Constantan

Nickel, used extensively to make coins and nickel steel for armor plates and burglar-proof vaults, and is also a component in Nichrome(R), Permalloy(R), and constantan. 

Chromel Chromel / Alumel Chromel-alumel Chromel-constantan... 

Copper compounds Copper concentrates Copper-constantan Copper-constantan Copper-Count-N Copper cyanide... 

Temperature measurements ranging from 760 to 1760°C are made usiag iron—constantan or chromel—alumel thermocouples and optical or surface pyrometers. Temperature measuriag devices are placed ia multiple locations and protected to allow replacement without iaciaerator shutdown (see... 

Not all elements of the industrial thermocouple need to be wine. For example, if a copper pipe contains a flowing fluid whose temperature is to be measured, a constantan wine attached to the pipe will form a T, or copper—constantan, thermocouple. Such arrangements ate difficult to caUbrate and requite full understanding of the possible inherent problems. For example, is the copper pipe fully annealed Homogeneous Pure, or an alloy Many ingenious solutions to specific measurement problems ate given in Reference 6. 

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... 

Figure 3.13. Fractional change in resistance as a function of shock stress for manganin and constantan.

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. 

Copper/constantan Iron/constantan ChromeFconstantan ChromeF alumel... 

Thermocouples of platinum with an alloy of platinum and 10 per cent, of rhodium or iridium are used at higher temperatures, and of copper and constantan at lower temperatures. 

For very low temperatures a lead wire is used as thermometer, with a heating coil of constantan wire (Kamerlingh Onnes). 

Calvet and Persoz (29) have discussed at length the question of the sensitivity of the Calvet calorimeter in terms of the number of thermocouples used, the cross section and the length of the wires, and the thermoelectric power of the couples. On the basis of this analysis, the micro-calorimetric elements are designed to operate near maximum sensitivity. The present-day version of a Tian-Calvet microcalorimetric element, which has been presented in Fig. 2, contains approximately 500 chromel-to-constantan thermocouples. The microcalorimeter, now commercially available, in which two of these elements are placed (Fig. 3) may be used from room temperature up to 200°C. 

Calvet and Guillaud (S3) noted in 1965 that in order to increase the sensitivity of a heat-flow microcalorimeter, thermoelectric elements with a high factor of merit must be used. (The factor of merit / is defined by the relation / = e2/pc, where e is the thermoelectric power of the element, p its electrical resistivity, and c its thermal conductivity.) They remarked that the factor of merit of thermoelements constructed with semiconductors (doped bismuth tellurides usually) is approximately 19 times greater than the factor of merit of chromel-to-constantan thermocouples. They described a Calvet-type microcalorimeter in which 195 semiconducting thermoelements were used instead of the usual thermoelectric pile. 

Calvet, standard model (16) Chromel to constantan couples 30-200 60 (30°C)... 

Data of Fig. 3.8 show that the use of wires of manganin or constantan is to be avoided in some applications as wiring of detectors (see Chapter 15). 

Unfortunately, the thermoelectric power vanishes when temperature tends to zero pairs as the classic Cu/constantan thermocouples show very low sensitivity below 10 K (see Fig. 9.2). 

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