GB2097538A

GB2097538A - Solid electrolyte probes and their manufacture

Info

Publication number: GB2097538A
Application number: GB8113184A
Authority: GB
Original assignee: Associated Engineering Ltd
Current assignee: Associated Engineering Ltd
Priority date: 1981-04-29
Filing date: 1981-04-29
Publication date: 1982-11-03

Abstract

A solid electrolyte probe is used for the measurement of the concentration of a specified substance in a fluid mixture such as a molten metal. The probe comprises a reference electrode 18 spaced from a sensitive electrode 21. The sensitive electrode comprises a reference material 19 shielded by a solid electrolyte 20 which is applied to the reference material by a plasma spraying technique. In use, the probe is dipped into the fluid mixture and the substance in the fluid mixture results in an electrical potential between the reference electrode and the sensitive electrode which is a function of the concentration of the substance in the fluid mixture. <IMAGE>

Description

SPECIFICATION Solid electrolyte probes and their manufacture The invention relates to solid electrolyte probes for use in the measurement of the concentration of a specified substance in a fluid mixture, and to methods of manufacturing such probes.

Such a probe commonly comprises a reference electrode and a sensitive electrode such that when dipped into the fluid mxture, the substance in the fluid mixture results in an electrical potential between the reference electrode and the sensitive electrode which is a function of the concentration of the substance in the fluid mixture. In one known form of probe, the sensitive electrode is formed by a ceramic tube with a conductor passing down the tube to make electrical contact with a pellet of material forming the sensitive electrode and pressed into the end of the ceramic tube.

It is a disadvantage of such a probe that it is difficult to manufacture and that its construction may produce errors in the measurement of concentration. It is an object of the invention to overcome these disadvantages.

According to a first aspect of the invention, there is provided a probe for use in the measurement of the concentration of a specified substance in a fluid mixture, comprising a reference electrode spaced from a sensitive electrode, the sensitive electrode comprising a reference material shielded by a solid electrolyte applied over the reference material by a plasma spraying technique, the reference electrode and the sensitive electrode being such that, in use, the substance in the fluid mixture results in an electrical potential between the reference electrode and the sensitive electrode which is a function of the concentration of the substance in the fluid mixture.

The sensitive electrode preferably includes a solid elongate electrical conductor, the reference material being applied on the surface of an end of the conductor by a plasma spraying technique.

Preferably, the solid electrical conductor is a steel rod having a radiussed and to which the reference material and the solid electrolyte are applied. Where the fluid mixture is at very high temperature, the conductor may be another suitable material, such as titanium.

The probe may include a lance into which the reference electrode and the sensitive electrode are a push-fit to connect electrically the electrodes to respective electrical terminals at an end of the lance remote from the push-fit connections.

The probe may be for use in the measurement of phosphorus concentrations in a fluid mixture and in this case the reference material may be iron phosphate.

According to a second aspect of the invention, there is provided a method of manufacture of a probe according to the first aspect of the invention and comprising preparing the reference material on an electrode and then applying the solid electrolyte over the reference material by a plasma spraying technique.

The step of preparing the reference material preferably comprises coating the end of an elongate electrical conductor with the reference material by a plasma spraying technique.

Preferably the solid electrolyte and/orthe reference material are fed to the plasma gun in the form of a powder, during plasma spraying.

The following is a more detailed description of one embodiment of the invention, by way of example, reference being made to the accompanying drawing which is a schematic vertical cross-section through a solid electrolyte probe for determining the concentration of a specified substance in a fluid mixture including the substance.

The probe comprises a lance 9through which pass two electrically conductive wires 10, 11 each extending between an associated push-fit connector 12, 13 and a terminal 14, 15 for connection to a high impe dence voltmeter 16. Two rods 17, 18 are push fitted into connectors 12, 13 respectively.

