US2536464A

US2536464A - Thermocouple

Info

Publication number: US2536464A
Application number: US25911A
Authority: US
Inventors: William A Ray
Original assignee: General Controls Co
Current assignee: General Controls Co
Priority date: 1944-09-16
Filing date: 1948-05-08
Publication date: 1951-01-02
Anticipated expiration: 1968-01-02

Description

Jan. 2, 1951 w, A, RAY 2,536,464

'H'IERHOCOUPLE Original fined Sept. 16, 1944 3noentor:

WILLIAM A. RAY,

Gttomeg.

Patented Jan. 2, 1951 UNITED STATES PATENT OFFICE THERMOCOUPLE William A. Bay, North Hollywood, Calif., assignor to General Controls 00., acorporation Original application September 16, 1944, Serial N0. 554,472, now Patent No. 2,443,641, dated June 22, 1948'.

Divided and this application" May 8, 1948,8erial No. 25,911

electromagnetic fuel-controlling valve either. of

the automatically-operated type, or of the manual-reset type wherein an electromagnet is em ployed to maintain the valve closure in open position only after it is first brought to that position by manual means; the valve being so arranged and biased that in the event of extinguishment of the pilot burner, and resultant cessation. of generation of energy, the supply of fuel to the main burner is obstructed so that the danger of Y explosion when the pilot burner is relighted is prevented. In large heating installations, and when burners of the sectional type are employed, it is necessary to provide a plurality of pilot burners; it then being essential that if any one of the pilot burners is extinguished, the suppy of fuel to its associated main burner should be stopped. If, as is customary, the main burners are supplied through a single valve, the thermoelectric generating device for energizing that valve must be influenced by the flames of all the pilot burners. However, due to the fact that the amount of electrical energy necessary to maintain an electromagnetic device in active condition is re atively small, the reduction of energy resulting from extinguishment of one of the piot burners may not be sufficient to effect closing of the valve; and that is so even when only two pilot burners are'employed and the amount of energy becomes one half of that normally ava lable. To eliminate such deficiency I provide a thermoelectric generating device influenced by heat from a plurality of separate sources, whereby the amount of electrical energy supplied to a load is disproportionately reduced upon failure of any one of the sources.

It is an object of this invention to provide thermocouples of novel construction which, while capable of other use, are especially adapted for use in thermoelectric generating. devices of the character described above.

For full understanding of the invention reference is to be had to the following detailed description and accompanying drawing, wherein:

Figure 1 is a sectional view of a thermoelectric a claim; (on. lac-4) generating device embodying the present invention; and

Figures 2, 3 and 4 are views of modified forms of thermocouples adapted for use in thermoelectric generating devices of the character disclosed inFig. 1. Referring first to Fig. 1 of the drawing, the numeral ll indicates a thermocouple comprising a tubular element l2 within. which is a rod-like element l3 which is welded or fused to the outer end of element l2 to form the "hot junction 23 of the thermocouple Indicated at H is another thermocouple which is identical in construction with thermocouple II, the elements of thermocouple ll therefore having been assigned the same numerals with a prime mark added. Connecting thermocouples II and H in parallel, to form a complete thermoelectric generating device, is a pair of conductors l5 and I5 of lowresistivity .metal such as copper. The effective cold Junction of each thermocouple is at the points (indicated by the numerals 32 and 32) where its elements 12-43 join the conductors l5l6. The thermoelectric generating device is shown connected by another pair of conductors I! to an electrical load It which may be the coil of an e ectromagnetic valve or relay. The thermocouples are arranged for heating by the flames I9 and 20 of a pair of gas burners 2i and 22, the size of the thermocouples being such that a sub stantial or major portion of the thermocouple elements adjacent the hot junctions thereof are heated by the flames.

The dissimilar elements of thermocouples of the type employed in connection with heating apparatus are usually constructed of the al oys 'ChromeP' and "Copel" or their equivalents, since the thermoelectric power of such alloys, in combination, is the highest of present-known materials capable of continuously withstanding the 7 heat of a gas flame.

