GB2303247A - Improvements in thermocouples - Google Patents

Abstract

A thermocouple for the continuous measurement of the temperature of a molten metal, or a molten alloy, comprises a positive electrode 2 which is formed from a refractory metal e.g. Mo, W, and a negative electrode 1 which comprises a graphite rod, the junction between the positive electrode and the negative electrode being provided, in use, by the molten metal or molten alloy the temperature of which is being measured by the thermocouple. The electrodes 1,2 are protected by graphite powder 11, an aluminium sheath 10, a cermet disc 13 and a cermet cup 6. The thermocouple may be used to measure the temperature of molten aluminium, copper, iron or steel.

Description

IMPROVEMENTS IN OR RELATING TO THERMOCOUPLES The present invention relates to thermocouples and, in particular, to a thermocouple for use in the measurement of the temperature of molten metals.

Thermocouples are widely used devices for the measurement of temperature and are used in many applications including the measurement of the temperature of molten metals during steel making and alloy making. Thermocouples typically consist of two wires of dissimilar electronic conductors joined together at the point of measurement which produce a voltage differential across their other ends. This voltage differential is a direct function of the difference in temperature between the hot junction and cold junction. Many different types of thermocouples are commercially available, suitable for operation over different temperature ranges and in different environments. For example, a thermocouple with a bimetallic junction of platinum and a platinum/rhodium alloy may be used for the measurement of the temperature of molten metals. A thermocouple of this type is disclosed in EP-A-0329323.The major disadvantage of thermocouples based upon platinum and platinum/rhodium alloys is their expense, particularly since in the aggressive environments encountered in, for example, the steel making industry thermocouples based on platinum and platinum/rhodium have a very short life typically of less than 30 seconds.

It has been proposed that thermocouples based on molybdenum/graphite could be used for the measurement of the temperature of molten metals, such as iron or steel, but a major disadvantage is that graphite is a very brittle material and it is therefore difficult to make an electrical connection of a graphite rod to a molybdenum wire at the hot thermocouple junction.

Furthermore, the thermocouple junction will oxidise in an air atmosphere when hot.

We have now developed a thermocouple based on a refractory metal and graphite in which the difficulties and disadvantages of the prior art systems are overcome.

Accordingly, the present invention provides a thermocouple for the continuous measurement of the temperature of a molten metal, or a molten alloy, which comprises a positive electrode which is formed from a refractory metal and a negative electrode which comprises a graphite rod, the junction between the positive electrode and the negative electrode being provided, in use, by the molten metal or molten alloy the temperature of which is being measured by the thermocouple.

In the thermocouples of the present invention the positive electrode will generally be in the form of a wire of a refractory metal such as molybdenum or tungsten. It will be understood that the refractory metal for the positive electrode should be a metal which is chemically inert in the molten metal or alloy. The positive electrode is generally protected against chemical dissolution on immersion into the melt by an electrically conductive cermet. The cermet is preferably in the form of a cermet cup such as a cup comprising a composite of molybdenum and zirconia which has a good resistance to dissolving in molten metals and molten alloys, whilst providing good electrical contact to the melt. The cup is packed with a metal powder and the positive electrode inserted into powder to give good electrical contact.

It may also be necessary to protect the graphite rod with a cermet for use in aggressive environments, such as molten steel, to prevent the dissolution of the graphite rod in the melt.

In the thermocouples of the present invention the junction between the refractory metal positive electrode and the graphite negative electrode is provided by the molten metal or molten alloy, the temperature of which is to be measured. The molten metal or molten alloy may thus be, for example, aluminium, copper, iron or steel.

In the use of the thermocouples of the present invention the molten metal or molten alloy acts in the manner of a third wire joining the positive and negative electrodes together. Provided that the bath of the molten metal or molten alloy is at a uniform temperature then the bath will have no effect on the emf. The emf which is produced depends only upon the hot/cold junction and the difference between these temperatures.

The thermocouple may be provided with a protective end cap, for example an end cap of iron or steel, which encases the end of the thermocouple which is immersed in the molten metal or alloy to provide the initial protection against chemical attack from the aggresive slags on the surface of the melt.

The advantages of the thermocouple of the present invention are that (i) it is made of relatively cheap materials, the graphite/molybdenum combination being much cheaper than the platinum/rhodium or tungsten/rhenium combinations of the prior art; (ii) the brittleness of the graphite electrode is overcome because no join to the graphite electrode is required to be made at the hot end and at the cold end which is at room temperature or below the connection can readily be made by vacuum brazing or mechanical means; (iii) the thermocouple may be cast into a refractory material to provide strength and protection against the molten metal and slag; (iv) the thermocouple has high sensitivity to temperature changes and a larger response in terms of millivolts/degree than the previously used thermocouples.

