Iron-constantan

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... 

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 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... 

BL TEMPKRATURF- Thermocouples are the most commonly used temperature sensing devices. The two dissimilar wires produce a millivolt signal that varies with the hot-junction temperature. Iron-constantan thermocouples are commonly used over the 0 to 1300°F temperature range. 

The temperature of the reaction vessel was measured with an iron-constantan thermocouple and controlled at 3 K. The pyrolysis experiments were performed at temperatures of 575, 625, 675, 725, 775, 825 and 925 K. The steam gasification experiments were carried out at temperatures of 825, 875, 925, 975, 1025, 1075, 1125, 1175 and 1225 K (Demirbas, unpublished work). 

The most common thermocouple type used in a laboratory situation is the Type J iron/constantan thermocouple. Low cost, high thermopower, and a use-... 

Thermocouples are based on the thermoelectric Seebeck effect, which generates a voltage at the junction between two metallic conductors, which depends on temperature [13]. Thus, in the measuring circuit, two junctions are created, namely, a sensitive (or hot) junction at the point where temperature has to be measured and a nonsensitive (cold) junction, kept at a constant known temperature, where the voltage established between the conductors can be easily measured [19]. Different typologies of thermocouples exist for application in a wide range of conditions they essentially differ by the materials, the most common being J (iron/constantan), K (chromel/alumel), T (copper/constantan), and E (chromel/constantan). 

The error limits of an iron-constantan thermocouple manufactured to meet the "special" limits of ISA. 

J Iron Constantan 0.014-0.035 32 1,500 Good nearly linear from 300 to 800°F Reducing Most economical Becomes brittle below 32°F... 

Y Iron Constantan 0.022-0.033 -200 1,800 About same as typej Reducing — Not industrial standard... 

Fig. 2.33 An iron-constantan thermocouple using an isothermal block and an ice bath. This illustration is from The Temperature Handbook 1989 by Omega Engineering, Inc. All rights reserved. Reproduced with the permission of Omega Engineering, Inc., Stamford, CT 06907...

Iron-Constantan thermocouples, which had been calibrated against an N.B.S.-standardized platinum resistance thermometer, measured both the sample temperature and the difference in temperature between sample and reference tubes. The thermocouple wires were embedded in magnesia and electrically insulated from their Inconel sheaths. The signal of the thermocouple in the sample tube could be determined either by a recording potentiometer or by a manual potentiometer and null meter. 

An iron-constantan thermocouple (constantan is an alloy of copper and nickel) is calibrated by inserting its junction in boiling water and measuring a voltage V = 5.27 mV, and then inserting the junction in silver chloride at its melting point and measuring V = 24.88 mV. 

For moderate and for low temperatures iron-constantan (an alloy of 60 per cent, copper with 40 per cent, nickel) is generally used. 

The pyrolysis unit consisted of an insulated 316 stainless steel preheater tube (1.3 cm i.d. X 50 cm length) which extended 1 in. into a 316 stainless steel fixed bed tubular reactor (2.5 cm i.d. x 46 cm length), which was heated by a cylindrical block heater. Two type J (iron-constantan) thermocouple probes were used to both monitor the internal catalyst bed temperature and maintain a consistent reactor wall temperature in combination with a temperature controller, A syringe pump, condenser, vacuum adapter, receiving flask, nitrogen cylinder, and gas collection system were connected as shown in Fig uTe 2. The reactor midsection was packed with 40 g of activated alumina, which was held in place by a circular stainless steel screen. The preheater and reactor were operated at 180-190 and 450 C, respectively. The entire process remained at normal atmospheric pressure throughout the mn. 

For general work at higher temperatures, several different types of couples are employed in this country. Up to 360°C. for extreme precision or to 500°C. for a precision of 5 to 10°C. the couple may consist of one wire of copper and the other wire of constantan. Iron-constantan or nichrome-constantan may be employed for technical processes below 900°C. For operation below 1,100°C. special patented alloys of chromium and nickel and of aluminum and nickel, chromel-alumek or nichrome-alumel are very satisfactory even for continuous service. For the temperature range 300 to 1,500°C. the Le Chatelier couple should be employed. This couple consists of one wire of platinum and the other wire an alloy containing 90 per cent platinum and 10 per cent rhodium. Other alloys and metals may be employed for special work but the above combinations are sufficient for almost all technical processes carried on in the temperature range 0 to 1,500°C. No satisfactory couple has been developed for operation much above 1,500°C. 

B represents mean calibration by Bureau of Standards of Iron-con-stantan couples from all sources. Z represents mean calibration of Leeds and Northrup s Iron-constantan couple. 

As already mentioned thermal probes of several different types have been used for acoustic power measurements. In 1954 Fry published a very detailed study on the determination of absolute sound levels and acoustic absorption coefficients [32], The probe which was used is shown in Figure 9 and is made of a thermocouple junction (copper constantan or iron constantan 0.0005 inch diameter) imbedded in a thin disk of absorbing liquid. The absorbing liquid is separated from the medium... 

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