Thermocouple contact potential

When two different metal surfaces are brought into contact, the surface space charges that were present at their interfaces with a vacuum will be modified. The electrons from the metal of lower work function will flow into the other metal until an interface potential develops that opposes further electron flow. This is called the contact potential and is related to the work-function difference of the two metals. The contact potential depends not only on the materials that make up the solid-solid interface but also on the temperature. This temperature dependence is used in thermocouple applications, where the reference junction is held at one temperature while the other Junction is in contact with the sample. The temperature difference induces a potential (called the Seebeck effect), because of electron flow from the hot to the cold Junction, that can be calibrated to measure the temperature. Conversely, the application of an external potential between the two Junctions can give rise to a temperature difference (Peltier effect) that can be used for heat removal (refrigeration). 

Of the variety of contact sensors that exist for measurement of temperature, thermocouples are among the more commonly implemented probes. Temperature measurement by thermocouple is based on the principle that the contact potential generated when two dissimilar metals are in contact varies with temperature. Therefore, by measuring the difference in the contact potential of a measurement junction that forms the probe tip, relative to that of the reference junction, the temperature difference between the junctions is obtained. Thermocouples can be manufactured from many different combinations of metals, depending on the working temperature range, chemical sensitivity of the probe, and desired sensitivity and accuracy. In general, the use of thermocouples is preferred over other contact sensors because of their relatively short response times and low cost. However, the temperature sensitivity of thermocouples can be quite low... 

The transducers shown in Figure 3-13 arc a pair of thermocouple junctions, one of the transducers is immersed in the sample, and the other transducer is immersed in a reference solution (often an ice bath) held at constant temperature. A temperature-dependent contact potential develops at each of the two junctions formed from wires made of copper and an alloy called constantan (other metal pairs are also used). The potential difference v, - v, is roughly 5 mV per 100°C temperature difference. 

The temperature dependence of a contact potential between two different metals is the basis of the operation of a thermocouple or thermopile to measure temperature (Sec. 2.3.5). 

A similar diffusion of electrons between metals produces the contact potential used in thermocouples. Vhi is zero volts if both sides of the junction are undoped and increases with Na and Nq- The built in voltage can be determined from a diagram of the band edges of a junction at equilibrium. It is simply the amount by which either band edge bends, see Figure 3.2. 

There are a number of other types of measurement made in soil that involve electrodes that are not directly in contact with the soil. An example is the thermocouple psychrometer, which involves a Thomson thermocouple in a ceramic cell buried in soil. The thermocouple cools when a current is passed through it, causing water to condense on the thermocouple. When the electricity is turned off, the condensate evaporates at a rate inversely proportional to the relative humidity in the soil. A voltage generated by the cooling junction is measured and related to the soil moisture content. This moisture content is related to both the matrix and osmotic potentials of the soil being investigated. 

The thermocouple utilizes the Seebeck effect. Copper and constantan are the two metals most commonly used and produce an essentially linear curve of voltage against temperature. One of the junctions must either be kept at a constant temperature or have its temperature measured separately (by using a sensitive thermistor) so that the temperature at the sensing junction can be calculated according to the potential produced. Each metal can be made into fine wires that come into contact at their ends so that a very small device can be made. 

A variety of optical techniques have been used to measure gas temperatures in combustion applications, particularly in flames. There are potentially some important advantages of optical techniques compared to contact techniques such as suction pyrometers (see Figure 5.7). Optical measurement techniques do not disturb the flow, where thermocouples may have a significant impact on the fluid dynamics. Optical techniques can potentially measure higher temperatures as there are not the materials issues compared to thermocouples. For some optical techniques, temperature profiles can be measured at one point in time without the need to make multiple individual measurements over some length of time. Optical techniques often have a much faster response time compared to contact methods. This is particularly important in turbulent and transient flows. 

Instrumentation developed in 1970 and in subsequent years has been used till nowadays and has created the basis of the present instrumentation. Intensive efforts have been devoted to the development of more sensitive detectors than the thermocouple or the thermistor detector . Thus, the currently available very sensitive high-resolution detectors have been obtained. These include, the contact conductivity detector which senses the electric conductivity of zones by means of two metallic (Pt) microelectrodes protruding into the separation capillary and being in direct contact with the electrolyte the potential gradient detector in which the electric gradients in zones are sensed with the aid of two similar electrodes placed in at certain small distance from one another in the direction of migration the UV detector where a narrow beam of UV light passes perpendicularly... 

Figure 3.8 (a) Ceramic cup for potential measurements in the alloy system Ag u, . (1) Ag plate with connecting Pt wire (8), (2) Ag electrode, (3) glass film melted on the Ag electrode (thickness ca. 0.1 mm), (4) Ag Ati alloy electrode with platinum contact wire (8) in a ceramic cup, (5) pressed on the electrolyte layer by the cover of the ceramic cup (6), and an alumina tube (7). (b) Apparatus used for potential measurements in the alloy system Ag u, with the ceramic cup (5) shown in detail in Figure 3.8a. (7) alumina tube, (8) platinum wire connections to the electrodes, (9) wire to hold the ceramic cup, and (10) thermocouple. ... 

A thermocouple consists of wires of two dissimilar metals (e.g., constantan alloy and copper) connected in series at soldered or welded junctions. A many-junction thermocouple is called a thermopile (Fig. 2.7). AYhen adjacent junctions are placed in thermal contact with bodies of different temperatures, an electric potential develops that is a function of the two temperatures. 

A third type of frequently-used thermometer is the thermocouple. Its discussion is summarized in Fig. 3.5. The thermocouple is based on the Seebeck effect. At the contact points of two dissimilar metals, a potential difference is created because some of the electrons in the material of the lower work function drift into the metal with the higher work function. 

F. 8.1 Cross section of zirctmia reference tube electrode. Only the bottom part of the zirconia tube contains the platinum contact and the thermocouple both buried in zirconia grit and in intimate ctmtact with the zirconia tube. The reference potential is thus strictly isothermal. Because it is easily broken, the use of the zirconia tube is restricted to laboratory measurements... 

To provide reliable, simultaneous heating of numerous samples, an aluminum culture plate on a magnetically stirred hotplate was used to heat the synthesis solutions. The temperatnre of the aluminum block can be monitored nsing a thermocouple in contact with the aluminum block. This provides safe, stirred heating of up to twelve samples simultaneously. To minimize exposure to chemicals, dilute stock solutions are provided to the student. The preparation of these dilnte solntions use potentially hazardous chemicals including metallic seleninm, trioctylphosphine or tribntylphosphine, and cadmium oxide (12-14) that are easily managed by the professional preparatory room staff. 

A 01 is the Galvani potential at the phase boundary between two different metals, for example at the contact point between the common silver wire shunt element and the copper instrument lead. Ignoring the surface potentials (see Ax Chap. 1.1), the EMF at such a contact point is known as the contact Volta potential. These can be highly temperature dependent, as demonstrated by the analogous thermocouples. The Ag/Cu contact has a temperature coefficient of 10 mV/°C, the Cu/tin solder contact 10" mV/°C, and the Cu/CuO contact an observable coefficient of 1 mV/°C. This last contact may arise if corroded copper plugs are used. In precise measurements striving for accuracies and sensitivities <0.1 mV, clean contacts are a must, and temperature variations should be avoided as much as possible. 

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