One rod 17 is formed of an electrically conductive material, for example B.S. EN31 steel of 1/8" diameter. The end of the rod 17 remote from the connector 12 is radiussed and carries a reference material 19 covered with a solid electrolyte 20 to form a sensitive electrode 21. This electrode is prepared as follows: First, a reference material 19 in the form of a powder is coated onto the end of the rod 17 by plasma spraying. The reference material 19 is a material which is sensitive to the substance of which the concentration is to be measured. Next, a solid electrolyte 20, also in powder form, is applied over the reference material by a similar suitable known plasma spraying technique. The solid electrolyte 20 covers completely the reference material 19, as can be seen from the Figure, and allows ionic, but not electronic, conduction.

The other rod 18 is made from a suitable inert material, to form an inert reference electrode.

In one example, for measuring the concentration of phosphorus in a fluid mixture, such as molten metal, the composition of the electrodes is asfollows: Inert electrode 18 - Graphite Reference material 19 - Iron phosphate Solid electrolyte 20 - Calcium phosphate.

It will be appreciated that, for the inert reference electrode 18, suitable materials other than graphite may be used. In addition, where the probe is for measuring the concentration of a material other than phosphorus, for example sodium, the materials forming the sensitive electrode 21 will be chosen accordingly; in the case of sodium, for example, the reference material 19 may be plasma-sprayed sodium (3aluminate and the solid electrolyte may be plasma-sprayed sodium aluminate.

In operation, the terminals 14, 15 of the probe are connected to a high impedence voltmeter 16 and the probe is dipped into the substance containing the material of which the concentration is to be measured, for example into a molten metal. The solid electrolyte 20 allows phosphorus ions to penetrate to the reference material 19 thus creating an electric potential between the inert reference electrode 18, and the sensitive electrode 21 through the molten metal. The magnitude of this potential, which is measured by the voltmeter 16, depends on the con centration of the phosphorus in the molten metal and thus can be used to determine this concentration. The voltmeter 16 can be calibrated to allow concentrations to be read directly from the voltmeter 16.

The plasma spraying of the reference material 19 over the end of the rod 17 provides a large area of this material to give maximum contact area of the sensitive electrode 21 with the material, thus eliminating the likelihood of misleading readings which might be obtained as a result of localised high concentrations of the substance adjacent to a sensitive electrode of small area.

The push-fit of the rods 17, 18 into the lance 9 allow good electrical connection to the highimpenence voltmeter and also allows easy removal and replacement of the rods 17, 18.

Claims

1. A probe for use in the measurement of the concentration of a specified substance in a fluid mixture, comprising a reference electrode spaced from a sensitive electrode, the sensitive electrode comprising a reference material shielded by a solid electrolyte applied over the reference material by a plasma spraying technique, the reference electrode and the sensitive electrode being such that, in use, the substance in the fluid mixture results in an electrical potential between the reference electrode and the sensitive electrode which is a function of the concentration of the substance in the fluid mixture.

2. A probe according to claim 1 wherein the sensitive electrode includes a solid elongate electrical conductor, the reference material being applied on the surface of an end of the conductor by a plasma spraying technique.

3. A probe according to claim 1 or claim 2 wherein the solid electrical conductor is a steel rod having a radiussed end to which the reference mat erial and the solid electrolyte are applied.

4. A probe according to claim 3 wherein the conductor is titanium.

5. A probe according to any one of claims 1 to 4 wherein the probe includes a lance into which the reference electrode and the sensitive electrode are a push-fit to connect electrically the electrodes to respective electrical terminals at an end of the lance remote from the push-fit connections.

6. A probe according to any one of claims 1 to 5 and for use in the measurement of phosphorous concentrations in a fluid mixture, the reference material being iron phosphate and the solid electrolyte calcium phosphate.

7. A probe substantially as hereinbefore described with reference to the accompanying drawing.

8. A method of manufacture of a probe according to any one of claims 1 to 7 and comprising preparing the reference material on an electrode and then applying the solid electrolyte over the reference material by a plasma spraying technique.

9. A method according to claim 8 wherein the step of preparing the reference material comprises coating the end of an elongate electrical conductor with the reference material by a plasma spraying technique.

10. A method according to claim 8 or claim 9 wherein the solid electrolyte and/or the reference material are, during plasma spraying, fed to the plasma gun in the form of powder.

11. A method of manufacture of a probe substantially as hereinbefore described with reference to the accompanying drawing.