Rhodium platinum thermocouples, temperature

The standard instrument used from —259.34 to 630.74°C is the platinum-resistance thermometer, and from 630.74 to 1064.43°C the platinum-10 percent rhodium/ platinum thermocouple is used. Above 1064.43°C the temperature is defined by Planck s radiation law. 

Calibration at fixed points is a complex process. Standard platinum resistance thermometers and standard platinum-rhodium-platinum thermocouples are calibrated at fixed points for use as primary standards. It is recommended that calibration be done by the NBS or other qualified laboratory. The narrow-band optical pyrometer is another primary standard its range over the fi%ezing point of gold is obtained through extrapolation. Ordinary calibration of temperaturemeasuring instruments is effected by comparison of their readings with those of primary or secondary standards at temperatures other than fixed points. Comparators are used to produce those temperatures. 

Thermocouples Temperature measurements using thermocouples are based on the discovery by Seebeck in 1821 that an electric current flows in a continuous circuit of two different metalhc wires if the two junctions are at different temperatures. The thermocouple may be represented diagrammaticaUy as shown in Fig. 8-60. A and B are the two metals, and T and To are the temperatures of the junctions. Let T and To be the reference junction (cold junction) and the measuring junc tion, respectively. If the thermoelectric current i flows in the direc tion indicated in Fig. 8-60, metal A is customarily referred to as thermoelectricaUy positive to metal B. Metal pairs used for thermocouples include platinum-rhodium (the most popular and accurate), cmromel-alumel, copper-constantan, and iron-constantan. The thermal emf is a measure of the difference in temperature between To and T. In control systems the reference junction is usually located at... 

Thermocouples—of copper and constantan at moderate temperatures, or platinum with an alloy of platinum and 10 per cent, of iridium or rhodium at high temperatures (cf. Pfund, Phys. Rev. 1912). 

For our system we chose the simplest approach — a fast sample conduit with quick quench into an evacuated sample container. For temperature measurements we used a similar probe outfitted with platinum/6% rhodium-platinum/30% rhodium thermocouple. For pressure measurements the same general type probe mentioned was employed but without extracting samples. This probe had one hole opening perpendicular to the longitudinal axis of the probe such that when inserted into the reactor it could be rotated 360°. In this manner the pressures were read from a precision pressure gauge with the opening facing 0°, 90°, 180°, and 270° relative to the direction of flow in the reactor. 

A platinum-10% rhodium to platinum thermocouple was operated with hot and cold junctions, respectively, in the alloy and condensed cadmium regions. Reference tables (16) were used to convert e.m.f. to absolute temperature. However, in the tables, temperatures are rounded to the nearest 0.1° C., which is too rough for our temperature differences. The tables are based on earlier work (13) with the values adjusted to the 1948 international temperature scale. To evaluate the difference between alloy and cadmium condensate temperatures, we have used without adjustment two equations from the earlier work... 

Secondary standards are liquid-in-glass thermometers and base-metal thermocouples. They are calibrated by comparing them with primary-standard platinum-resistance thermometers or standard platinum-rhodium versus platinum thermocouples at temperatures generated in comparators. These secondary standards are used in turn for the calibration of other devices, such as liquid-in-glass thermometers, bimetallic thermometers, filled-system thermometers, and base-metal thermocouples, in which the highest degree of accuracy is not required. Optical pyrometers as secondary standards are compared with primary-standard optical pyrometers, and they are then used for calibration of r ular test pyrometers. 

The noble metal thermocouples, Types B, R, and S, are all platinum or platinum-rhodium thermocouples and hence share many of the same characteristics. Metallic vapor diffusion at high temperatures can readily change the platinum wire calibration, hence platinum wires should only be used inside a nonmetallic sheath such as high-purity alumina. 

Turbine inlet temperature. Thermocouple is constructed of platinum-platinum rhodium with the junction enclosed with ceramic insulation. Typically, 9-12 units are required at this stage. 

