US2012465A

US2012465A - Thermocouple of high electromotive force

Info

Publication number: US2012465A
Application number: US618112A
Authority: US
Inventors: Godecke Wilhelm
Original assignee: Individual
Current assignee: Individual
Priority date: 1931-06-20
Filing date: 1932-06-20
Publication date: 1935-08-27
Anticipated expiration: 1952-08-27
Also published as: GB381137A

Description

Aug. 27, 1935. V w. GOD ECKE 2,012,465

THERMOGOUPLE OF HIGH ELECTROMOTIVE FORCE Filed June 20, 1932 \NVELNTOR William Gb'olecke ATTO NE-Y Patented Aug. 27, 1935 THERMOCOUPLE OF HIGH ELECTRO- MOTIVE FORCE Wilhelm Giideeke, Hanau-on-Main, Germany Application .iime 20, 1932, Serial No. 618,112 In Germany June 20.1931

Claims.

This, invention relates to thermocouples and aims at providing improved thermocouples of high electromotive force and thus rendering possible the use of very cheap measuring in stru ments, which possess a low sensitiv'eness.

So far as thermocouples having limbs made of noble metals are concerned, the Le Chatelier couple has hitherto been most generally used, this alone having a measuring range up to 1600 C. The E, M. F. of this couple is, however, so small that it is necessary to use very highly sensitive measuring instruments which are very expensive.

The E. M. F. of thermocouples made from base metal components, such for example as constantanand chrome-nickel couples and the like, is substantially greater, but their measuring ranges only extend up to about 900-1000 C., so that the extent to which they can be used is limited.

For a long time, therefore, attempts have been made to find thermocouples which, like the Le Chatelier couples,..are suitable for temperatures up to about 1600 C. but which owing to increased E. M. F. can be connected to less sensitive and, therefore, more accurate and cheaper measuring instruments, and which able measuring instruments of substantially hi her total resistance to be used with particular accurate measurements.

It has been found that couples of higher E. M. F. and substantially higher heat resistance can be produced by making one limb from platinum or one of the platinum metals such as rhodium or of an alloy thereof, and the second limb from an alloy containing platinum and rhenium.

Even with rhenium contents of about 3%, such a couple has a substantially higher E. M. F. than one with limbs containing respectively platinum and platinum and rhodium. In general the rhenium-may amount to between 3 and of the total alloy, while it may advantageously be about 10%. Rhenium contents outside these limits may, however, sometimes be used.

Tests have shown that in comparison with the E. M. F. of 16.6 millivolts at 1600 C. produced by the Le Chatelier couple, the E. M. F. of a couple according to the invention and having a limb containing about 10% rhenium in addition to platinum is 45 millivolts, while an increase of the rhenium content leads to an increase in the E. M. F. to 60 millivolts.

By alloying further metals, such as rhodium, iridium, iron, nickel or cobalt, to the platinumrhenium alloy, besides increasing the sensitivity and heat-resistance, this has the further advantage that the strong tendency of the platinum metals to recrystallization can be avoided. Thus, for example, an addition of 3% rhodium is enough to prevent the alloy becoming brittle owing to recrystallization. A limit is set to the amount of further elements which may be added 5 by the resultant increase inrthe specific resistance.

The heat resistance of the couples can be increased further if rhodium is included in the second limb. Thus for example thermocouples in 10 which one limb contains rhodium and platinum and the other platinum and rhenium are more heat-resistant than those in which one limb consists of platinum alone and the otherof platinum and rhenium. in

With the use of a rhodium limb besides a platinum-rhenium limb, the heat-resistance is increased still further, that is'to say up to' about 1800" C., but in this case theE. M. F. is somewhat smaller and amounts to about 25 millivolts at It has further been found that the combination of one limb made of a platinum-rhenium alloy with another limb made of palladium gives a thermocouple which has an E. M. F. at temperatures of 1000 C., and over which lies uniformly. about 15 to 20 millivolts higher than that of a combination consisting of platinum on the one hand and platinum-rhenium on the other hand. The use of palladium for the'second limb also makes the thermocouple cheaper. because the much smaller specific weight of palladium makes possible a. reduction in the weight of the limb in comparison with a platinum limb.

