Chromel-constantan thermocouples, temperature

All the samples were further purifred by removing dust particles through 0.2 pm Millipore filter and sealed in fused silica cells or P x cells. The sample cell was embedded in a specially designed home-made cryostat or furnace. The temperatures were measured with a chromel-constantan thermocouple closely attached to a cell. The accuracy of the temperature control is within 0.1 K. The thermocouples were prepared at Chemical Thermodynamics Laboratory, Osaka University. 

A Ni(lll) single crystal, oriented to within 0.2° of the (111) plane, is mounted on a manipulator which rotates it 360° around an axis parallel to its surface and translates it in three mutually perpendicular directions while maintaining the ultrahigh vacuum conditions in the main chamber. The crystal can be cooled to 8 K by contact with a liquid He reservoir and can be heated to 1400 K. A chromel-constantan thermocouple is spot-welded to the crystal for temperature measurements. The procedure for cleaning the crystal by Ar ion sputtering, oxidation and reduction has been discussed previously (ref. 5). 

The operating temperature range is typically 100 -1000 K using samples of area 30-50 mm and thickness 0.01-0.3 mm. The temperature resolution is 0.0025 K for T< 770 K and 0.025 K for r> 770 K. The sample holder is purged with a dry inert gas. Alumel-chromel or chromel-constantan thermocouples of 0.002 mm diameter are placed in a paper frame and soldered to metal samples to measure. Polymers are dissolved in an organic solvent and the solution is spread on a thin metal... 

Thermopower measurements used the differential technique [48,49] two isolated copper blocks were alternately heated with the sample mounted between the copper blocks with pressure contacts. The heating current was accurately controlled by computer. The temperature difference between the two copper blocks was measured by a chromel-constantan thermocouple and did not exceed 0.5 K for each thermal cycle. The voltage difference across the sample was averaged for one complete cycle. Any temperature difference between sample and thermocouple was less than 10% of the temperature gradient across the sample the thermometry was carefully calibrated for the entire temperature range (5 K < T < 300 K). The absolute thermopower of the sample was obtained from the absolute scale for lead [48,49]. 

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... 

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. 

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 metallic wires if the two junctions are at different temperatures. The thermocouple may be represented diagrammatically as shown in Fig. 8-63. There A and B are the two metals, and I) and T2 are the temperatures of the junctions. Let 7) and T2 be the reference junction (cold junction) and the measuring junction, respectively. If the thermoelectric current i flows in the direction indicated in Fig. 8-63, metal A is customarily referred to as thermoelectrically positive to metal B. Metal pairs used for thermocouples include platinum-rhodium (the most popular and accurate), chromel-alumel, copper-constantan, and iron-constantan. The thermal emf is a measure of the difference in temperature between T2 and Ij. In control systems the reference junction is usually located at the emf-measuring device. The reference junction may be held at constant temperature such as in an ice bath or a thermostated oven, or it may be at ambient temperature but electrically compen-... 

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). 

Two principal DSC designs are commercially available—power compensated DSC and heat flux DSC. The two instruments provide the same information but are fundamentally different. Power-compensated DSCs heat the sample and reference material in separate furnaces while their temperatures are kept equal to one another (Fig. IB). The difference in power required to compensate for equal temperature readings in both sample and reference pans are recorded as a function of sample temperature. Heat flux DSCs measure the difference in heat flow into the sample and reference, as the temperature is changed. The differential heat flow to the sample and reference is monitored by chromel/ constantan area thermocouples (Fig. IC). ... 

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-... 

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 temperatures are determined with a thermocouple such as Chromel-Constantan calibrated at three temperatures the boiling point of water and the melting points of lead and antimony. 

In heat-flux DSC, the difference in heat flow into the sample and reference is measured while the sample temperature is changed at a constant rate. Both sample and reference are heated by a single heating unit. Heat flows into both the sample and reference material via an electrically heated constantan thermoelectric disk, as shown in Figure 31-12. Small aluminum sample and reference pans sit on raised platforms on the constantan disk." Heat is transferred through the disks and up into the material via the two pans. The dififerential heat flow to the sample and reference is monitored by Chromel-constantan area thermocouples formed by the junction between the constantan platform and Chromel disks attached to the underside of the platforms. The differential heat flow into the two pans is directly proportional to the difference in the outputs of the two thermocouple junctions. The sample temperature is estimated by the Chromel-alumel junction under the sample disk. 

