes forms the oxygen sensor when an end cap is fitted to the probe body, the improvement comprising an end cap having a charge of gel and a semi-permeable membrane which are retained within the cap by means of a strip of sheet material which is normally adhesively secured to the end of the cap through which the electrode end of the probe is introduced into the end cap for fitting the latter to the probe body, and optionally by a second strip of removable material covering the membrane covered window in the opposite end of the end cap.

By sealing both ends of the end cap in the manner provided by this aspect of the invention, so a charge of gel within the end cap can be kept away from the atmosphere and both the membrane and the charge of gel can be kept in perfect condition for an extended period of time comprising the shelf-life of the precharged end cap.

When the cap is to be used, the strip covering the first mentioned end of the cap is peeled away and the cap is screw fitted to the end of the probe body after which the strip at the other remote end of the end cap can likewise be peeled away to expose the membrane covered window. The sensor is then ready for use.

Where the protection afforded by the membrane is sufficient, the second mentioned strip of removable material can be dispensed with.

The end cap is preferably formed from a plastics material and conveniently a screwthread or bayonet fitting is provided for securing the end cap to the probe body which is configured accordingly.

In order to relieve the pressure within the end cap as the electrode assembly and end of the probe are inserted therein during the initial fitting of the precharged end cap to the probe body, the end cap preferably includes at least one passage communicating between the interior of the end cap and the end thereof which is remote from the windowed end of the end cap so that any excess gel exists remote from the window is kept clear of the end face of the end cap and the cylindrical external surface thereof.

According to a preferred feature of the invention, an end cap for fitting to a probe body containing anode and cathode electrodes at the end to which the end cap is to be fitted, includes an internal screw thread within the end of the cap which is to be fitted to the probe body and a complementary screw thread profile around the end of the probe only containing the electrodes for receiving and having fitted thereto the said end cap, and wherein the internal screw thread profile of the end cap is formed with at least one axial furrow or groove which does not interfere with the interengagement of the male and female thread forms but does provide an axial passage above the peaks of the thread form on the probe body through which excess gel can penetrate to relieve the pressure of the gel within the end gap as the latter is screwed down onto the probe body.

It will be seen by incorporating the excess gel outlet in the screw-thread, the outlet is wholly within the circular opening defined by the internal screw-threaded opening at the end of the cap which is to be fitted to the probe so that a circular disc of sheet material such as plastics film or foil or appropriately impregnated paper, when secured to the annular end wall of the cap at the internally threaded end thereof, will totally cover the furrow or groove in the screw thread profile and thereby seal the excess gel outlet from the atmosphere. This is important as compared with previous excess gel outlet provision in which the excess gel outlets communicated directly with the interior of the end cap by means of radial ports or the like which would need to be separately sealed in order to retain a gel charge within the end cap in good condition.

Where reliability and reproducability are of paramount importance, the end cap may be removed and thrown away after each use of the sensor and before a new end cap is fitted the electrode assembly may be cleaned and a fresh cleaned charged end cap may be fitted to the probe body after removing the end seals of foil or paper or the like as previously described. The provision of a supply of ready charged clean end caps thus allows each different operation to be performed using a fresh end cap and charge of gel with consequent improved reliability of such oxygen sensing apparatus.

It will be appreciated that in addition to using a fresh gel and membrane, the opportunities presented to the technician to clean the electrode assembly as each end cap is replaced with another and indeed the technician should be encouraged to do so thereby extending the useful life of the probe by preventing a build-up or corroded material on the surface of the anode.

Where it is not considered desirable to make the whole of the end cap expendable the membrane and gel charge may be formed as an insert located within a cylindrical sleeve itself adapted to be fitted with a slipping fit within the interior of the end cap so that the replacement of the gel and membrane is now achieved by simply unscrewing the end from the probe body and removing a cylindrical capsule containing the spent charge of gel and stretched membrane and replacing the same with a fresh gel filled capsule before replacing the end cap on the probe body assembly.