Furthermore, the thermocouple of the present invention may be incorporated into a probe for measuring the content of an impurity or the like in a molten metal or molten alloy bath. For example, if the probe is of the type in which the counter electrode is formed from a refractory metal then the same electrode may also be used as the positive electrode for the thermocouple of the present invention. In practice, all that may be required is to modify the sensor probe design by incorporating a graphite rod into it. A probe which may be modified in this manner is the probe for the measurement of trace elements as described in PCT/GB94/00045 (W094/16318), in which a counter electrode, such as a molybdenum zirconia cermet, is already incorporated.

The present invention will be further described with reference to the accompanying drawings, in which:- Figure 1 illustrates a first embodiment of the graphite/molybdenum thermocouple of the present invention; Figure 2 illustrates a second embodiment of the graphite/molybdenum thermocouple of the present invention; Figure 3 is a graph of the response of the thermocouple of Figure 1 when placed in a molten aluminium bath as described in Example 1 and also compares the response with that of a platinum/rhodium thermocouple as described in Example 1; Figure 4 compares the response of the thermocouple of Figure 1 when placed in a molten aluminium bath with that of a platinum/rhodium thermocouple as described in Example 1; and Figure 5 is a graph of the emf v.temperature for a molybdenum/graphite thermocouple tested in a furnace.

Referring to the drawings, Figure 1 is an illustration of a graphite/molybdenum thermocouple in accordance with a first embodiment of the invention.

The thermocouple comprises a graphite rod 1 which forms the negative electrode and a molybdenum wire which forms the positive electrode of the device. The molybdenum wire is encased in an alumina sheath. The graphite rod 1 and molybdenum wire 2 are housed within a steel tube 4 which is filled with a castable refractory material 5,for example, Plibrico Petrolite 39. The end of the molybdenum wire which is to be dipped into the molten metal or metal alloy is protected by a cermet cup 6 which is filled with a molybdenum powder 7 into which the molybdenum wire 2 extends. The cermet cup 6 containing the molybdenum wire positive electrode and the tip 8 of the graphite rod 1 protrudes beyond the boundary of the castable material 5.

In use, the molybdenum/graphite thermocouple is placed in a bath of a molten metal or molten alloy, the metal or alloy acting as a third wire between the tip 8 of the graphite rod 1 and the molybdenum wire 2 encased in the cermet cup 6.

Figure 2 illustrates a molybdenum/graphite thermocouple in accordance with a second embodiment of the present invention. Like parts are detailed in Figure 2 of the drawings with like reference numerals.

In this embodiment of the invention the graphite rod 1 forming the negative electrode is surrounded by an alumina sheath 10 and the space between the alumina sheath and the graphite rod is filled with graphite powder 11. The end 12 of the graphite rod is, in this embodiment of the invention, protected from the molten metal or molten alloy into which the probe is inserted by an electrically conductive cermet disc. The cermet disc 13 is formed with a blind hole into which the tip 12 of the graphite rod extends. The operation of the graphite/molybdenum thermocouple as detailed in Figure 2 is substantially the same as the mode of operation of the thermocouple described with reference to Figure 1, except that the end of the graphite rod is protected from the melt and the life time of the thermocouple is increased.

The present invention will be further described with reference to the following Examples.

EXAMPLE 1 A junctionless thermocouple probe containing a graphite rod and a molybdenum wire as the two conductors as illustrated in Figure 1 of the accompanying drawings was prepared according to the following procedure.

A Mo-Zr02 composite cermet cup was filled with molybdenum powder having a particle size in the range of from 4 to 8Am. A preformed length of molybdenum wire was immersed into the cup. The wire was insulated with an alumina sheath. The cermet cup and graphite rod were placed in a rubber bung and a ceramic cement (Aremco 569) was used to seal the area at the open end of the cermet cup to prevent any loss of molybdenum and the ingress of air or moisture. The assembly was then placed in a steel mould 0.5 m in length which fitted over the rubber bung and a castable mixture of Plibrico Petrolite 39 was then prepared. A length of chicken wire was inserted into the mould and the castable mix was then poured into the mould whilst the mould was vibrated in order to remove trapped air.The protruding molybdenum wire and graphite rod were moved to their desired positions and secured with a template whilst the castable mixture was allowed to set.

After approximately 12 hours the rubber bung was removed and the probe was transferred to an oven and fired. A ramp rate of 100/minute was used to increase the temperature from 250C to 3500C. The thermocouple was held at this temperature for 12 hours and then returned to the starting temperature of 250C at a ramp rate of 100C per minute.