The relationship between the aforementioned trade-marks and the particular alloys identified thereby is believed to be clear in view of the accepted use of these marks in the art and by the National Bureau of Standards. For instance, in Pyrometric Practice, Technologic Paper No. of the Bureau of Standards, the alloys "ChromeP and Cope'." are referred to without any identification of their compositions, and their thermoelectric E. M. F.s are given in a chart appearing on page 307 of thatpaper. On page 396 of the June 1940 issue of the Journal of Applied Physics, in an article entitled Thermoelectric Thermometry" and written by William F.

Roeser, senior physicist of the National Bureau of Standards, the composition of "Chromel-P (commonly known merely as Chromel") is given as approximately 90% Ni and 10% Cr, and that of Copel" is approximately 55% Cu and 45% Ni.

The change of electrical resistance of a "Chromel" v. Cpel" thermocouple upon heating or cooling is very small since the temperature coefllcient of resistivity of Chromel" is of the order of +0.0001 per C. and that of Copel" substantially zero. Hence, if the thermocouples of Fig. 1 were constructed of these alloys, the voltage across the load l8 would become one half the normal voltage if one of the flames i9-20 were extinguished.

According to this invention, the decrease of voltage across the load upon deenergization of one of the parallel thermocouples is augmented by forming at least one of the elements "-43 of a material having a high positive temperature coeflicient of resistivity. For example, the element i3 may be of nickel and element l2 of Chromel thermoelectrically dissimilar to nickel in a high degree. The temperature coeificient of nickel has an average value of +0.005 per C. so that a nickel element having a resistance of, say, 1 ohm at 20 C. will have a resistance of approximately 3 ohms at 420 C. and 4 ohms at 620 C. Therefore, if one of the flames is extinguished, the resistance of the cold thermocouple will be considerably less than that of the hot one so that the cold thermocouple forrns a relatively low-resistance shunt across the higher-resistance hot thermocouple, and the voltage across the load is reduced to considerably less than one half the normal voltage.

If one of the elements of the thermocouple is of a material having a low temperature coefficient, the resistance of that element is made small by increasing its cross-sectional area with respect to that of the high-temperature-coeflicient element so that even if the resistivity of the material of the low-temperature-coefficient element is high (as it is of Chromel) there will be a considerable overall change of resistance of the thermocouple with change of temperature. As shown in Fig. l, the cross-sectional area of the tubular Chromel element [2 is approximately eight times that of the nickel element i3. To further enhance the change of resistance, a substantial or major portion (beyond the hot junctions 23-23) of each thermocouple is subjected to the flame, as illustrated.

In order to obtain maximum transfer of power to the load, the resistance of the same is made substant ally equal to the resistance of the thermocouples in parallel, and when heated. For example, if the hot resistance of thermocou le ii is 4 ohms and that of thermocou le it likewise 4 ohms, the resistance of the load should be approximately 2 ohms. (Obviously, the resistance of each thermocoup e should be substantially the same.) The re i tance of the load may advantageously be slightly greater than that of the paral el thermocouples since, as can readilv be verified by calculation. there is then maximum change of resistance w th change of tem erature. However, the mismatch of resistance should not be gre t because of the overall loss of power which results.

The electrical device, represented by the load 08, is so constructed that it is necessary for both thermocoup es to be heated in order for the device to operate; i. e., for it to pull-in" if it is an or some other material which is automatic valve or relay, or to. hold-in" if it is of the manualreset" type. Upon extinc-- tion of one of the flames, "drop-out of the device is assured by the substantial reduction or power due to the decrease of resistance of the unheated thermocouple.

Combinations oi materials in addition to that mentioned above, suitable for construction of the thermocouples are, for example: nickel against iron, or nickel against copper, in which combinations both of the materials have high temperature coeflicients; or iron against "Copel," which combination has a high ther .oelectric power.