The capsule-like inserts may be formed from a thin walled cylindrical sleeve of plastics material or the like having a semi-permeable membrane stretched across one end face thereof and containing a charge of gel which can be kept in place until required by means of a strip of sheet material such as sheet plastics material or paper or foil itself stretched across the other end of the cylindrical sleeve from the membrane end and where required, a similar seal of sheet material may be stretched across the membrane end of the capsule, both being readily removable prior to insertion of the capsule into the end cap or breakable as the end cap is fitted to the probe and electrode assembly so as to allow the electrodes to come into contact with the gel as the end cap is fitted.

A particular advantage of this arrangement is that the charge of gel can be controlled very accurately if this is required but in any case can be kept to a volume which is sufficient to just full the interior of the end cap when the latter is fully fitted to the electrode assembly end of a probe body so that there is no significant pressure increase in the gel as the end cap is fitted to the probe body thereby reducing the quantity of excess gel which will be generated as the end cap is fitted to the probe body.

Such an arrangement should be compared with the previously mentioned arranged where the whole of the end cap is filled with a gel charge and the ends of the cap are sealed by means of peel-off or tear-off strips or the like and wherein as a result of the large quantity of gel there will be a considerable build-up of pressure within the gel in the end cap as the latter is fitted to the probe body.

According to a preferred feature of this aspect of the invention, the replaceable capsule may be sealed at the end through which he electrodes are to penetrate by means of a thin film sheet material such as plastics material having an array of radiating virtual perforations which break open to allow the electrode assembly nose to penetrate into the capsule as the latter, within an end cap, is moved axially relative to the electrode assembly.

Preferably the capsule adapted by means of its shape and/or dimensions so as to be capable of being fitted only one way into the end cap to prevent the accidental insertion of the capsule into the end cap with the semipermeable membrane end thereof adjacent the open end which is to be fitted to the probe.

In its simplest form a "protected" capsule may be formed with a radial protrusion which may be an annular rib or simply one or more radial lugs adjacent the end of the capsule remote from the end containing the semipermeable membrane and by providing a shoulder around at least part of the internal surface of the end cap past which the end of the capsule containing the radial protrusion or rim or the like cannot pass.

Alternatively the capsule may be in the form of a truncated cone and the interior of the end cap may be formed with a complementary truncated conical passage so that the capsule can only be inserted into the end cap with the smaller diameter end first. By ensuring that the latter corresponds to the end containing the semi-permeable membrane, it will be seen that such a capsule can never be fitted into the end cap the wrong way round.

Where the fit between the capsule and the end cap is such that after insertion it is difficult to remove the capsule from the end cap, means is preferably provided to facilitate the removal of the capsule.

Such means may be in the form of a tool for insertion through the windowed end of the end cap to push against the semi-permeable membrane end of the capsule.

Alternatively and preferably (since the semipermeable membrane tends to be a very fragile structure,) the internal wall of the capsule may be formed with an internal screw-thread profile and the removing tool may comprise a screw-driver-like member having a complementary screw-thread profile in place of the blade for fitting into the inerior of the capsule to allow the latter to be axially withdrawn from the end cap.

In a further embodiment the end cap may include one or more pins which are activatable either directly or by means of relative rotation of one part of the end cap relative to another, the pins being axially displaceable in a direction so as to force the capsule out of the end cap.

In a further arrangement the end cap itself may be formed in two parts which are relatively rotatable or slideable so as to enable relative axial movement between a part which grips the exterior of the capsule and a part which acts on the semi-permeable membrane end of the capsule can be effected to eject the capsule from the end cap.

It will be seen that by providing a pack of pre-sealed pre-gelled capsules, so the oxygen sensor can be recharged with a fresh semipermeable membrane and gel charge both quickly and with minimal mess and fuss thereby facilitating the replacement of gel and semi-permeable membrane at the beginning of each test or operation or the like.

Where it is desired not to cover the semipermeable membrane with a peel-off strip or the like, the sealed capsule may of course be retained in a sealed pack of for example plastics film material or foil material or the like so as to keep the contents of the capsule away from air to prevent degradation of the gel as by drying out.