A protective steel cap was welded to the end of the probe to give initial protection against chemical attack from the slags on the surface of the melt upon immersion into the melt during use.

The probe was tested in molten aluminium into which it was inserted at approximately 7100C. A platinum/rhodium thermocouple was used as a reference and inserted into the molten aluminium at 7100C before the insertion of the molybdenum/graphite probe.

Temperature measurements were checked against a commercial dip sensor.

The emf signals from the molybdenum/graphite thermocouple and the platinum/rhodium thermocouple were recorded on a chart recorder at a speed of 1 cm/minute. The molybdenum/graphite probe responded more quickly to temperature changes than those recorded by the reference thermocouple. Figure 3 of the accompanying drawings illustrates the increased sensitivity of the molybdenum/graphite thermocouple in comparison to the platinum/rhodium reference thermocouple (type B).

A signal comparison between the molybdenum/ graphite thermocouple and the reference thermocouple was made. At several positions on the chart recording a common difference was found to separate the two thermocouples. Respective signal values approximately 14mV higher than those observed for the reference thermocouple were observed continually for the molybdenum/graphite thermocouple as shown in Figure 4 of the accompanying drawings.

EXAMPLE 2 A junctionless thermocouple probe containing a graphite rod and a molybdenum wire as the two conductors as illustrated in Figure 2 of the accompanying drawings was prepared according to the following procedure.

A Mo-ZrOz composite cermet cup was filled with molybdenum powder having a particle size in the range of from 4 to 8hum. A preformed length of molybdenum wire was inserted into the cup and the wire was insulated with an alumina sheath. A 20g cermet disc of molybdenum/zirconia was pressed with a diameter of 25mm and a 5mm diameter blind hole then green machined half way into the disc. The disc was placed in an Astro furnace and heated to 18500C. After firing the diameter and hole size had decreased by approximately 13%.

A graphite rod was then inserted into the hole and an alumina sheath was fitted over the rod. The cermet cup and cermet disc were then placed into a rubber mould and a ceramic cement (Aremco 569) was used to seal the areas which were open to the air.

Graphite powder was also introduced to fill the space between the graphite rod and the alumina sheathing.

The remainder of the preparation of the probe was in accordance with the procedure of Example 1, except that the mould was 1.OM in length.

The probe was inserted into a bath of molten pig iron at a temperature of approximately 14600C and the power to the furnace was turned off. The signal showed an increase in gradient until the probe reached thermal equilibrium with the melt. The emf value was approximately 30mV t 0.5 mV during this time and the temperature was approximately 13700C.

The power was turned on again and a dip temperature measurement was taken approximately 8 minutes later. The temperature was approximately 1410 C and the emf value at this time was approximately 3lmV + 0.5 mV.

The emf's recorded in this experiment were consistent with the emf results obtained using a molybdenum/graphite thermocouple in which the junction between the molybdenum and the graphite was positioned in an Astro furnace. The temperature profile for the control experiment is given in Figure 5. In this control experiment the cold junction of the molybdenum/graphite thermocouple was maintained at OOC using a calibrated ice point reference cell.

Claims (1)

1. CLAIMS:

   1. A thermocouple for the continuous measurement of the temperature of a molten metal, or a molten alloy, which comprises a positive electrode which is formed from a refractory metal and a negative electrode which comprises a graphite rod, the junction between the positive electrode and the negative electrode being provided, in use, by the molten metal or molten alloy the temperature of which is being measured by the thermocouple.

   2. A thermocouple as claimed in claim 1 wherein the positive electrode is in the form of a wire of the refractory metal.

   3. A thermocouple as claimed in claim 1 or claim 2 wherein the refractory metal is molybdenum or tungsten.

   4. A thermocouple as claimed in any one of the preceding claims wherein the positive electrode is protected by a cermet cup comprising a composite of molybdenum and zirconia.

   5. A thermocouple as claimed in any one of the preceding claims wherein the graphite rod which forms the negative electrode is protected by a cermet.

   6. A thermocouple as claimed in claim 5 wherein the cermet protecting the graphite rod is in the form of a sheath or a disc with a blind bore.

   7. A thermocouple as claimed in any one of the preceding claims wherein the molten metal or molten alloy is aluminium, copper, iron or steel.

   9. A thermocouple as claimed in any one of the preceding claims wherein the thermocouple has protective cap encasing the end immersed in the molten metal or alloy to provide initial protection against chemical attack upon immersion into the molten metal or alloy.

   10. A thermocouple as claimed in any one of the preceding claims which is incorporated into a sensor for the measurement of impurities in molten metals or molten alloys.

   11. A thermocouple as claimed in claim 10 wherein the positive electrode of the thermocouple also acts as the counter electrode in the sensor.