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76 AIRCRAFT ENGINEERING March, 1934 Furthe r Details of a Simple Method Previously Described, wit h Results of Some Researches followed, and the fact that the human eye H E original application of specially The problem was tackled from two different could detect with precision different tints as angles. On the one hand it was observed that prepared paints for the purpose of the y formed meant that accurate control could the great covering power of certain paints, such recording temperature changes dates back some thirty years. be established. as those of titanium oxide, might be useful, since, although their opacity was considerable Even by applying the earlier double iodide compounds, using the chameleon principle, in the ordinary way, this factor might be Th e Original Application a much greater measure of accuracy can be obviated and the covering power alone made At that time, breakdowns of machinery were secured, as a range of colour changes is involved, use of. more the rule than the exception, and any and not merely that of a simple change of, say, thing in the nature of an overheated bearing A New Line of Thought brown to yellow, or yellow to red. or section was viewed with suspicion and On the other hand, modern developments in accorded, if possible, immediate attention. Th e First Tests th e electro-deposition of compounds, using Modern improvements in the construction and specially constructed cells, and superposed The first attempts at employing the chame design of mechanical plant, and also of the alternating current at low frequencies on leon principle a few years ago consisted of lubricants employed were largely responsible specially diluted electrolytes, opened up quite applying the thinnest conceivable layers of the for these paints falling into disuse. a new avenue. Most of this work has been different compounds, one on top of the other, Recent developments in the applications of devoted to operating at higher densities, and in a pre-arranged order. In this way it was aluminium alloys have demonstrated the circulating heated electrolytes, t o obviate undue hoped that the deeper tint produced below necessity for a means of recording temperature separations occurring from anolyte and catho- tha t of one of lighter colour would be detected, changes with comparative accuracy. The lyte, but conditions can be so controlled that a and further, would successively alter the various pyrometers of different types used uniform deposit of the finest possible nature (optical, resistance, etc.) are without exception can be precipitated. instruments of very fine mechanism, but Decomposition of the compounds would be In an article on "Improved Salt Baths for require a certain amount of care and attention fatal, and hence it is necessary in the first place Duralumin, " published in AIRCRAFT in their use. t o see that over-voltages are appreciably ENGINEERING , Vol. v, October, 1933, A prepared paint which may be replenished reduced by applying alternating current in p . 242, reference was made to specially pre by a fresh coating provides an alternative which pare d compounds used for temperature conjunction with the direct current, and work ha s the advantage that the colour of the com control. This aroused considerable in ing a t low frequencies using a variable frequency pound changes as the temperature rises. terest , and some controversy , an d produced a alternator. numbe r of inquirie s for further information. A Series of Skins I n view of this, we publish here another An Early Experimenter articl e giving more details on this subject Now, reverting to the compounds which which is undoubtedly interesting and pro When first used, different temperature change their colour at different tempera vides a novel application of an old process. changes could only be noted by applying tures, these are caused to deposit in the form different coatings of the respective paints; of a slight skin or film. The one is followed while th e accuracy was questionable. Probably by the other in a pre-arranged order until the the first authority to write on the subject was appearance of the surface as portrayed to the ultimate deposit consists of some six different Tonner, of America, who used copper mercuric eye. The opacity of these compounds, how compounds. After drying, this is suitably pro iodide as an indicator of overheating on the ever, was such that at first this appeared to be tected by the earlier method of using white axles of railway rolling stock. At a later date almost an impossibility. shellac spirit varnish, such as is used on elec Pinnock used the double iodide of silver and Experiments were persevered with, and a trical machinery. mercury, and different applications of this range of some six different colour-changing The electro-deposited compounds present a compound. paints were coated side-by-side at some thinness which no human hand could copy This was used for testing the temperature of convenient point on the bath, or heated plant. I n other words, the deposits resemble those of overheated bearings, and was applied with a great covering power, such as titanium oxide, A Metal Refining Analogy variety of different varnishes, and under differ without possessing the opacity of the latter. en t conditions, so as to display the colour Assistance was gained by following the change t o best advantage. practices used in another direction, namely, Successive Changes of Colour th e enamels employed for detecting the depth When heated, th e grey mass possesses a botto m of layers of aluminium and lead, and of zinc Indicatin g Water Temperature layer which develops a colour, and can be seen and lead, as used by continental metal refining quite clearly despite the presence of other fine B y dint of carefully protecting the exposed concerns. These are based on the principle layers, and these changes of colour continue surface of the paint, it was claimed that the tha t the amount of a heavy metal such as successfully from the bottom layer to that on pain t was in good condition after many years of lead, which will not appreciably alloy with th e surface. This was further augmented by continuous use. Amongst the earlier applica either aluminium or zinc, can be ascertained experiments made on infra-red, ultra-violet and tions quoted was that of paint which changed to within a few pounds by dipping into the other rays. The assistance gained by use of its colour between 90 deg. C. to 100 deg. C., molten mass a rod coated with specially th e ultra-violet rays was considerable. and was used to draw attention to the fact prepared vitreous enamel. In the case of I t was then finally ascertained that the tha t water was boiling. heterogeneous masses of zinc and lead scrap, "chameleon " type of paint could be made up Other double iodide compounds indicated these are melted, and the rod inserted. as one material. The composition of the paint the point at which the air supply of producer On withdrawing the rod, one section is is not given. The experiments proved quite gas plant became saturated with water vapour. definitely coloured by its association with the conclusively that , by employing the electrolytic These were in use some twelve years ago, lead, above which is a different tint, indicating system of deposition outlined, a number of bu t appeared since then to have fallen some the section of the bath occupied by zinc-lead different compounds could be made use of for wha t out of favour. alloy, while the lighter zinc floating above the th e foregoing purposes—in fact, there is more lead marks the enamel in a totally different work in suitably depositing the constituents Chameleon Paints manner. A similar method is employed, using tha n in the selection of the compounds them Later researches showed that iodides of other enamels for testing the depth of layers selves. thallium and other rarer metals could accord of other metals and alloys. more convincing results. It was then realised Variation and Range I t was realised that, if anything in the way tha t a paint which on heating would change of success was to accrue from a series of layers These paints appear to be of appreciable its colour through different shades (namely, a being coated one on top of the other, these service to the makers of aluminium alloys, and "chameleon") , would be invaluable for this would require to be of such an order that the a change of colour is denoted for some 5 deg. C. work. Instead of one temperature change opacity would not interfere with change of rise or fall in temperature, the range embracing being recorded a complete short range could be colour transmitted. some 30 deg.
Aircraft Engineering and Aerospace Technology – Emerald Publishing
Published: Mar 1, 1934
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