This alternative approach to protecting the gel charged is preferred since it is then impos sible to accidentally leave the end cover on the semi-permeable membrane on the end of the capsule as the capsule is introduced into the end cap, any such end cover over the semi-permeable membrane of course prevent ing subsequent operation of the sensor.

Where the probe body is to be fitted into a sleeve-like member forming part of for example a T or elbow connector in an airline or other fluid line or the like, and where it is essential that the probe once inserted cannot be accidentally removed nor can possibly slip out as a result of a lubricating action produced by excess jel, the external body of the probe is formed with a locking device as by a groove or protrusion and the sleeve-like member into which the probe body is fitted is provided with a complementary interlocking member or aperture or the like so that the probe, when fitted into the sleeve, can be locked in position by means of the interengagement of the co-operating regions or members or the like of the probe and sleeve.

In a particular preferred embodiment of this aspect of the invention, the external surface of the probe may be formed with an external screw-thread profile and the sleeve may be provided with a complementary internal screw-thread profile enabling the probe to be screwed into the sleeve during fitting and unscrewed after use. The screwthreaded engagement of the two parts is of course exemplary of many different types of inter-engage ment and this aspect of the invention is not in any way limited to the use of a screw-thread profile on the probe body only. Indeed for quick release, a bayonet type fitting or the like may be preferred.

The invention will now be described by way of example with reference to the accompany ing drawings in which: Figure 1 is a cross sectional view through a probe constructed in accordance with the invention.

Figure 2 is an end view of the probe assembly in the direction of arrow A in Fig. 1, Figure 3 is an enlarged cross sectional view of a removable end cap with replaceable capsule, and Figure 4 is an end view of the end cap of Fig. 3 when viewed in the direction of arrow B, and Figure 5 is a perspective view of a capsule having certain aspects of the invention incor porated therein, and Figure 6 is an end view of another capsule constructed in accordance with another em bodiment of the invention.

Figure 7 is a cross-section through an im proved replacement end-cap containing a charge of gel with a throw-away bung.

Detailed description of the drawings Referring to Fig. 1, an oxygen sensor com prises a cylindrical probe body 10 having a central through bore 1 2 along which extends conductors for connection to a thin walled cylindrical anode 1 4 formed from silver and a concentric rod like cathode 1 6 of gold which are separated by a cylindrical sleeve of electri cally insulating dielectric material 1 8.

The extreme right-hand end of the cylindri cal anode and rod-like cathode occupy substantially the same radial plane and can therefore be brought into close axial proximity to a stretched semi-permeable membrane 20 which forms an end wall of a removable capsule of cylindrical configuration 22.

Within the capsule 22 is located a charge of electrolyte gel suitable for use with gold and silver electrodes to produce a variable conductance cell depending on the amount of oxygen which migrates through the semi-permeable membrane 20.

An end cap of general cylindrical configuration 24 secures the capsule 22 in position and itself is formed with an internal screwthread at 26 for engaging an external screwthread form 28 at right-hand end of the body 10 as shown in Fig. 1.

At the left-hand end of the body 10 is a second end cap 30 bonded or scrweed or otherwise secured to the body 10 and to facilitate the fitting of the probe into a sleevelike member (not shown) the shouler between the end cap 30 and the body 10 is formed with a screw-thread profile 32.

Leads generally designated 34 serve to connect the electrode assembly to apparatus for measuring the conductance of the cell and indicating the latter as a measure of oxygen molecules permeating the semi-permeable membrane.

Within the body 10 is located a thermistor 36 connected to a second pair of leads generally designated 38 for introducing temperature condensation into the measured values of oxygen permeation.

A window 40 in the end of the end cap 24 allows the semi-permeable membrane 20 to be exposed to a fluid containing oxygen.

Fig. 2 is an end view in the direction of the arrow A in Fig. 1 and illustrates the semipermeable membrane 20 which can be seen through the window 40 in the otherwise solid end of the end cap 24.