Thermocouples are primarily based on the Seebeck effect In an open circuit, consisting of two wires of different materials joined together at one end, an electromotive force (voltage) is generated between the free wire ends when subject to a temperature gradient. Because the voltage is dependent on the temperature difference between the wires (measurement) junction and the free (reference) ends, the system can be used for temperature measurement. Before modern electronic developments, a real reference temperature, for example, a water-ice bath, was used for the reference end of the thermocouple circuit. This is not necessary today, as the reference can be obtained electronically. Thermocouple material pairs, their temperature-electromotive forces, and tolerances are standardized. The standards are close to each other but not identical. The most common base-metal pairs are iron-constantan (type J), chomel-alumel (type K), and copper-constantan (type T). Noble-metal thermocouples (types S, R, and B) are made of platinum and rhodium in different mixing ratios. 

The international temperature scale is based upon the assignment of temperatures to a relatively small number of fixed points , conditions where three phases, or two phases at a specified pressure, are in equilibrium, and thus are required by the Gibbs phase rule to be at constant temperature. Different types of thermometers (for example, He vapor pressure thermometers, platinum resistance thermometers, platinum/rhodium thermocouples, blackbody radiators) and interpolation equations have been developed to reproduce temperatures between the fixed points and to generate temperature scales that are continuous through the intersections at the fixed points. 

Several features of the early model (Fig. 6) have been modified in the present-day, high-temperature version of this calorimeter (Fig. 7) (37). Depending upon the temperature range envisaged, the block is made of refractory steel, alumina, or beryllium oxide and is machined to house the calorimeter itself. The thermoelectric pile (about 50 platinum to platinum-rhodium thermocouples) is affixed in the grooves of an alumina plate (A), which is permanently cemented to two cylindrical tubes of alumina (B). Cylindrical containers of platinum (C) ensure the uniformity of the temperature distribution within the calorimeter cells. 

The built-in thermocouples can vary depending on the temperature range platinum — platinum/rhodium (10% Rh) 25 - 1600 °C nickel — chrom/nickel 25 — 800 °C... 

To measure temperatures not exceeding 800 °C, one should use thermocouples made from copper and constantan (the latter is an alloy of 45-60% copper, 40-55% nickel, and 0-1.4% manganese it usually also contains about 0.1% carbon), Alumel (an alloy of 95% nickel, 2 % aluminium, 2% manganese, and 1% silicon), and Chromel (90% nickel and 10% chromium), or iron and constantan. Platinum-platinum/rhodium thermocouples are generally used for measuring high temperatures (up to 1600 °C). 

B Platinum - Platinum 30%, Rhodium 50 to 1700 Oto 100 These thermocouples are excellent for vacuum use. The output between 0 and 50°C is virtually flat and therefore is useless at these temperatures. The reference junction can be between 0° to 40°C. Should only be used inside nonmetallic sheaths such as alumina. 

The use of the thermopile in the form of Le Chatelier Js 3 platinum, platinum-rhodium thermocouple, a so-called pyrometer, has obtained especial importance. This can be used to measure temperatures up to 17000.4 The electromotive force is measured either by one of the well-known methods, or else direct reading precision-voltmeters (or galvanometers), whose scales are divided both into millivolts and into the corresponding degrees Celsius or Fahrenheit, are employed. The determination of the... 

Since the specimens were supported from the balance by a 2-mil Nichrome wire, a direct measurement of the specimen temperature was impractical. Instead, the temperature of the inner ceramic tube between the heater wires and under a surrounding ceramic coating at a point opposite the specimen in the tube was taken as the sample temperature. Previous observations for our system showed that the tube and the specimen had a temperature difference of less than 1°C. when the system was heated at a rate of 3° to 4°C. per minute at temperatures of 800° to 950"C. The temperature was measured, using a platinum and platinum-10 % rhodium thermocouple with a White potentiometer. 

The main source of uncertainty was the absolute temperature measurement, which was made with a platinum, platinum-rhodium thermocouple, calibrated by the National Bureau of Standards to i but thought to be accurate to 1 ° G. AZ/q was obtained l y substituting the experimentally determined equilibrium constants and the spectroscopically derived free energy functions into equation 3.1.6. The results obtained are shown in Table 3.2.1. 

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