Thermoelectric sensors

In 1821 Seebeck discovered that, in an electric circuit consisting of two different materials X and Y in the form of wires, when the two junctions are at different temperatures 0, and a potential exists at the terminals on open circuit and, if the circuit is closed, a current flows 271. At each junction there exists a contact potential E r and E x respectively which depends on the type of metal employed and the temperature of the junction. When the system is on closed circuit the electromotive force (emf) is given by  

DOEBELIN 4) lists five laws of thermocouple behaviour  

The temperature of a gas oil product flowing through a pipe is monitored using a chromel/alumel thermocouple. The measurement junction is inserted into the pipe and the reference junction is placed in the plant control room where the temperature is 20°C. The emf at the thermocouple junction is found to be 6.2 mV by means of a potentiometer connected into the thermocouple circuit adjacent to the reference junction. Find the measured temperature of the gas oil. 

Thermojunctions may be formed by welding, soldering or pressing the materials together. Such junctions give identical emfs (by law (iii)), but may well produce different currents as the contact resistance will differ depending on the joining process utilised. Whilst many materials exhibit thermoelectric effects, only a small number are employed in practice. The characteristics of the more common thermocouple materials are listed in Table 6.4. 

A typical thermocouple installation for an industrial application is shown in Fig. 6.23. Instead of placing the reference junction in a temperature controlled environment (which is often inconvenient), an automatic reference junction compensation circuit is fitted. This provides a second source of emf Sj,° in series with the thermocouple emf E. The meter thus measures 0 = E 0 where E%-0 

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B Transducers, from nonelectrical to electrical quantity or vice versa Thermoelectric sensor, thermo cell, photoelectric cell, dynamometer, crystal transducer, microphone, pick-up... 

Thermoelectric Flow Sensors, Fig. 2 Thermoelectric flow sensor consisting of an electrical heater on a thin membrane with parallel meandering thermoelectric sensors, two thermopiles with 15 thermoelements [4]... 

Due to its use in microelectronics, silicon is by far the most common and best understood semiconductor material. It is cheap and a large variety of fabrication processes are available. Consequently, sihcon has become a common material for thermoelectric sensors. The material properties of silicon depend heavily on the doping concentration and type as well as on the fabrication process [9]. 

Thermocouples/Thennopiles. Thermocouples operate on the basis of the Seebeck effect. If wires fabricated from two different metals or semiconductors are soldered together to form a circuit, any temperature difference that exists between the joined points leads to a measurable potential difference, the magnitude of which depends on the extent of the temperature difference and the materials involved. The use of several thermocouples connected in series thermopiles) increases the sensitivity of the sensor, but if a single thermocouple is damaged the entire sensor is affected. It is more difficult to miniaturize thermoelectric sensors than resi.stance devices, and the long-term stability of thermocouples is often not good, A typical response time is less than one second. 

Malcovati P (1996) CMOS thermoelectric sensor interfaces. Ph.D. thesis, Physical Electronics Laboratory, Swiss Federal Institute of Technology (ETH)... 

Nesheva D, Aneva Z, Reynolds S, Main C, Rtzgerald AG (2006) Preparation of micro- and nanocrysteilline CdSe rmd CdS thin films suitable for sensor appUcations. J Optoelectron Adv Mater 8(6) 2120-2125 Nishibori M, Shin W, Houlet L, Tajima K, Izu N, Itoh T, Murayama N, Matsubara 1 (2006a) New structural design of micro-thermoelectric sensor for wide range hydrogen detection. J Ceram Soc Jpn 114 853-856 Nishibori M, Tajima K, Shin W, Izu N, Itoh T, Matsubeua I (2006b) CO oxidation catalyst of Au-TiOj on the thermoelectric gas sensor. J Ceram Soc Jpn 115 34-41... 

Fig. 11.5 Aging process of the hydrogen thermoelectric sensors with thick Pt/AljO, catalyst at operating temperature of 100 °C (a) fresh catalyst and (b) after aging treatment at 160 °C for 14 days (Reprinted with permission from Nishibori et al. (2008). Copyright 2008 Elsevier)...

The presence of temperature gradients is a cracial problem for the accuracy of measurements leading to errors due to the thermoelectric effect. Thermoelectric sensors have been made on this principle. In this case, the cell can schematically be represented by... 

