Platinum Metals Review

PLATINUM METALS REVIEW A quarterly survey of research on the platinum metals and of developments in their application in industry VOL. 10 APRIL 1966 NO. 2 Contents Recent Advances in Industrial Platinum Resistance Thermometry 42 Petrochemicals by Platinum Reforming 47 Prevention of Corrosion in Paper Making Machines 48 Expansion in Platinum Production 52 Platinum-wound Furnaces in the Manufacture of Semiconductors 53 The Wetting of Platinum and its Alloys by Glass 54 Electrodeposition of Iridium 59 The Reaction between Hydrogen and Oxygen on Platinum 60 Catalysis of Olefin-to-Olefin Addition 65 Abstracts 66 New Patents 73 Communications should be addressed to The Editor, Platinum Metals Review Johnson, Matthey & Co Limited, Hatton Garden, London ECI Recent Advances in Industrial Platinum Resistance Thermometry By J. S. Johnston, B.s~.,A.R.C.S. Rosemount Engineering Company Limited, Bognor Regis The modern platinum resistance thermometer provides the most accurate and versatile method of industrial temperature measurement and control. This article gives details of a new design of platinum resistance thermometer element of small dimensions and good stability. It also describes a range of complete thermometers based on these elements together with a resistance-bridge system used for signal conditioning when the thermometers are used in data-logging or computer controlled systems. Two principal factors have contributed to a thermometer confers the advantages of better recent large increase in the use of platinum reproducibility and larger output signal resistance thermometers in industry. On the coupled with the ability to scale the output one hand new techniques of manufxture to fit the requirements of the instrumentation. have produced thermometers that are smaller and more robust than was possible in the Thermometer Designs past so that it becomes feasible to specify Modern methods of refining platinum resistance thermometers for applications have put materials of extreme and dependable which have hitherto been the province of the purity at the disposal of the manufacturer of thermocouple. On the other hand the con- resistance thermometers in a wide range of tinual striving after improved accuracy of wire diameters down to 0.0004 inch or even measurement and control has received an smaller. These small diameters were often, in additional impetus with the introduction of the past, associated with high impurity central data-logging and computing equip- content as a result of material picked up from ment. For these applications the resistance the drawing dies; this problem has now been Platinum Metals Rev., 1966, 10, (2),4247 42 Fig. 2 A range of modern industrial platinum resistance thermometers suitable either for direct immersion or for use in pockets largely overcome, although some surface problem is usually solved by some sort of etching is still normally necessary to achieve compromise since, in general, higher vibra- the highest temperature coefficients. tion levels will require greater degrees of The problem facing the thermometer manu- support, with inevitably poorer electrical facturer is to maintain the high temperature stability. At one extreme are found inter- coefficient and good stability inherent in the polation standard thermometers with almost pure platinum, while at the same time unsupported coils, since the ultimate in providing the mechanical support needed to stability is required under conditions of enable the element to survive the vibration careful laboratory handling, while at the other and shock conditions of military and industrial extreme thermometers for use in aircraft use. This support is the more necessary in engines may have the platinum encapsulated that pure annealed platinum is extremely over the whole length. Many compromise con- soft and hence mechanically weak. This structions between these extremes have been Platinum Metals Rev., 1966, 10, (2), 43 Fig. 4 The resistance thermometer shown on the left is suitable for measuring the temperature of metal bodies such as bearings by insertion into an oversize hole in the metal so as to be tip-sensitive. That on the right has a sensitive portion mrresponding in length to the threaded part of the thermometer firmly attached to withstand the levels of vibration normally en- countered in industrial processes. The fact that part of the coil is encapsulated necessarily means that there is some reduction in temperature coefficient and degradation in stability as compared with the proposed; for example in some aircraft unsupported platinum wire. It might be thermometers the platinum coil is wound on somewhat naive to suppose that the charac- an insulated platinum tube to avoid differen- teristics should fall roughly one-sixth of the tial expansion effects (an example of this is way between those of the unsupported and shown in Fig. I), but the construction des- the fully encapsulated thermometer, but the cribed below (I) has proved extrcrnely value obtained for a) the mean temperature useful and