Paul’s Recorder.—In the recorders previously described, the motive power is furnished by clockwork. R. W. Paul has introduced an instrument in which all the moving parts are actuated by a motor driven with power from the mains. This recorder is shown in fig. 25. The motor is furnished with a special type of governor to ensure constant speed, and is connected by suitable gearing to the mechanisms moving the chart, presser-bar, and inking ribbon, provision being made to vary the speeds of these movements by changing the gear. The galvanometer is of uni-pivot pattern, and the pointer is pressed at intervals on to a typewriter ribbon which lies above the chart. Immediately beneath the ribbon is placed a thin metal rod over which the paper passes, and the result of the contact is to produce a small dot. As in the thread recorder, the chart is divided into rectilinear coordinates, the ribbon in this case serving the same purpose as the thread in the former instrument. The lower part of the recorder is prolonged so as to display a considerable length of the chart, which is in the form of a roll, and is drawn forward by the mechanism. When two records are taken simultaneously the ribbon consists of two strips, one moistened with black ink, and the other with red; and it is arranged that each strip in turn is over the thin rod on to which the pointer is pressed, so that the records appear in separate colours. This recorder can also be arranged for multiple records, or fitted with a scale-control. With a view to workshop use, all the covers are fitted with faced metal joints, which are much better for keeping out dust than wooden ones. A further useful feature is that the various units in the recorder—galvanometer, motor, feed and record mechanism, and reducing gear—are all separate and interchangeable. By introducing a suitably divided chart this recorder will also serve for a radiation pyrometer, or, as will be shown later, for a resistance pyrometer.
The Leeds-Northrup Recorder.—The Leeds and Northrup Company, of Philadelphia, manufacture a recorder which is largely used in the United States. As in Paul’s recorder, all the mechanism is motor driven; but the other arrangements are entirely distinct. Instead of measuring the deflection of the pointer, a zero deflection method is used. The pyrometer forms part of a potentiometer circuit, and the function of the mechanism is to oppose an E.M.F. equal to that of the pyrometer, from which the temperature is known. This has the advantage that the measurement is independent of the resistance of the leads, and is capable of great accuracy. The manner in which the adjustment of the opposing E.M.F. is controlled may be understood from fig. 26, in which the galvanometer coil is shown at the top of the figure. The shaft from the motor carries four cams, B, C, D, D, and at each revolution the cam B raises the bar (5) so as to press it against an arm attached to the galvanometer coil. At the same moment the cam C pushes against the bar (3), and thereby releases a clutch (2) from the disc beneath. As shown, the boom from the coil is to the right of the central position, and is gripped between a bar (5) and the lever (4) when the former rises, causing an angular movement of the clutch-arm (2). As the rotation continues the cam C leaves the bar (3), which then springs back and engages the clutch on the disc. The cam D then descends and presses on the projection of the clutch-arm to the left, causing the disc to rotate. The movement of the disc is conveyed to an arm which moves over the slide wire of the potentiometer; and this movement continues until the galvanometer boom is in the central or zero position, when neither of the levers 4, 4 is gripped, and consequently the disc is not fed in either direction. If the boom swing to the left, the movement of the disc will evidently be in the converse direction to that described.
In this recorder considerable power is available to drive printing or other mechanisms. The arm moving over the potentiometer wire carries a pen which marks the moving chart, or, when several records are taken simultaneously, a stamping machine is used which impresses the number of the pyrometer on the chart. The same galvanometer mechanism serves also for use with resistance pyrometers, as will be explained later.
Control of Furnace Temperatures.—Many attempts have been made to secure the automatic regulation of furnace temperatures by means of mechanisms controlled by an indicator or recorder. In the arrangement employed by the Brown Company of Philadelphia, movable stops are provided, which may be brought to any part of the scale, the mark between the stops representing the temperature it is desired to maintain. The indicator (or recorder) is provided with a presser-bar which descends periodically; and if the temperature be too low the depressed pointer completes a circuit through the inner stop, whilst if too high the circuit is through the outer stop. Both circuits contain a relay which brings a mechanism into operation, the result being to increase the supply of electricity or gas if the temperature be too low, or to diminish the supply when too high. When correct, the depression of the pointer fails to complete either circuit, and in this manner control between small limits may be ensured. In the case of large furnaces the relay circuits are employed to light lamps of different colours, the adjustment then being made by the man in charge of the furnace. Arrangements of this kind effect a considerable saving in fuel by preventing unnecessary heating, and are particularly valuable in cases where overheating would be deleterious to the articles in the furnace. The future will probably witness considerable developments along these lines.
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.
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.
The determination of the “critical” points of steel call for special mention. In cooling down a mass of steel the fall of temperature is arrested at one or more points, observations of which are frequently of service in deciding the subsequent treatment of the steel. A method commonly employed is known as the “differential method,” and is indicated in fig. 28. The sample of steel, A, is placed side by side with a piece of nickel, B, of equal dimensions, in the tube of an electric furnace. A naked junction, C, is placed in a hole drilled in A, and is connected to the galvanometer G, which is calibrated to read temperatures. A two-junction circuit, formed of a junction D placed in the hole in A, and another junction E located in the hole in B, are connected to a delicate galvanometer H. The furnace is heated until the galvanometer G indicates 900° C., when the arrangement is allowed to cool. As A and B, under normal circumstances, cool at an equal rate, the junctions D and E will be at the same temperature, and no deflection will be observed on H. When, owing to recalescence, the cooling of A is arrested, B, not being thus affected, will continue to cool, thus producing a difference between the temperatures of D and E, and consequently a deflection on H. The temperature of A at the time this occurs is read off on G.