In the enlarged view of Fig. 3, it will be seen that the capsule 22 has stretched across one end of the semi-permeable membrane 20 and can have stretched across its opposite end a retaining seal of foil or the like identified by reference numeral 42, which seal can be penetrated by the electrode assembly to allow the electrode assembly to come into contact with the gel normally held captive within the capsule 22 by the seal 42 at one end and the semi-permeable membrane 20 at the other.

In the event of excess gel existing within the end cap 24, a furrow 44 is formed through the internal thread profile 26 formed in the open end of the end cap 24 so that excess gel can penetrate through the tightly fitting screwthreads to exist at the annular region 46 shown in Fig. 1.

Fig. 4 which is an end view in the direction of arrow B in Fig. 3, shows how two such furrows at 44 and 48 can be formed in the screw-threaded end of the end cap 24. For clarity the insert capsule 22 is shown without an end seal 42 in place and without any gel, in Fig. 4.

In order to prevent the accidental insertion of a capsule such as 22 the wrong way round, an improved capsule design may be used as shown in Fig. 5 in which the outer surface of the capsule is in the form of a truncated cone as denoted by reference numeral 50. By providing a complementary truncated conical surface within the interior of the end cap 24, so the capsule 52 of Fig. 5 can only be inserted small end first into the end cap 24.

Simply to illustrate how the end of the capsule can be sealed, (irrespective of the external shape thereof) a disc of foil 54 is shown partially peeled away from the end face 56 of the capsule 52 of Fig. 5. Complete removal of this foil disc would be required before the capsule 52 is inserted into the end cap 24.

The charge of gel is not shown within the capsule 52 but an internal screw-thread profile at 58 is shown. The purpose of this is to facilitate the withdrawal of the capsule 52 when it is to be replaced. To this end a tool having an appropriate external thread profile can be inserted into the open end of the capsule 52 and twisted into engagement with the screw-thread profile to allow the capsule 52 to then be withdrawn out through the open end of the end cap 24.

The taper of the truncated conical external surface of the capsule 52 is shown exaggerated in Fig. 5 to demonstrate the principle. In practice only a very shallow taper is required, just sufficient to prevent incorrect entry of the capsule into the end cap.

Where it is desired that the insertion of the electrode assembly through the sealing disc such as 54 will automtically break the latter, a sealing disc such as 60 and shown in Fig. 6 may be employed in which a series of radial half-cuts or virtual perforations are formed as at 62. Each such half-cut or virtual perforation comprises a groove formed in the internal or external or external surface of the disc which is insufficient to penetrate the material of the disk but is sufficient to produce a line of severe weakness such that in the event of an object hitting the central region of the disc, the material forming the latter will part along the lines of weakness and allow the penetration through the disc of an electrode assembly or the like.

Fig. 7 shows an alternative end cap 66 somewhat similar to that of Fig. 3 which is adapted by means of the internal screw thread 68 to be fitted to the threaded end 26 of the probe body 10 of Fig. 1. However instead of the capsule 42 of Fig. 3 being replaceable, the charge of gel in Fig. 7 is contained in a cylindrical sleeve 70 which is a tight fit within the inside of the cap 66 and the entire end cap is made disposable after use. To this end the sleeve 70 is securely held in place by means of the membrane 72 stretched therearound, which covers the end of the sleeve 70 adjacent the opening 74 in the cap 66 and is sandwiched between the outer surface of the sleeve 70 and the inside of the cap 66.

The gel is kept in place by the membrane 72 at the one end and by the bung 76 at the other, the bung being threaded at 78 to enable it to be screwed into the cap thread 68. The enlarged diameter head 80 of the bung may be knurled.

The assembly of cap and bung is sealed into a plastics wallet or the like (not shown) and when required for use is simply removed from the wallet, the bung is unscrewed and the cap screwed onto the the threaded probe end in place of a spent cap which can be disccarded together with the bung.

Claims (14)

1. In a sensor in which a generally cylindrical anode surrounds a concentric radially spaced cathode to form at one end a sensitive annular region the improvement wherein the end of the anode and the end of the cathode defining the said sensitive annular region occupy substantially the same radial plane and a semi-permeable membrane is stretched across the said sensitive region in a second radial plane axially spaced apart from the first with a film of electrolytic gel sandwiched between the ends of the electrodes and the said membrane.

