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Alloys for electrical heating elements
94
NATIONAL BUREAU OF STANDARDSNOTES.
[J. F. I.
corrosion process. The design of copper-copper sulfate and cadmium-cadmium sulfate half-cells ha& been improved to correct some of the faults of the half-cells previously used. (See Technical News Bulletins 264 and 265, March and April, 1939.) A brief .description of these improved half-cells was given, together with temperature correction curves. With these half-cells the potential difference between two points in the soil can be measured with an error not greater than 2 mv. A consideration of the potentials and currents in a corroding pipe line indicates that in some cases it should be possible to apply cathodic protection to bare pipe lines of small diameter. The protective current should be drained from the pipe line in places where the reference half-cell has a large positive potential with respect to the pipe line. In such places the pipe is usually found to be discharging current into the soil. By locating anodes so as to take advantage of the galvanic currents in the pipe line, a much greater length of line can be protected with one anode. The potential of the pipe with respect to a reference half-cell is not appreciably changed except in the originally anodic area near the anode. This is probably the only part of the line that really needs protection. The feasibility of the method depends upon the distribution and stability of the originally anodic areas along the pipe line. Whether it is economical or not will depend upon the relative costs of cathodic protection and probable future reconditioning. ALLOYS FOR ELECTRICALHEATING ELEMENTS. The manufacturers of electric heating devices, from toasters to furnaces, as well as the users of these devices, have always been vitally interested in the life and maximum operating temperature of the heating unit. Various types of tests, such as the “Accelerated Life Test for Metallic materials for Electrical Heating” B76-38 of the American Society for Testing Materials, have been devised with the view of predicting the life expectancy of a heating element The operating under specified conditions of operation. temperature of the material probably has more effect upon
July,1939.1 NATIONAL
BUREAU
OF STANDARDS
NOTES.
95
the results obtained from these tests than any other single Most tests of this nature variable which must be controlled. are conducted by heating the materials electrically, and in such cases the temperature of the materials (usually in the form of small wires) is most conveniently measured with an optical pyrometer. The temperature observed when an optical pyrometer is sighted upon the surface of a material heated In order in this way, is known as the apparent temperature. to obtain the true temperature which determines the life of the material, some knowledge of the radiating characteristics of the surface is required. A paper presented before the American Society for Testing Materials by William F. Roeser of the Bureau’s Pyrometry Section, gives the relation between apparent temperature and true temperature of the various types of heating element When samples of the different materials used in this country. types of materials were heated to the same apparent temperature of 2200~ F. in air, the true temperatures of the different materials ranged from 2221' to 2253' F. Although the difference between 2221~ 22.53'F. is not large, the information available indicates that the life of an element in air may be changed by 25 to 50 per cent. by changing the temperature of the material 2.5’ F. When these same materials were operated at the same apparent temperature of 2200’ F. in purified hydrogen, the true temperature of the different materials ranged from 2260’ to 2402’ F. Results on some of the alloys in commercial hydrogen and nitrogen are also included. This paper gives data needed to devise tests for determining the relative performance of different types of materials, not only in air, but also in atmospheres which are generally considered reducing.
NATIONAL BUREAU OF STANDARDSNOTES.
[J. F. I.
corrosion process. The design of copper-copper sulfate and cadmium-cadmium sulfate half-cells ha& been improved to correct some of the faults of the half-cells previously used. (See Technical News Bulletins 264 and 265, March and April, 1939.) A brief .description of these improved half-cells was given, together with temperature correction curves. With these half-cells the potential difference between two points in the soil can be measured with an error not greater than 2 mv. A consideration of the potentials and currents in a corroding pipe line indicates that in some cases it should be possible to apply cathodic protection to bare pipe lines of small diameter. The protective current should be drained from the pipe line in places where the reference half-cell has a large positive potential with respect to the pipe line. In such places the pipe is usually found to be discharging current into the soil. By locating anodes so as to take advantage of the galvanic currents in the pipe line, a much greater length of line can be protected with one anode. The potential of the pipe with respect to a reference half-cell is not appreciably changed except in the originally anodic area near the anode. This is probably the only part of the line that really needs protection. The feasibility of the method depends upon the distribution and stability of the originally anodic areas along the pipe line. Whether it is economical or not will depend upon the relative costs of cathodic protection and probable future reconditioning. ALLOYS FOR ELECTRICALHEATING ELEMENTS. The manufacturers of electric heating devices, from toasters to furnaces, as well as the users of these devices, have always been vitally interested in the life and maximum operating temperature of the heating unit. Various types of tests, such as the “Accelerated Life Test for Metallic materials for Electrical Heating” B76-38 of the American Society for Testing Materials, have been devised with the view of predicting the life expectancy of a heating element The operating under specified conditions of operation. temperature of the material probably has more effect upon
July,1939.1 NATIONAL
BUREAU
OF STANDARDS
NOTES.
95
the results obtained from these tests than any other single Most tests of this nature variable which must be controlled. are conducted by heating the materials electrically, and in such cases the temperature of the materials (usually in the form of small wires) is most conveniently measured with an optical pyrometer. The temperature observed when an optical pyrometer is sighted upon the surface of a material heated In order in this way, is known as the apparent temperature. to obtain the true temperature which determines the life of the material, some knowledge of the radiating characteristics of the surface is required. A paper presented before the American Society for Testing Materials by William F. Roeser of the Bureau’s Pyrometry Section, gives the relation between apparent temperature and true temperature of the various types of heating element When samples of the different materials used in this country. types of materials were heated to the same apparent temperature of 2200~ F. in air, the true temperatures of the different materials ranged from 2221' to 2253' F. Although the difference between 2221~ 22.53'F. is not large, the information available indicates that the life of an element in air may be changed by 25 to 50 per cent. by changing the temperature of the material 2.5’ F. When these same materials were operated at the same apparent temperature of 2200’ F. in purified hydrogen, the true temperature of the different materials ranged from 2260’ to 2402’ F. Results on some of the alloys in commercial hydrogen and nitrogen are also included. This paper gives data needed to devise tests for determining the relative performance of different types of materials, not only in air, but also in atmospheres which are generally considered reducing.