Chromel resistances

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. 

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. 

Chromel, 35-60% Ni, 16-19% Cr, generally with the balance Fe, also is used as resistance wire and for thermocouples. 

Specimens were mounted on transferable copper or stainless steel holders mounted on precision XYZ rotary manipulators with provisions for the crystal temperature to be varied between ca. 100 and 1500 K. Samples were heated either resistively or by electron bombardment from the rear, and their temperature monitored with an alumel-chromel thermocouple either spot-welded to the edge of the sample or affixed by other means. 

The XPS measurements were carried out on a VG ESCALAB Mark II electron spectrometer. TPD data was also collected on the same system using VG SX 200 Mass spectrometer. The Ru(OOl) crystal was fixed on a liquid N2 cooled manipulator and its temperature was measured by chromel/alumel thermocouple spot-welded on the side. The crystal temperature could be varied from 95 to 1500 K. The crystal surface was cleaned initially by Ar-ion bombardment and then annealed at 1200 K for about 20 min before flashing to 1500 K. The Sm was deposited by resistive heating of Sm sample inside the preparation chamber. CO doses to the crystal surface were measured in Langmuir units (1L=10 Torr-sec). A linear heating rate of 5 K/sec was applied for TPD measurements. 

One of the first precise vacuum or inert-atmosphere instruments was designed and constructed by Whitehead and Breger (37). The furnace was constructed from an alundum core, 9 in. in length by 2 in. ID, wound with Chromel A resistance wire. The core was shielded by four sheet-nickel cylinders, mounted on three posts, and the entire assembly was placed inside a 12 x 24-in. Pyrex bell jar. All electrical connections were made through the bottom of the bell jar mounting base. The sample block was made in the dimensions shown from Type 446 or 309 stainless steel. The furnace heating rate was controlled by a Leeds and Northrup Micromax controller the differential temperatures were recorded on a Beckman Photocell recorder. 

C, has been described by Wendiandt (112). The sample holder and furnace arrangement are shown in Figure 11.28. The sample A, in the form of a pressed disk (1x5 mm) was placed between two platinum electrodes (7.0 mm in diameter). Leads to the electrodes were led out of the furnace area by one-holed ceramic insulator tubes. To maintain a constant tension on the sample disk by the electrodes, one electrode is spring-loaded at H. The furnace consists of a Nichrome resistance wire heater element on a Vycor tube suitably insulated with a ceramic material- A clamp G secures the tube furnace to the base. Furnace temperature, Tf, is detected by a Chromel-Alumel thermocouple located at D. The other components of the EC apparatus are the same as those previously described. The apparatus was used oil pure... 

A.ll experiments were conducted at atmospheric pressure in a quartz-glass flow tube reactor (2.-5 cm diameter. 20 cm length). The reaction gases were premixed and flowed perpendicular to the catalytic foil in a stagnation point flow configuration (inset fig. 1).. All experiments were conducted at total gas flow rates between 1 slpm and 6 slpm. which did not influence the results within experimental error. The high-purity platinum foils were resistively heated and the foil temperature was determined by a chromel/alumel thermocouple spotwelded to the back of the foil. Temperature measurements were reproducible within 10 K on the same foil and within 30 K in independent runs with different foils. 

In power compensated DSC the small size of the individual sample and reference holders makes for rapid response. The temperature sensors are platinum (Pt) resistive elements. The individual furnaces are made of Pt/Rh alloy. It is important that the thermal characteristics of the sample and reference assemblies be matched precisely. The maximum operating temperature is limited to about 750 °C. High temperature DSC measurements (750-1600°C) are made by heat flux instruments using thermocouples of Pt and Pt/Rh alloys. The thermocouples often incorporate a plate to support the crucible. The use of precious metal thermocouples is at the expense of a small signal strength. Both chromel/alumel and chromel/constantan are used in heat flux DSC equipment for measurements at temperatures to about 750 °C. Multiple thermocouple assemblies offer the possibility of an increased sensitivity - recently a 20-junction Au/Au-Pd thermocouple assembly has been developed. Thermocouples of W and W/Re are used in DTA equipment for measurements above 1600°C. The operating temperature is the predominant feature which determines the design and the materials used in the con-... 

ELECTRICAL RESISTANCE RATIO OF CHROMEL-P WIRE BETWEEN 4.2° AND 600°K... 

The measurement and control of the temperature of experimental apparatus in cryogenic environments has been widely explored p]. Problems in such measurement and control by thermoelectric and thermal resistance effects are receiving constant attention. However, the application of Chromel-P vs. constantan thermocouples to cryogenic temperature measurement and control has not become widespread. The reason for this limited usage is not clear, especially since the sensitivity and potential 2. 3] fQj. his thermocouple system are higher than for the more popular copper vs. constantan thermocouple system. Furthermore, the use of low-thermal-conductivity Chromel-P P] wire, instead of copper wire, would reduce heat leaks into cryogenic systems. 

The numerator of the above ratio can vary with changes in either resistivity or thermal expansion. The resistivity changes with temperature are small for most of the resistive alloys thus, the effect of dimensional changes with temperature cannot be ignored when determining the resistivity ratio. In the case of Chromel-P thermocouple wire, the thermal expansion data are not known below room temperature. Therefore, we have elected to determine the resistance ratio instead of the resistivity ratio the electrical resistivity ratio can be calculated using (1), the data presented and thermal expansion data when available. 

---