2. A sensor as claimed in claim 1 in which the cylindrical anode is formed from a thin walled cylindrical member of silver or silverplated material.

3. A sensor as claimed in claim 1 in which the annular space between the anode and cathode is filled with an electrical insulating material which serves as a dielectric material between the two electrodes.

4. A sensor as claimed in claim 1 in which the electrode assembly is protected by a cylindrical end cap which extends beyond the end of the protruding electrode assembly and the remote end wall of which is provided with a central aperture, and a membrane is located within the cap so as to cover the window formed in the remote end thereof by the said aperture and to trap a charge of electrolytic gel in contact with the electrode assembly.

5. A sensor as claimed in claim 4 wherein the end cap is formed from a plastics material and a screw-thread or bayonet fitting is provided for securing the end cap to the probe body which is configured accordingly.

6. A sensor as claimed in claim 4 wherein the end cap includes at least one passage communicating between the interior of the cap and the end thereof remote from the windowed end so that any excess gel can exit from the cap at a point remote from the window and is kept away from the end face of the cap and the cylindrical external surface thereof.

7. A sensor as claimed in claim 1 in which the body is formed with an external screwthread profile in combination with an end cap which is formed internally with a complementary internal screw thread for securing the cap to the probe body the improvement wherein the internal screw thread profile of the end cap is formed with at least one axial furrow or groove along which excess gel can penetrate to relieve the pressure of the gel within the end gap as the latter is screwed onto the probe body.

8. A sensor as claimed in claim 1 in combination with a removable end cap which latter includes a generally closed end containing a window that is fitted internally with a sleeve dimensioned to just fit within the cap and containing a charge of gel and wherein a membrane is stretched across the end face of the sleeve and sandwiched between the end face thereof and the generally closed end of the cap, behind the window.

9. A sensor as claimed in claim 8 wherein the end of the sleeve through which the nose containing the electrodes penetrates on fitting the cap to the body, has stretched thereacross a thin sheet material having an array of radiating vertual perforations which upon the cap being fitted to the body are broken open to allow the nose to penetrate into the gel.

10. A sensor as claimed in claim 8 wherein the sleeve is adapted by means of its shape and/or dimensions to be capable of being fitted only one way into the end cap to prevent its accidental insertion with the semi permeable membrane end thereof adjacent the open end which is to be fitted to the probe.

11. A sensor as claimed in claim 10 wherein the sleeve is in the form of a trun cated cone and the interior of the end cap is formed with a complimentary conical passage so that the sleeve can only be inserted into the end cap with the smaller diameter end first and the membrane is stretched across the smaller diameter end.

1 2. A sensor as claimed in claim 8 wherein the cap is fitted with a plug to trap a charge of gel therein and keep it out of contact with the atmosphere until required for use, to which end the plug is removable to facilitate the fitting of the cap to the probe body.

1 3. A sensor as claimed in claim 1 2 in which the membrane is stretched across the inner most end of the sleeve containing the gel and is trapped between not only the end face of the sleeve and the inner end face of the cap but also between the outer wall of the sleeve and the inner wall of the cap so as to assist in holding the sleeve in position and keep the membrane stretched tightly across the end of the sleeve.

14. A sensor as claimed in claim 10 wherein the cap is adapted to facilitate the removal of the sleeve.

1 5. A sensor as claimed in claim 1 which is adapted to be fitted into a sleeve-like receiver and wherein the probe body is formed with means for locking it into the receiver so that the probe when fitted into the receiver can be locked in position.

1 6. A sensor as claimed in claim 1 5 in which the external surface of the probe body and a co-operating internal surface of the receiver are formed with complimentary screw threads whereby the probe body can be screwed into the receiver for locking therein.

1 7. A sensor as claimed in claim 1 constructed arranged and adapted to operate substantially as herein described with reference to and as illustrated in the accompanying drawings.