Practical Forms of Thermocouples.—When expensive junctions are employed, wires of the minimum thickness consistent with strength and convenience of construction are used, a diameter of No. 25 standard wire gauge being suitable. A common arrangement is shown in [fig. 4], in which J is the hot junction, the wires from which are passed through thin fireclay tubes which serve as insulators (or through twin-bore fireclay) to the reels R R, in the head of the pyrometer, upon which a quantity of spare wire is wound to enable new junctions to be made when required. Two brass strips, S, are screwed down on to the wires at one end, and are furnished with screw terminals at the other end, from which wires are taken to the galvanometer or indicator. A protecting-tube, T, surrounds the wires and hot junction. The head, H, may be constructed of wood, fibre, or porcelain, and should be an insulator for electricity and heat. There are various modifications in use, but the general method described is adopted by most makers. In order to guard against errors arising from alterations in the temperature of the cold junctions in the end of the pyrometer, some firms construct the head so as to leave a hollow space, through which cold water is constantly circulated ([fig. 5]), the arrangement being known as a “water-cooled head.” In some forms the supply of spare wires is made to take the form of two spiral springs in a hollow head, the upper ends of the springs being taken to terminals.
Fig. 5.—Pyrometer with Water-cooled Head.
The choice of a protecting-tube is a matter of considerable importance. Obviously, such a tube should not soften at the highest temperature attained, and when expensive metals are used to form the junction the sheath should not be permeable to gases or vapours. It should also, if possible, be a good conductor of heat, so that the junction may respond quickly to a change of temperature in its surroundings, and should be mechanically strong. It is difficult to secure all these properties in any single material, and the choice of a sheath is decided by the conditions under which the couple is to be used. The substances employed, and their properties and special uses, may be enumerated as follows:—
1. Iron or Mild Steel.—For temperatures not exceeding 1100° C. iron or mild steel covers are cheap and efficient from the standpoint of conductivity, although liable to deteriorate owing to oxidation. The tendency to oxidise is greatly diminished by “calorising” the exterior by Ruder’s process, in which the iron is heated in a mixture of metallic aluminium and oxide of aluminium, a surface alloy being formed which resists oxidation. A result nearly as good may be obtained by smearing the surface with fine aluminium powder, and bringing to a white heat. This treatment greatly prolongs the life of an iron sheath. Some makers employ an inner steel tube round the wires, and an outer tube which comes into contact with the furnace gases, corrosion of the latter being detected before the inner tube has given way and exposed the junction. Some makers do not consider it safe to expose heated platinum to an iron surface, with only air intervening, and hence use an inner cover of silica or porcelain, which the outer iron or steel tube protects from mechanical damage. For ordinary work seamless steam or hydraulic steel tubing, with a welded end, is satisfactory; but for dipping into molten lead or other metals the tube should be bored from the solid. The great advantage of an iron or steel sheath is its mechanical strength, which protects the couple from damage in case of rough usage.
2. Nichrom.—Certain alloys of nickel and chromium, and especially that known as Nichrom II, may be kept at 1100° C. without oxidising to any appreciable extent; and hence sheaths of this material may be used up to the temperature named. In addition to being more durable than iron, nichrom possesses the same advantages of strength and good conductivity; on the other hand, it is more costly.
3. Molybdenum.—This metal, which possesses a melting point of about 2500° C., may be dipped in molten brass, bronze, copper, etc., without being attacked, and has been used to form the tip of a protecting-tube designed to measure the temperature of molten alloys. A junction covered only by a thin tube of molybdenum quickly attains the temperature of its surroundings.
4. Graphite and Graphite Compositions.—Carbon has the highest melting point of all known substances, and in the form of artificial or Acheson graphite may be easily machined to any desired shape. Graphite sheaths are sometimes used for immersion in molten metals, but at 1000° C. and higher Acheson graphite oxidises easily and becomes friable. It is a good conductor of heat, but is easily broken. Compositions of natural graphite and refractory earths, such as Morgan’s “Salamander,” are inferior to pure graphite in conductivity, but are stronger and not readily oxidised, and may be used to form sheaths for temperatures up to 1400° C. or possibly higher, when penetration of furnace gases to the junction is not of moment.
5. Porcelain.—This material, in its best forms, may be used up to 1400° C., but must be efficiently glazed to prevent the ingress of furnace gases to the junction. It is easily broken by a blow, and when circumstances permit should be protected by an iron covering-sheath. The variety known as “Marquardt” has been found very satisfactory for high-reading thermal couples. Porcelain is not a good conductor of heat, and a junction encased in it does not respond quickly to external changes in temperature.