METHOD

Abstract

The present invention relates to a Pt vs. RhPt thermocouple (such as a Type R or Type S thermocouple), and to the modification of the electrical properties of the same, while in service. More especially there is provided a method for reducing the drift of a Pt vs. RhPt thermocouple while the thermocouple is in use in an oxidising environment, wherein the Pt limb of the thermocouple is doped platinum comprising an effective amount of one or more dopants selected from the group consisting of yttrium, zirconium and samarium.

Claims

1. A method for reducing the drift of a Pt vs. RhPt thermocouple while the thermocouple is in use in an oxidizing environment, wherein: the Pt limb of the thermocouple is doped platinum comprising: (a) platinum; and (b) an effective amount of one or more dopants selected from the group consisting of yttrium, zirconium and samarium.

2. A method according to claim 1, wherein the platinum to which the dopant is added is 99.997 wt % pure.

3. The method according to claim 2, wherein the platinum is 99.999 wt % pure.

4. The method according to claim 1, wherein the doped platinum comprises about 0.001 to about 0.01 wt % each of any one or more dopants selected from the group consisting of yttrium, zirconium and samarium.

5. The method according to claim 1, wherein the dopant is zirconium.

6. The method according to claim 4, wherein the doped platinum comprises about 0.0025 to about 0.0075 wt % of zirconium.

7. The method according to claim 1, wherein the oxidizing atmosphere comprises oxygen, NOx or a combination thereof.

8. The method according to claim 1, wherein the doped platinum further comprises: (c) an effective amount of one or more oxides selected from the group consisting of yttrium oxide, zirconium oxide and samarium oxide.

9. The method according to claim 8, wherein the doped platinum comprises about 0.001 to about 0.01 wt % each of any one or more oxides selected from the group consisting of yttrium oxide, zirconium oxide and samarium oxide.

10. The method according to claim 8, wherein the oxide is zirconium oxide.

11. The method according to claim 10, wherein the doped platinum comprises about 0.0025 to about 0.0075 wt % of zirconium oxide.

Description

[0033] The invention will now be described by way of the following non-limiting Examples and with reference to the following FIGURE in which:

[0034] FIG. 1 illustrates a bamboo structure resulting from grain growth in a 0.5 mm diameter pure Pt wire from a failed Pt limb.

EXAMPLE

Example 1

Preparation of the Pt Limb

[0035] The example is produced by partially oxidizing an alloy of platinum and zirconium to give both metallic zirconium and oxidized zirconium in the platinum.

[0036] The alloy contains approximately 100 ppm by weight of zirconium prior to partial oxidation. 0.6 g of zirconium is added to 6.5 kg of high purity platinum (99.997 wt % pure); 92 ppm by weight of zirconium was added.

[0037] The platinum used is of thermocouple quality. It is analysed before alloying with zirconium and is found to contain 33 ppm by weight of impurities by direct measurement). The emf output of the platinum before alloying with zirconium is measured in comparison with NIST SRM 1967a Pt reference material (0 V) and is found to be between 8 V and 10 V at approximately 1064 C. and between 15 V and 18 V at approximately 1554 C.

[0038] The alloy is melted under a reduced pressure protective atmosphere of inert gas to prevent oxidation of the zirconium. The resulting ingot is hot forged, cold rolled and drawn to form wire. The wire is flame sprayed to form a slab of partially bonded flakes with porosity between the flakes; this provides a continuous but fragile structure. Some zirconium oxidation occurs during the spraying process.

[0039] The slab is heated in air to promote further but not total oxidation. The slab is compacted to a fully dense bar by hot forging. The bar is cold rolled and drawn to a 0.5 mm diameter wire.

[0040] After annealing using electrical resistive heating to heat the wire to 1100 C. for 10 minutes the emf output of the wire is measured in comparison with NIST SRM 1967a and found to be 32 V at approximately 1064 C. and between 53 V and 54 V at approximately 1554 C.

Example 2

Positive Drift

[0041] A sample of the wire prepared according to Example 1 is heated in air in an electric furnace at 1200 C. for 140 hours to simulate service conditions. The wire is again measured in comparison with NIST SRM 1967a and the output is 27 V at approximately 1064 C. and 49 V at approximately 1554 C.

[0042] A reduction in output of the Pt limb in a thermocouple comprising Pt and 10% RhPt (Type S) or Pt and 13% RhPt (Type R) increases the indicated temperature of the thermocouple. The measured reduction in the emf of the Pt limb equates to a 0.4 C. increase in the indicated temperature of a Type R or Type S thermocouple at both the test temperatures.

[0043] The output of a sample of the wire prepared according to Example 1 is measured before and after heating at 1200 C. in air for 140 hours:

TABLE-US-00001 Before After Change Temperature (V) (V) (V) ( C.) Approximately 1064 C. 32 27 5 0.4 Approximately 1554 C. 54 49 5 0.4

[0044] Without wishing to be bound by theory, the measured reduction in the Pt limb output relative to pure Pt reference material is significant because the Pt limb of a thermocouple normally becomes increasingly contaminated in use with metallic impurities both from the local environment and from the RhPt alloy thermocouple limb (referred to as Rh migration or Rh drift). The observed effect acts to counter the normal drift.

[0045] The reduction in output has been caused by the further oxidation of metallic zirconium in the wire. Increasing the level of metallic contamination in the Pt limb always reduces the temperature indicated by the thermocouple. In contrast adding oxide inclusions to Pt does not change the emf output by the same amount and the effect of adding ZrO.sub.2 may be negligible. Converting metallic zirconium to oxidized zirconium in use therefore acts to reduce the emf output of the Pt and compensate for other acquired contamination.

[0046] Compensating for normal downward drift in the thermocouple indicated temperature is important because otherwise the drift would be greater and the thermocouple would have a shorter service life.