The Indicating and Recording of Correct Temperatures, and their Regulation.
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A Temperature Indicator and Recorder, suitable either for Case-hardening or Superheated-steam Requirements.
Common Forms of Thermometers.
Most people, when they desire to ascertain the temperature of a room, chamber, or ease, or of any liquid, are prepared to accept, as sufficiently accurate for their purposes, the indication of a mercury thermometer ; such apparatus depends for its action on the expansion or contraction of the metal. At all ordinary temperatures, the ratio of increment in volume to increment in absolute temperature is practically constant in the case of therein-2,-, but it must be remembered that all bodies expand by heat, and contract by cold ; the vessel containing the mercury of a thermometer, therefore, is also affected by changes of temperature, though not to the same extent as the mercury itself, and, unless such thermometer be calibrated with careful reference to the rate of expansion of the containing vessel, the reading can only be approximately correct. Even if the precaution named be taken, there is always the possibility that, in practice, the whole of the containing vessel would not be equally affected by the liquid, or other medium, the temperature of which was being taken. Glass is commonly employed as the material of the containing vessels for thermometers, but, for some purposes, metal is employed ; it will easily be understood that, should such a vessel become dented, its capacity would be reduced, and, consequently, the whole thermometric scale elevated. The reading, too, can only be taken near to the source of heat, or the effects of external temperatures become a source of considerable error. Then, again, the mercurial thermometer is limited in its application, because high temperatures cannot. conveniently he read, on Recount of the height of the column of mercury, due to its expansion under the influence oi great heat. Thermo-electric Pyrometers.
Air thermometers, in which the expansion of a permanent gas is employed to indicate any rise of temperature, are more correct than those of the mercury type, but the scope of their employment is not wide. Thermo-electric pyrometers, of the types described in an article on " The equipment of case-hardening rooms," in our issue of the 18th of June, 1908, are extensively employed for high temperatures, and in the hands of practical men they give fairly-accurate results, but such instruments are extremely delicate, and they will not stand up long against abuse. Any of the forms of temperature-indicating and recording devices, which we have mentioned, give readings with limits of error that can be overlooked for many purposes within the scope of the respective types, but, when absolutelycorrect readings are necessary, the most-elaborate precautions against external influences must be taken, and intricate calculations be made to discount the irregularities inherent in all systems which depend upon the laws of expansion of metals or fluids.
Saturated-vapour Thermometers.
If absoliite accuracy be required, some means which are independent of expansion of liquids or metals must be found. M. Joseph Borbe Foornier, of Fournier et Cie., 11, Rne Campagne-Premier, Paris NIVe, for which house Messrs. F. Mellin and Co., of 60, Sainsbury load, Kilburn. NW., are acting as sole agents in Great Britain, Ireland, and the Colonies, has devised an instrument that appears to solve many of the difficulties which attend the indicating, recording, and regulating of clutuges of temperatures. One of our Editorial representatives, for sonic weeks past, has been investigating the claims of M. Fournier, and we are now perfectly satisfied that the device gives a degree of accuracy impassible of attainment by any other method at present known. M. Fournier's instrument is based on the laws of the tension of saturated vapours. A saturated vapour, by the way, is a vapour which is in contact with an excess of its own liquid. Its tension, or surface pressure, depends upon the nature of the liquid and its temperature, but has no relation to the volume of its atmosphere within the limit-s of saturation. Unlike steam, or gases that follow—more or less approximately—Boyles's law, the modern rendering of which law is " the pressure on a given mass of gas at constant temperature is inversely proportional to its volume," saturated vapours do not obey any common law; the law relating to the vapour of any particular liquid, however, may be determined by experimental research.
Fourniees Invention. •suitas; Fournier's device consists of a curved tube (of flattened oval section, similar to those employed in Bourdon pressure gauges), a cylinder containing the saturated vapour, and a connecting tube of very small bore—a capillary tube. These parts are shown on Fig. 1, where A represents the Bourdon tube, B the capillary tube, and C the vapour cylinder. When the pressure within a Bourdon tube is increased, the tube tends to straighten out, whilst, if the internal pressure be reduced, the two ends approach more closely to each other. These movements are utilized in pressure gauges, to indicate the pressure within any vessel to which the gauge may be connected. One end of the Bourdon tube, in such a gauge, is fixed, whilst the other is connected to a toothed quadrant that meshes with a small pinion on the spindle of the indicating finger; in this manner, the movements of the free end of the tube are magnified by the gearing, as is shown in Fig. 2.. Such a Bourdon tube is employed by Fournier, to indicate, to record, or to regulate, all changes of temperature in the vapour contained within the cylinder (C). The Bourdon tube is filled with dried sand, or some other inert matter, so as to reduce its capacity as a receptacle for liquid. From the fixed end of this tube, a capillary tube leads to the vapour cylinder, and projects into the last-named part as far as the centre of its length. The interstices between the grains of sand, the capillary tube and more than one-half of the vapour cylinder are filled with mercury, or some other liquid (D), with a high boiling-point, whilst the remaining space within the cylinder is occupied by the saturated vapour of a liquid of low boiling-point, together with a small quantity of its own liquid. An examination of Fig. 1 will show that, in whatever position the cylinder be held, the open end of the capillary tube is always covered with the lesservolatile liquid, by reason of the ratio of volumes; the more highly-volatile fluid and its vapour always occupy a spa.ee remote from the capillary tube.