It has also been found that, in thermocouples of the-kind described so far in this specification, the rhenium of the platinum-rhenium alloy to be used for one limb can be replaced by metals which are adjacent to rhenium in the periodic system and especially by those of the fifth and 4 sixth groups of the periodic sys'iem. Thermocouples of which one limb consists of platinum or palladium or another platinum metal while the 1 and"4% vanadium, between about2an'd 10% molybdenum, and from 3 up to about "1% manga'nese... I

.The E. M. F. of such thermocouples, when one 5 limb is made of platinum and the other of an alloy as described are as follows at 1200 C. I

Composition of the alloy E. M. F

Platinum-i17% tantalum"; .Q 23 my. Platinum+2,6% vanadium my Platinum 7% molyb' enum... 24 my. Platinum 9% tungsten- 36 my.

' enclosed in anon-metallic; preferably gas-tight,

materiaL, Examples of suitable materials are highly fire-resistant porcelain, quartz and the like.

limb instead of a'platinum limb, the thermocou-' The mkbr oxidation due to the base metal character of the metals such inter alia' as tungsten r and molybdenum, as well as the volatility of the resultantioxides and any absorption or taking up of gas by the wires of thethermocouples, can be avoided if that limb which consisted an alloy of platinum with the said easily oxidizable metals is In order to prevent the access of oxygen to the enclosed wire; the latter may advantageously .be

' 'iused into the material. Preferably a ceramic tube is used to encase the wire and the space within the tube around the'wire is evacuated or l pies have substantially higher E. M. F. In general the increase in E. M. F. at 1200" C., relatively to that or a thermocouple having the second limb of platinum, is about 15 millivolts. 4 'When a palladium limb is used it has been found to be particularl advantageous to encase it gas-tightly in a'ceramic material in the way described, above, in'order ,to counteract the tendency of palladium to take-up gas on being maintainedior sometime at a high temperature and thus-to bring about variations in the E. M. F. curve of the couple.-

Ii both limbs are encased gas-tightly, combina- 'tions such as .pa1ladium-platinum-!tungstenor 5 palladium-platinum-molybdenum have proved to be. completely resistant.

' owing to their high a. M. n, have the substantial advantage that cheaper-and less. sensitive measur-. ing instruments can be used for indicating the E. M.F. than has been possible hitherto with noble metal thermocouples.

The drawing shows in cross section a thermocouple according to my invention.

Such thermocouples,

In the drawing, 2 2 represent the two limbs oi. the thermocouple, which are encased in a covering-element 4 of refractory material such as porcelain or quartz. The covering may be closed at its lower endby a sealing element 8 .0! .insulatin'gmater-ial of any suitable type.

The use of heat-resistant alloys with rhenium, which also have the property'of not being at-- tacked, affords in addition to the large range over which they can be employed, thesubstantial advantage that cheap, i. e. measuring instruments oflow sensitiveness can be. used, and

metalthermocouples What 1' claim is 1. Athermocouple one limb of which consists essentially of a metal of the platinum group while the'other consists essentially of platinum' and rhenium 2. A thermocouple one limb of which consists essentially of platinum and the other essentially of an alloy .of platinum with rhenium.

3. A thermocouple one limb of which consists essentially of palladium'while the other consists essentially of platinum and rhenium,-

4. A thermocouple one limb of which consists essentially of a metal of the platinum group and the other essentially of an alloy of platinum .with

5. -A thermocouple one limb of which consists essentially of a metal of the platinum group and ,the other; essentially of an alloy of platinum containing about 10% of rhenium.

6. A thermocouple one limb of which consists essentially of a metal of the platinum group while the other consists essentially of platinum and rhenium, to which ametal selected from the group consisting of rhodium, iridium, iron,-nick el and cobalt is alloyed which is capable of preventing recrystallization.

7'. A thermocouple one limb oi which consists essentially of a metal of theplatinum group while this hasnot hitherto been possible with noble rhenium containing from 3 to 15% of rhenium."

the other consists essentially of platinumand I rhenium, to which a metal selected from the group consisting. oi! iridium, iron and nickel is alloyed which is capable of preventing recrystallization.

8. A thermocouple essentially of a metal of the platinum-group while the other comprises essentially platinum, rhenum and'rhodium. 1

I 9. A thermocouple one limb "01 which consists the other 'consistsessentially of platinum, rhen- .ium and substantially, 3% of rhodium.

10. A thermocouple comprising two the one limb of which consists essentially or a metal of the platinum group while major; portion of one ofwhich limbs consists of a platinum metal and the major portion of the.

' other of which .li'mbs consists of pi tinum and rhenium. I v v I wmr-mmabnncxla