Modules 910,2910, and2920 The TA Instruments Q series DSCs evolved from their 910,2910, and 2920 modules. The DSC 910,2910, and 2920 cells use a thermoelectric heat leak made of constantan (a copper/nickel alloy) as noted in Hg. 2.2. The sample and reference pans sit on raised platforms or pods with the constantan disk at their base. The temperature sensors are disk-shaped chromel/constantan area thermocouples and chromel/alumel thermocouples. The thermocouple disk sensors sit on the underside of each platform. The AT output from the sample and reference thermocouples is fed into an amplifier to increase their signal strength. The heating block is made of silver for good thermal conductivity and also provides some reflectivity for any emissive heat. 

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). 

This instrument utilises a silver block chamber with an external heater. The chamber contains a constantan disc with raised platforms for the sample and reference containers. The temperature difference between sample and reference is monitored by area thermocouples formed by the constantan disc and chromel wafers under the platforms. Amplification and electronic compensation of the differential temperature signal provides a linear calorimetric response over a wide temperature range. The theory of this instrument is discussed by Lee and Levy 6). Other available examples of... 

There are two main types of DSC instrumentation, heat-flux DSC and power-compensated DSC. A schematic of a commercial heat-flux DSC is presented in Figure 16.19. In a heat-flux instrument, the same furnace heats both the sample and the reference. In heat-flux DSC, the temperature is changed in a linear manner, while the differential heat flow into the sample and reference is measured. The sample and reference pans sit on the heated thermoelectric disk, made of a Cu/Ni alloy (constantan). The differential heat flow to the sample and reference is monitored by area thermocouples attached to the bottom of the sample and reference positions on the thermoelectric disk. The differential heat flow into the pans is directly proportional to the difference in the thermocouple signals. The sample temperature is measured by the alumel/chromel thermocouple under the sample position. This temperature is an estimated sample temperature because the thermocouple is not inserted into the sample itself. The accuracy of this temperature will depend on the TC of the sample and its container, the heating rate, and other factors. As shown in Figure 16.19, the sample and reference pans both have lids and the reference pan is an empty pan. A schematic of a power-compensated DSC is presented in Figure 16.20. The major difference in power-compensated DSC... 

The most important type of thermometer, the thermocouple, uses the electrical effects of temperature. It consists of two wires of dissimilar metals, joined at the ends and connected to a current-measuring instrument. When one of the two junctions is heated an electrical current flows between them and the measuring instrument is calibrated to read the temperature of the hot junction directly. Common thermocouples are copper-constantan and chromel-alumel. 

Constantan kan(t)-ston- tan [ft. the fact that its resistance remains constant under change of temperature] (1903) n. An alloy containing about 55% copper and 45% nickel and having a low thermal coefficient of resistivity. Its main use in the plastics industry is in thermocouple wire with either iron or cooper as the mating element. Iron-constantan, Type J, and chromel-alumel. Type K, are widely used to sense temperatures in plastics-processing equipment. 

Iron versus constantan and chromel versus alumel thermocouples are not very suitable for use at low temperatures because of their relatively low sensitivity and high inhomogeneities. 

Danley 2003,2004). In addition to the sample and reference sensors, an additional center thermocouple, denoted To (Tzero), is utilized for the heat flow measurements. Similar to the 910,2910, and 2920 modules, there are two raised platforms for the sample and the reference on a constantan disk, which acts as a heat leak. The sample and reference disk thermocouples are attached to the underside of each platform. Two AT measurements are made. The first is taken between the chromel wires that are attached to the chromel disk area detectors. In addition, ATo is measured between chromel wires attached to the sample chromel disk and the To sensor. A chromel wire is looped between the sample chromel disk and the Tq sensor, which measures the sample temperature at the raised pod. The Tq sensor temperature is measured at the junction of the constantan and chromel wires attached at the center of the heat leak base. 

Temperature measurement. The most commonly used metliod of temperature detection in the HRE-2 is the thermocouple measurement of vessel and pipe wall temperatures the couples arc spot-welded directly to the wall and then covered with insulation. When faster response is desired, thermocouples are spring-loaded into thin thermowells. Chromel-Alumel wire is generally used because its resistance to corrosive attack by moisture is better than that of iron-constantan alloys. 

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