It is the cylinder (C) only which is placed witldn the heated or cooled space under observation, mid the capillary tube may be of any length. For instance, a suitable vapour-cylinder may be placed inside a case-hardening furnace. whilst the pressure gauge, by which the temperature is indicated, may be placed in the manager's office, possibly more than 1,000 yds. distant. Instead of a pressure gauge. the Bourdon tube may be caused to operate a recording derive, and any number of vapour cylinders, in different parts of a works or Odp, may be connected up to a single recorder, one type of which is shown in Fig. 4. Further combinations may he made, of whirl' the following examples are typical :— One vapour cylinder may he connected to one or any number of indicators. one or more recorders may also be connected to the same vapour cylinder. The various indicating and recording instruments may be installed in various parts of a 'factory.
External Conditions do not Affect Results.
Neither the temperature of the capillary tube, nor the amount of expansion of the mercury or other liquid, has more than a momentary effect on the indication of the temperature of the vapour cylinders. The mercury merely acts as an incompressible medium for transmitting, to the Bourdon tube, the pressure of the saturated vapour, which pressure, as we have already stated, is dependent only upon the temperature of the vapour and net upon its rolewe. In order to illustrate this point, let us assume that we have 4 c. in. of saturated vapoar, to gether with a small quantity of its liquid, within a cylinder, and that this vapour, while still at the same temperature, is compressed into a space of only 2 c. in. Its pressure would remain the same, because, as the vapour was saturated at the outset, i.e., holding as much liquid in the gaseous form as it could retain when occupying the larger volume, no more can be held when its volume is reduced ; hence, the excess of saturation is thrown dawn, or condensed. If, however, while occupying the larger volume, the temperature of the vapour were increased, a. larger quantity of its own liquid would be volatilized, and the tension or pressure would be' increased, according to the law of the particular volatile fluid in use.
An Interesting Experiment.
Our representative recently made an interesting experiment, and this goes to prove that external influences do not affect the correct indication of the temperature of the vapour. The experiment consisted in the placing of a vapour cylinder in a tub of water, at 40 degrees Fahrenheit, which temperature was indicated by a pressure gauge ; a large roil of the connecting capillary tube was then plunged into a bath of boiling water, and tin' rapid expansion of the mercury inside the capillary tube reused the iedicator needle to make a sudden " kick," after which it quickly went back to its old position (indicating the temperature of the water), and stopped there as long as the vapour cylinder remained in the cold water. The duration of the higher reading—not more than three seconds—represented die time taken for the condensation of the vapour proportional to the reduction of the capacity of the vapour chamber, upon whose space, of course, the mercury had at once encroached. Such a test is an extreme one. When the temperature of the capillary tube is raised slowly to that of boiling water, the rate of displacement of the mercury being more gradual, there is no indication by the gauge of the external temperature change.
Nature of the Vapours Employed, According' to the range of temperature for which the instrument is required, the vapours of various acids and other agents, or combinations of the same, are employed, whilst for very-high temperatures, sueli as those existing in a blast furnace, a metal is the' medium employed, in which case the arrangement shown in Fig. 3 is adopted. In this diagram, E and H are two cylinders of porcelain or other refractory material, inside the furnace; they are connected, by means of a 1: tube, and a tube (F) leads out from the cylinder (H), through the wall of the furnace, to a bulb (not shown) which contains
a quantity of mercury, or other intermediary medium, and thence, by a capillary tube, as already described for Fig. 1, to an indicator or recorder. The cylinder (E) is filled, and the cylinder (II) partly filled, with solid iron or steel (G). When the furnace Leaches a temperature equal to the fusing point of the steel, the metal becomes molten; from this point, any rise of temperature causes some of the metal to volatilize, and the " saturated vapour " conditions to obtain, the tension or pressure of which will be transmitted, through the liquid in the capillary tube (not. shown) to the Bourdon tube.
Its Application to Motor Vehicles.
fn this article, we have avoided the use of technical phrases, as far as possible, and have endeavoured to explain in simple language the fundamental principle which governs this ingenious
and practical invention. 'We would
now direct our readers' attention to one important use to which the device might be applied for motor vehieles: the degree of superheat in the steam supplied to an engine is an all-important factor for successful working, and with the aid of an instrument of the type shown in Fig. 5, the supply of liquid fuel to the burners of a steam generator could easily he regulated by the temperature of the steam. We are well-a a are that thermostats have already been used for that purpose, Lot we cannot call to mind a reallysuccessful application, for thi& purpose, of any instrument which depends on the laws of expansion, such devices aro too delicate for use, except in the laboratory, and are not sufficiently sensitive. In Fournier's apparatus, however, the vapours are so sensitive to changes of temperature, that the placing of a finger on a cylinder will give a visible indication of temperature change; yet, the device is so robust, that the Bourdon tube may be made to exert a force of three tons. without the employment of any multiplying levers or gears. It is, virtually, hydraulic transmission_ In the, device, shown in Fig. 5, any change of pressure within th.J vapour cylinder and the Bourdon tube causes the freeend of the latter to act on a small valve. Experiments with one of these regulators are now being made on an English-built steam car_ Similar apparatus, Of varying sizes, is in constant employment by a large number of industrial concerns on the Continent, and apart from all the It-rench railway companies, the State Railways of Italy and Belgium—its use is now compulsory on all trains in the last-named country—both the French and Italian Navies are now making increased use of M. Founder's. invention. Mr. Mellin will at all times be pleased to give demonstrations, and -to forward his interesting series of descriptive pamphlets to anyone who cares to write to him.