Chromel

The Type N thermocouple (Table 11.60) is similar to Type K but it has been designed to minimize some of the instabilities in the conventional Chromel-Alumel combination. Changes in the alloy content have improved the order/disorder h ansformations occurring at 500°C and a higher silicon content of the positive element improves the oxidation resistance at elevated temperatures. 

An alloy of chromel containing Ni, Fe, and Cr was analyzed by a complexation titration using EDTA as the titrant. A 0.7176-g sample of the alloy was dissolved in ITNOa and diluted to 250 mb in a volumetric flask. A 50.00-mb aliquot of the sample, treated with pyrophosphate to mask the Fe and Cr, required 26.14 mb of 0.05831 M EDTA to reach the murexide end point. A second 50.00-mb aliquot was treated with hexamethylenetetramine to mask the Cr. Titrating with 0.05831 M EDTA required 35.43 mb to reach the murexide end point. Einally, a third 50.00-mb aliquot was treated with 50.00 mb of 0.05831 M EDTA, and back titrated to the murexide end point with 6.21 mb of 0.06316 M Cu +. Report the weight percents of Ni, fe, and Cr in the alloy. 

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

Compressor inlet. Unit is eonstrueted of Chromel-Alumel (niekel alloy) and eharaeterized by an exposed junetion eonsisting of a bare wire with eeramie insulation. One unit is required here. 

Turbine exhaust. Thermocouple is constructed of Chromel-Alumel with an exposed junction. Nine to 12 units are required at this stage. 

The composition of the filament is of importance in determining the yield and the performance of the apparatus. With nickel-chromium alloys excessive carbonization occurs and the yield is poor. Much better results are obtained with the nickel-iron-chromium alloys called Chromel C and Nichrome AUoy Wire. 

It is necessary to provide an efficient stirrer driven by a powerful motor. The yields reported were obtained using a Hershberg Chromel wire stirrer (Org. Syn. 17, 31) with other stirrers the yields were lower by about 10 per cent. 

Distillation was performed using a 30-cm. vacuum-jacketed column wrapped with heating tape and containing a spiral of wire (Nichrome or Chromel) in the center bore. The receiver portion of the assembly contained two flasks, one to collect the material that distilled below 72° and another for the desired fraction. An air-cooled condenser was used with this apparatus, since water cooling often causes the product to solidify and clog the condenser tube. 

Figure 1. Schematic Diagram of the off-axis radiant heated reactor. A. cell body B. linear/rotary motion feedthrough C. transport rod D. projector bulb E. reflector F. insulated stainless steel enclosure G. air cooling port H. gas inlet I. gas outlet/pumping port J. chromel/alumel thermocouple K. high vacuum gate valve L, sample mount.

That oxidized surfaces appear to yield higher CHFs than clean metallic surfaces was also reported by Ivey and Morris (1965). They found little difference in the CHF for wires that are not prone to severe oxidation. Results of tests with 0.020-in. wires of platinum, Chromel, silver, stainless steel, and nickel yielded CHFs of 350,000 + 20% BTU/hr ft2 (1.1 X 106 + 20% W/m2). The scatter in the data for a given wire, as well as for different materials, was within the 20% band. 

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)... 

The submitters used a 7 X 300-mm. externally heated column packed with a helix of Chromel wire and fitted with a partial reflux head.3... 

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