versatile in a very wide range of coefficient between oo and IOO~C,of 0.003916 industrial applications. is in approximately this relationship with the The main features of the construction may typical figures of 0.003925 for unsupported be appreciated from Fig. 3. The helical wire and 0.003850 to 0.003880 for fully element is embedded in a glazing compound encapsulated constructions. Stability figures which attaches it to the high-alumina do not admit of such facile juggling, but these ceramic over approximately one-sixth of the also lie closer to those of unsupported than circumference of each turn. The result of those of encapsulated wires. This stability is this is that the greater part of the coil is illustrated by the following figures obtained free to expand and contract, but is sufficiently on a sample batch of four thermometers: Stability Tests on a Batch of Four Thermometers R"=Resistance in ohms at 0°C;R,,,=Resistance in ohms at 100°C Thermometer 1 2 3 4 Ro R,,, R, Rtoo Ro RlOO Initial Values 99.998 139.1 33 99.988 139.1 29 99.988 139.1 07 100.004 139.1 52 After 10 cycles I from 20°C to 400°C, and 250 99.995 139.126 99.982 139.109 99.985 139.104 100.008 139.142 hours at 400°C Platinum Metals Rev., 1966, 10, (2), 44 Fig. 5 The large unit on the lef, incorporating the basic element construction shown in Fig. 3, is used for sucface temperature sensing in wing- skin temperature measurements in an aircraft anti-icing SYS- tem. The two small units, although not using exactly the same element design, retain the concept of a partially ronstrained platinum helix The wide range of shapes and sizes of left a sensor incorporating the same basic platinum thermometers that these and other element construction for surface temperature techniques have made possible means that sensing in wing-skin temperature measure- an ever-increasing number of temperature ments in an aircraft anti-icing system, and measuring systems now use resistance tem- on the right two small surface temperature perature sensors. Some of the special shapes sensors which, though not using exactly the of resistance thermometer that have been same element construction, retain the concept produced for a variety of industrial, aircraft of an only partially constrained platinum and medical applications are shown in the illus- helix. Fig. 6 shows thermometers designed trations. Fig. 2 shows a range of industrial for medical and pathological work and based immersion thermometers, some suitable for on the standard range of hypodermic needles. direct insertion and some for use in pockets (thermo-wells). These thermometers all use Bridge Designs elements having the construction shown in When resistance thermometers are used Fig. 3 and can be produced in sheath in systems requiring the high accuracy diameters down to 0.093 inch. Fig. 4 shows called for in computer control, some of the thermometers for metal or bearing tempera- precautions normally associated only with ture sensing. The latter unit has found wide application in metal temperature sensing because of the extreme ease of installation and the wide temperature- range (up to 650°C) cov- ered. Fig. 5 shows on the Fig. 6 Two platinum resistance thermometers, for use in medical and pathological work, lased on the standard range of hypodermic needles Platinum Metals Rev., 1966, 10, (2), 45 The basic principle is that the leads in- corporated in the bridge arms are in series with fixed resistors having values very large compared to the lead resistances so that moderate changes in lead resistance have negligible effects on the total series resistance value. Thus N and J are large compared with L, and L, respectively, and L, is in the output circuit in series with the recordcr or indicator, which has a large input irn- pedance. The bridge is designed so that very little current flows through L, when lead resistances are at the design values and, when lead resistances vary, L, tends to equalise the voltage across the top of the bridge. This configuration makes it feasible to locate the sensor several thousand feet from the bridge and recorder or indicator. A particularly versatile conditioning unit of this type is shown in Fig. 8; this is a IO- channel bridge chassis to which the resistance thermometers are connected and which can precision laboratory measurements become accept up to ten plug-in Kelvin double necessary. The most important is the elimina- bridge modules. These are arranged to tion of lead resistance effects; this has, over convert the resistance change of the sensor, the years, received considerable attention and over a specified temperature range, to a systems using compensating loops and three- o to IoomV d.c. output signal. This standard wire connections have been widely used in signal is suitable for most data-loggers and industry, while four-wire, current and poten- computers and demonstrates one of the ad- tial lcad connections have normally only vantages of resistance thermometry; this been employed in standards work.

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