In Figs. 2, 3 and 4 there are illustrated various forms of thermocouples especially adapted for use in the system of Fig. 1, but obviously capable of other uses.

The thermocouple of Fig. 2 is similar to tho e of Fig. 1 and comprises a tubular element 24,-, conveniently of Chromel," within which is a coiled element 25, conveniently of nickel. By this :0:- rangement the effective length of the nickel "element is increased so that there is a greater cha' ge of thermocouple resistance with change oi! teperature.

In Fig. 3, the

thermocou le elements

28, and 21 form an interlaced double coil, thee convolus tions of which are spaced from each other. The fiame is arranged to heat a substantial portion of the elements beyond the hot junction 28 thereof, and since both elements are subiected directly to the flame the change of resistance of the thermocouple upon heating or cooling is relatively great.

The thermocouple of Fig. 4 com rises a straight element 29 around which, and in spaced relation thereto except at the junction 3|, is an open coil 30 which forms the other element of the thermocouple. This arrangement, generally similar to that of Fig. 3, is particularly suitable when the materials chosen are nickel and Chromel, or the like, since by employing nickel for the coil an the resistance and exposure to the flame of that element is increa ed: the short straight element 29 then being of "Chromel," its undesirable resistance is reduced to a minimum.

The embodiments of the invention herein shown and described are obviously susceptible of modification without departing from the spirit of the invention, and I intend therefore to be limited only by the scope of the appended claims.

I claim as m invention:

1. A thermocouple comprising a pair of elongated elements of thermoelectrically dissimilar materials connected together at one'of their ends to form a hot junction adapted to be subjected to a flame, the other ends of the elements being adapted to be connected to a load remote from the flame; a portion, closely adjacent said hot Junction, of one of said elements being so de formed as to make its length subjectable to said flame at least twice the length of the corresponding portion, adjacent said hot junction and subjectable to said flame, of the other of the elements; said portions being out of engagement with each other.

2. A thermocouple as defined in claim 1, and wherein said portion of said one of the elements is coiled to make its length long with respect to the length of said corresponding portion of the other of the elements.

3. A thermocouple as defined in claim 1, and

wherein the material of said one of the elements has a positive temperature coefilcient of resis- 5 tivity which is high with respect to that of said other of the elements.

4. A thermocouple as defined in claim 1, and wherein the material of said one of the elements has a positive temperature coeflicient of resistivity which is high with respect to that of said other of the elements, and also wherein said portion of said one of the elements is coiled to make its length long with respect to the length of said corresponding portion of the other of the elements.

5. A thermocouple comprisin a pair of elements of thermoelectrically dissimilar materials, one of said elements being of tubular form, the other of the elements being coiled within said tubular element and connected at one end there! to, the convolutions of said other of the elements being spaced from each other and from the tubular element.

6. A thermocouple as defined in claim 5, and wherein said coiled element is of material having a positive temperature coeflicient of resistivity which is high with respect to that of the material of said tubular element.

7. A thermocouple comprising a pair of elements of thermoelectrically dissimilar materials, one of said elements being substantially straight,

the other of the elements being joined at one end to one end of said straight element to form a hot junction and coiled around the straight element adjacent said junction, the convolutions of said other of the elements being spaced from each other and from the straight element.

8. A thermocouple as defined in claim 7, and wherein said coiled element is of material having a positive temperature coefficient of resistivity which is high with respect to that of the material of said straight element.

WILLIAM A. RAY.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 836,531 Pickard Nov. 20,1906 1,076,438 Marsh Oct. 21, 1913 1,221,270 Bard Apr. 3, 1917 1,761,392 Hase June 3, 1930 1,863,373 Harrison June 14, 1932 1,972,146 Johnson Sept. 4, 1934 2,156,853 Huggins May 2, 1939 2,305,585 Alfery Dec. 22, 1942 2,339,809 Ray Jan. 25, 1944 2,365,207 Moles Dec. 19, 1944 2,370,326 Ray Feb. 27, 1945