Chromel-constantan

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

Fig. 1.45.1. Artist s view of a DSC cell in Tzero technology as used in modulated DSC (MDSC) processes. 1, Sample and reference table made from one piece of constantan 2, chromel thermocouples directly connected to the constantan tables 3, Tzero sensor from chromel-constantan in the middle between sample and reference table (TA Instruments, New Castle, DE, USA...

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). 

Other materials may be used in addition to those shown in Figure 4. For example Chromel-Constantan is excellent for temperatures up to 2000°F Nickel/Nickel-Molybdenum sometimes replaces Chromel-Alumel and Tungsten-Rhenium is used for temperatures up to 5000°F. Some combinations used for specialized applications are Chromel-White Gold, Molybdenum-Tungsten, Tungsten-Iridium, and Iridium/Iridium-Rhodium. 

E Chromel Constantan 0.015-0.042 -300 1,800 Good Oxidizing Highest EMF/°F Larger drift than other base metal couples... 

These devices have a disk (e.g. constantan alloy) on which the sample and reference pans rest on symmetrically placed platforms. Thermocouple wire (e g. chromel alloy) Is welded to the underside of each platform. The chromel-constantan junctions make up the differential thermocouple junctions with the constantan disk acting as one leg of the thermocouple pair. 

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. 

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). ... 

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). 

E Chromel-constantan Nickel-chromium alloy7 (purple) vs. a copper-nickel alloy5 (red) -270 to 1000°C 67.5... 

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 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. 

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... 

Type E thermocouples are often referred to as chromel-constantan thermocouples. They are regarded as more stable than Type K, therefore often used where a higher degree of accuracy is required. 

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. 

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]. 

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