The furnace illustrated in fig. 29 is suited to this determination. It consists of a silica tube 1 foot long, wound with a special resistance wire and efficiently lagged, and may be heated in safety to 1000° C. for long periods, and to 1200° for a short time. It may be placed across the electric mains directly, and reaches 900° C. in less than half an hour. It consumes 600 to 700 watts at the highest temperatures, and the cost of re-winding is small. This furnace is useful as a general laboratory appliance, and may be kept at a given steady temperature by the use of an external resistance.
The wires in this experiment should be platinum and iridioplatinum or rhodioplatinum, or a good pair of base metals, and the junctions in A should be separated from each other and from the specimen by asbestos; the same precaution being taken to prevent the junction E from touching B. A thin layer of mica should be used beneath A and B, to avoid contact with the furnace tube, which, when hot, allows of leakage of current from the heating coil. Both A and B may be 1½ in. long, ¾ in. diameter, with a hole ¼ in. diameter drilled to a depth of ¾ inch.
An alternative method is to insert a junction in a hole in the specimen, and to take direct readings as the temperature slowly rises or falls, when an arrest in the movement of the pointer of the indicator shows that the change-point has been reached. Special sets are made for this purpose.
Measurement of Lower Temperatures by the Thermo-electric Method.—Many cases arise in practice in which a thermal junction and a sensitive galvanometer are preferable to a mercury thermometer; and below -39° C., at which temperature mercury freezes, a thermal junction is frequently better to employ than an alcohol or pentane thermometer. A number of practical examples of the use of thermal junctions for ordinary and low temperatures will now be considered.
Measurement of Surface Temperatures.—A mercury thermometer, when laid on a hot surface, only touches along a line, and does not show the true surface temperature. The construction of a thermal junction suitable for this purpose is described on page 41, and for steam-pipe surfaces, hot plates, and the exterior of furnaces, a specially calibrated millivoltmeter, giving a full-scale deflection with 20 millivolts, may be used. In making the temperature scale, boiling water (100°C.), boiling aniline (184° C.), and melting tin (232° C.) are convenient standards. If the surface temperature be less than 100° C. a mirror galvanometer should be used, and the junction standardized in paraffin wax (freezing point usually about 50° C., but should previously be determined with an accurate thermometer), absolute alcohol at boiling point (79° C.) and boiling water. The author has found that this method yields excellent results in the case of steam-pipes, the exterior of rotary cement kilns, and hot surfaces generally.
Measurement of Low Temperatures.—Junctions of iron and constantan, Hoskin’s alloys, copper and German silver, or copper and constantan, are suited to these measurements. In a laboratory the cold junction may be kept in ice in a Dewar vessel, the mechanically protected form known as the “Thermos” flask being very useful for this purpose. With a good mirror galvanometer precise readings may be secured, 1/10 of a degree C. being easily detected. Calibration between -40° and +40°C. may be effected by comparison with a standard mercury thermometer, a water-bath being used above 0°, and alcohol surrounded by a freezing mixture of ice and calcium chloride crystals below zero. For very low temperatures (-200°C. or less) the junction maybe calibrated in solid carbon dioxide (-78°C.) and liquid air (-184°C.). Dewar has found that copper and German silver form a reliable junction for very low temperatures, and the author has successfully used a couple of Hoskin’s alloys for special work down to -200°C., a pivoted indicator being employed. No couples tested show a linear relation between E.M.F. and temperature at these low ranges.
Owing to the magnitude of the error caused by changes in the cold junction, the thermo-electric method is not suited to the measurement of atmospheric temperatures, or for explosive magazines or cold stores. In such cases instruments of the resistance type, to be described later, are used.
Temperature of Steam, Exhaust Gases, etc.—For measuring the temperature of ordinary or superheated steam, the exhaust gases from internal combustion engines, etc., iron-constantan junctions, with suitable indicators, are satisfactory. When placed in a pipe the junction should be as nearly as possible in the centre, so as to avoid the cooling effect of the walls. Several junctions, situated in different parts of the pipe, may be used with a single indicator and suitable switchboard. The above remarks also apply to the hot-blast for blast furnaces, and similar instances where the temperature does not exceed 900° C.
Measurement of Differences of Temperature.—Cases frequently arise in practice in which the difference in temperature between two points is required, and if this difference be subject to rapid changes, a mercury thermometer, from its large mass, would not respond with sufficient rapidity to indicate these changes. In such cases a circuit is made after the manner of fig. 2, one junction being located at each point; thin wires of iron and constantan being used. For small differences—1° C. or less—a mirror galvanometer should be used. Calibration may be performed by placing one junction in hot water and the other in cold, the water temperatures being read with an accurate thermometer.
Advantages of the Thermo-electric Method of Measuring Temperatures.—Compared with other methods, the thermo-electric possesses the following points of superiority:—(1) Simplicity, no special experiment being necessary to obtain a reading; (2) cheapness of outfit; (3) adaptability to a variety of purposes; (4) ease of repair in case of damage; (5) robustness, not being liable to get out of order under workshop conditions; and (6) suitability to the purpose of a centrally controlled installation. The drawbacks are:—(1) Liability to error owing to fluctuations in the cold junction (which may be avoided with care); and (2) lack of sensitiveness at very high temperatures compared with the resistance method—a point seldom of great practical importance, as the limit of accuracy is usually within the amount by which an ordinary furnace fluctuates in temperature under working conditions.