Thermometry contact

Temperature either by contact thermometry (millisecond experiments only) or surface radiation for optical thermometry, J(t)... 

Temperature is undeniably the most important property for all calorimetric measurements, because it is the common denominator. Two different techniques for temperature measurements are used for pulse calorimetry contact thermometry (e.g. thermocouples) and radiation thermometry or pyrometry. Because pulse calorimetry is often used to handle and measure liquid materials, non-contact radiation thermometry is far more common in pulse-heating than contact thermometry. Other reasons for non-contact temperature measurement methods include the fast heating rates and temperature gradients (inertia of the thermocouples), difficulties mounting the contact thermometers (good thermal contact needed), and stray pick-up in the thermocouple signal because the sample is electrically self-heated. 

Contact thermometry is successfully used for e q)eriments in the millisecond to second range, [75], where tiny thermocouples are spot-welded to the sample and the additional voltage drop between the thermoelectrodes and the thermocouple junction is compensated for. 

The most common method of temperature measurement is contact thermometry, as demonstfated in Fig. 4.1. One brings a thermometer, C, a system with a known thermal property, into intimate contact with the to be measured system, A. Next, thermal equilibration is awaited. When reached, the temperatures of A and C are equal. The use of C as a contact thermometer is based on the fact that if the two systems A and B are in thermal equilibrium with C they must also be in thermal equilibrium with each other. This statement is sometimes called the zeroth law of thermodynamics. It permits to use B with a known temperature to calibrate C, and then use C for measurement of the temperature of system A. A calibration with B can be made at a fixed temperature of a phase transition without degree of freedom, as given by the phase rule of Sect. 2.5.7. Less common are methods of temperature measurement without a separate thermometer system. They make use of the sample itself. For example, the temperature of the sample can be determined from its length, the speed of sound within the sample, or the frequency of light emitted. 

The main drawback in this type of thermometry is the presence of spurious thermoelectric powers due to chemical inhomogeneity, stress in conductors, contact effects in switches if present, etc. 

The resistance thermometry is based on the temperature dependence of the electric resistance of metals, semiconductors and other resistive materials. This is the most diffused type of low-temperature thermometry sensors are usually commercial low-cost components. At very low temperatures, however, several drawbacks take place such as the low thermal conductivity in the bulk of the resistance and at the contact surface, the heating due to RF pick up and overheating (see Section 9.6.3)... 

Ge resistors are specifically produced for low-temperature thermometry carbon and Ru02 resistors are commercial products for electronics. Pure carbon is not a semiconductor. The negative slope R(T) is due to the production process which consists in pressing and sinterization of carbon particles with glue. The resulting resistance is probably determined by the contact resistance between the particles. The cost of the carbon resistor thermometer is very low. Manufacturers such as Speer, Allen-Bradley and Matsushita have produced in the past carbon resistors for many years. Most of firms have now ceased manufacture, although their products may still be found in the storerooms of research laboratories. 

Thermal electric noise thermometry 1. Josephson junction point contact 2 Conventional amplifier 0.001-1 4-1400 Mean square voltage fluctuation Nyquist s law oc fegT Other sources of noise serious problem for T > 4 K... 

There have been a number of applications of Cc-Gr thermometry to contact metamorphism. Wada (1978) studied Cc-Gr pairs in skams from the Kamioka Mining District and found that graphite in the skam was similar in 5 C to that of the country rocks. However skam calcite was 5-10%o lower than the equilibrium value with graphite and gradients of up to 10%o/2cm were measured in 5 C(Cc). Wada and Suzuki (1983) found a good correlation of A C(Cc-Gr) and temperature from 400 to 680°C, however, as discussed for (11), it is not certain if this correlation is controlled by equilibrium or kinetics. 

Figure 18. The results of calcite-graphite isotope thermometry (filled symbols) and calcite-dolomite solvus thermometry (open symbols) plotted against distance to the Tndor Gabbro contact, Ontario. The solvns thermometiy is reset by regional metamorphism at 490°C while the isotope thermometry preserves high temperatures due to earlier contact metamorphism. The sohd cnrved line is the predicted thermal gradient for contact metamorphism (from Dnim and Valley 1992).

Jaeger MS, Mueller T, Schnelle T (2007) Thermometry in dielectrophoresis chips for contact-fi-ee cell handling. J Phys D 40 95-105... 

Thermometry is based on the principle that the temperatures of different bodies may be compared with a thermometer. For example, if you find by separate measurements with your thermometer that two bodies give the same reading, you know that within experimental error both have the same temperature. The significance of two bodies having the same temperature (on any scale) is that if they are placed in thermal contact with one another, they will prove to be in thermal equilibrium with one another as evidenced by the absence of any changes in their properties. This principle is sometimes called the zeroth law of thermodynamics, and was first stated as follows by J. C. Maxwell (1872) Bodies whose temperatures are equal to that of the same body have themselves equal temperatures. ... 

Such contact measurements of temperature are appealing because they provide a direct measure of the bulk-cooling element temperature. Furthermore, the extraction of heat from an attached sensor is in itself a demonstration of cooling a thermal load. However, the use of thermocouples becomes difficult because of fluorescent heating and heat conduction through the wires. These drawbacks can be avoided with some of the noncontact thermometry methods reviewed in the following. 

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]. 

As indicated above, silicon sensors have become the standard in diode thermometry and an extensive amount of data exists for them. Typical long-term stability is on the order of 50 mK, while short-term stability can be as low as a few millikelvin. This stability makes the diode competitive with industrial grade platinum thermometers with the added benefit of being usable to as low as 1 K. The upper temperature limit for commercially available diode sensors is around 400 K. This limit is determined by the properties of silicon, the metallurgy of the contact areas, and also by the construction materials and techniques used in device packaging. 

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