Progress in the development of solid electrolytes is also being achieved from advances in several other fields of technology such as fuel and electrolysis cells, thermoelectric converters, electrochromic devices, and sensors for many chemical and physical quantities. 

Among all semiconductor NPs, metal selenides have been the focus of great attention due to their importance in various applications such as thermoelectric cooling materials, optical filters and sensors, optical recording materials, solar cells, superionic materials, laser materials and biological labels. Many synthetic methods have been developed for the preparation of relatively monodispersed selenide nanopartides (Murray et al., 1993 Korgel... 

Early bolometers used, as thermometers, thermopiles, based on the thermoelectric effect (see Section 9.4) or Golay cells in which the heat absorbed in a thin metal film is transferred to a small volume of gas the resulting pressure increase moves a mirror in an optical amplifier. A historical review of the development of radiation detectors until 1994 can be found in ref. [59,60], The modern history of infrared bolometers starts with the introduction of the carbon resistor, as both bolometer sensor and absorber, by Boyle and Rogers [12], The device had a number of advantages over the Golay cell such as low cost, simplicity and relatively low heat capacity at low temperatures. 

Thermoelectric flame failure detection Analog burner control systems Safety temperature cut-out Mechanical pressure switch Mechanic/pneumatic gas-air-ration control Thermoelectric flame supervision Thermal combustion products, discharge safety devices Electronic safety pilot Electronic burner control systems Electronic cut-out with NTC Electronic pressure sensor/transmitter Electronic gas-air-ration control with ionisation signal or 02 sensor Ionisation flame supervision Electronic combustions product discharge safety device... 

A thermopile can also be used as a chemical sensor if one of the two materials is a catalytic metal for a given volatile compound. In this case it is necessary to keep the warm and cold junctions at constant temperature. During absorption of the volatile compound on behalf of the catalytic material the thermoelectric power may change, giving rise to an output voltage which can be related to the concentration of the volatile compound. A typical example is the thermopile as hydrogen sensor, where one of the two materials is palladium, a standard hydrogen catalyzer. 

Examples of hydrogen sensors utilizing thermoelectric power and low frequency measurements are discussed below. 

Thermostats—bimetallic strip, snap-action disk, and thermostatic cable Continuous discrete conductors Thermistor sensors Rate of temperature rise Copper tubing heat-actuated devices (HADs) Thermoelectric Combined rate of rise/fixed temperature Rate compensation Oil mist ... 

Thermistor and resistance thermometer elements Thermoelectric-bolometric sensors Semiconductor-based elements... 

In a modern dew-point instrument, a sample is equilibrated within the headspace of a sealed chamber containing a mirror, an optical sensor, an internal fan, and an infrared thermometer (Figure A2.2.2). At equilibrium, the relative humidity of the air in the chamber is the same as the water activity of the sample. A thermoelectric (Peltier) cooler precisely controls the mirror temperature. An optical reflectance sensor detects the exact point at which condensation first appears a beam of infrared light is directed onto the mirror and reflected back to a photodetector, which detects the change in reflectance when condensation occurs on the mirror. A thermocouple attached to the mirror accurately measures the dew-point temperature. The internal fan is for air circulation to reduce vapor equilibrium time and to control the boundary layer conductance of the mirror surface (Campbell and Lewis, 1998). Additionally, an infrared thermometer measures the sample surface temperature. Both the dew-point and sample temperatures are then used to determine the water activity. The range of a commercially available dew-point meter is 0.030 to 1.000 aw, with a resolution of 0.001 aw and accuracy of 0.003 aw. Measurement time is typically less than 5 min. The performance of the instrument should be routinely verified as described in the Support Protocol. 

Electrolytic type sensors Uxt thick film techniques, e.g. capacitor coated in gl bonded on to a ceramic disc mounted on a thermoelectric (Peltier effect) cooler. Control is by a platinum resistance thermometer which adjusts the temperature of the cooler to regain equilibrium after a change in capacitance due to moisture deposit. Range depends on technique. Capable of high precision. Limitations are similar to those for AIjO) sensor. Capable of being direct mounted. Relatively cheap. Suitable for on-line use. 

Liess, M. and Steffes, H. (2000) The modulation of thermoelectric power by chemisorption a new detection principle for microchip chemical sensors. /. Electrochem. Soc. 147, 3151-3153. Tran-Minh, C. and Vallin, D. (1978) Anal. Chem. 50, 1874. 

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