Contact-Pen Recorders.—The force with which the pointer of an indicator is urged over the scale is relatively small, particularly in the case of pyrometers in which the platinum series of metals are used, as these furnish only a low E.M.F. If, therefore, the pointer terminate in a pen which is in continuous contact with the record-paper, the friction thus occasioned interferes considerably with the free movement of the pointer. When cheap-metal pyrometers are used, which yield a much higher E.M.F., the use of the pointer as contact-pen becomes more feasible, and if uniform friction at all parts of the paper can be ensured, records may be taken in this manner; and a recorder so constructed is simpler and cheaper than those of the intermittent type. Contact-pen recorders are used in America to some extent, being made by Bristol, Brown, and others; but so far British makers have not developed the manufacture of these instruments. At present, contact-pen recorders must be considered less accurate and reliable than those in which the contact is intermittent.
Installations of Thermo-electric Pyrometers.—When a number of furnaces in the same establishment are to be controlled, considerable economy may be effected by making one indicator serve for all the couples, which in this case must necessarily be made up of wires identical in thermo-electric value. Such an arrangement is shown in [fig. 27], in which H1 and H2 represent two couples, one wire from each being connected to one of the terminals of the galvanometer G. The other terminal of the galvanometer is connected to the arm D of a switch, and the remaining thermocouple leads are connected to the points 1 and 2 respectively on the circumference. As shown, H1 is connected to the galvanometer, and by turning the arm D to the point 2 the other couple would then be connected. Any number of junctions may thus be arranged with a single indicator. When this arrangement is adopted in a workshop, it is advisable to construct a small wooden building at a spot convenient for most of the furnaces, in which the indicator and switchboard are kept, and which could also contain a recorder if necessary; a spot as free as possible from vibration being preferable. Separate indicators are only necessary when a furnace is used for special work.
Fig. 27.—Connections for an Installation of Pyrometers.
In some instances a second indicator is kept in the shop office, to which all the pyrometers are wired, and which serves as a standard. The scale of the office indicator is checked daily at one point; and by connecting a given couple first with the shop indicator, and immediately afterwards with the office standard, any errors can be detected. It is also possible to ascertain the temperature of any given furnace in the office at any time, and so to control the whole. In fixing up such an arrangement it is necessary that each couple and its leads, up to the indicator, should possess the same resistance, or should not differ by an amount sufficient to affect the readings. The general experience of a properly managed installation is that the cost is saved in a few months in fuel alone; and, in addition, the work is carried out to much better advantage owing to complete control from the office.
Management of Thermo-electric Pyrometers.—Generally speaking, thermo-electric pyrometers give little trouble in practice, but the management should always be placed in skilled hands. It is advisable to test each instrument periodically at a fixed point near the working temperature, by the method explained on page 57; and if two or three pounds of material be used, the protecting shield need not be removed. A useful material for checking pyrometers near the critical range of steel is an alloy of 60 per cent. of copper and 40 per cent. of tin, which gives a well-defined freezing point at 738° C., and which may be used indefinitely in a reducing atmosphere Any serious error is easily detected by observing that the indications differ widely from those generally obtained under the same working conditions. If an error of 20° C. or more is noted, it is advisable to form a new junction, as the discrepancy will probably become greater, being due to a change at the hot junction. A small error, of the nature of 5 or 10° C., may be due to “creep” in the indicator, which may be adjusted accordingly, or a numerical correction may be made when taking a reading. An iron protecting sheath may be saved from rapid oxidation by black-leading once per week, which greatly prolongs its useful life, but should be replaced immediately it becomes dangerously thin in any part. Coating with aluminium powder also greatly prolongs the life of an iron sheath. When used in lead baths, the immersed part, if of iron or steel, should be bored from the solid, and left thick at the portion opposite the surface of the lead, where most corrosion occurs. A graphite tube, or one made of a composition containing graphite, is often useful in cases where iron is readily corroded, and can be used to much higher temperatures.
When a number of instruments are in use, it is advisable to keep a standard pyrometer for checking purposes, preferably one which has been certified by the National Physical Laboratory. In conducting a test, the couples, with protecting-tubes removed, may be placed in the tube of an electric furnace of the type shown in [fig. 29], in close proximity with the standard junction. On raising the temperature gradually, the readings of each working instrument may be compared with the standard, and the necessary corrections discovered. Care must be taken to prevent contact with the furnace tube, and this may be accomplished by passing the wires through an asbestos stopper fitted into the end of the tube.
When recorders are used the attendant should make himself thoroughly conversant with the details of the mechanism, so as to be able to remedy any minor ailments, which are, as a rule, easily cured. On no account should an unskilled workman be trusted with recorders; it is better and safer to keep these in the office, where they will not be likely to be damaged or tampered with. All records should be kept for future reference, properly dated, and labelled according to the operations represented.
Laboratory Uses of Thermo-electric Pyrometers.—Numerous operations carried out in muffle furnaces at prescribed temperatures require no special precautions beyond those previously given. In determining the melting points of metals or alloys, however, a porcelain or silica sheath is inadvisable, as they are easily corroded. An iron sheath is proof against some metals, but not against others, and it is always safer to fix a thin fireclay sleeve, closed at the end, over the part immersed. A sheath of graphite or graphite composition may be used for temperatures above 1100° C.; and occasionally a sheath bored from a thick arc-lamp carbon, coupled to an iron tube beyond the heated part, will be found useful at high temperatures. Alundum is useful up to 1600° C, and for temperatures of this order the higher refractories such as silfrax and zirkite may also be used to advantage.