Thermistor resistance

Let us remember that the bolometer noise consists of two contributions [82] the phonon noise caused by fluctuations in the transfer of thermal energy between the bolometer and the heat sink and the Johnson noise due to the thermistor resistance. 

These materials are often described as hopping semiconductors in which the mobility is proportional to the exponential of the energy required to liberate the charge carriers so that the resistance decreases exponentially with temperature. Thermistor resistance R is frequently written as... 

T = temperature in Kelvin, R = thermistor resistance, A, B and C fitting constants)... 

The enzyme thermistors suffer from a notoriously high detection limit when the generated heat is detected as the change of thermistor resistance. Suggest ways of mitigating this problem. 

W. R. Siwek, M. Sapoff, A. Goldberg, H. C. Johnson, M. Bolting, R. Lonsdorf, and S. Weber, A Precision Temperature Standard Based on the Exactness of Fit of Thermistor Resistance-Temperature Data Using Third-Degree Polynomials, in Temperature Its Measurement and Control in Science and Industry, vol. 6, pt. 1, pp. 491-496, American Institute of Physics, New York, 1992. 

Thermistors are specially prepared metal oxide semiconductors that exhibit a strong negative temperature coefficient, in sharp contrast to the weak positive temperature coefficient of RTDs. Nominal thermistor resistance, usually specified for 25°C, ranges from less than 1000 S2 to more than 1 Mf2, with... 

Temperature-sensitive resistors (thermistors) resistance varies with temperature... 

The Bathythermograph. The thermistor sensing probe of a disposable bathythermograph is coated with parylene. This instmment is used to chart the ocean water temperature as a function of depth. Parylene provides the needed insulation resistance and is thin and uniform enough to permit a rapid and accurate response to the temperature of the surrounding salt water (64). 

The development of active ceramic-polymer composites was undertaken for underwater hydrophones having hydrostatic piezoelectric coefficients larger than those of the commonly used lead zirconate titanate (PZT) ceramics (60—70). It has been demonstrated that certain composite hydrophone materials are two to three orders of magnitude more sensitive than PZT ceramics while satisfying such other requirements as pressure dependency of sensitivity. The idea of composite ferroelectrics has been extended to other appHcations such as ultrasonic transducers for acoustic imaging, thermistors having both negative and positive temperature coefficients of resistance, and active sound absorbers. 

Among nonmetallic resistance thermometers, an important class is that of thermistors, or temperature-sensitive semiconductkig ceramics (5). The variety of available sizes, shapes, and performance characteristics is very large. One manufacturer Hsts ki the catalog a choice of characteristics ranging from 100 Q at 25°C to 1 M Q at 25°C. 

The thermistor material is usually a metal oxide, eg, manganese oxide. Dopants, eg, nickel oxide or copper oxide, may be added to obtain a variety of resistance and slope characteristics. The material is usually skitered kito a disk or bead with kitegral or attached connecting wkes. Figure 4 shows a typical series of steps ki the production of a disk thermistor. 

Whereas it is no longer an iaterpolation standard of the scale, the thermoelectric principle is one of the most common ways to transduce temperature, although it is challenged ia some disciplines by small iadustrial platinum resistance thermometers (PRTs) and thermistors. Thermocouple junctions can be made very small and ia almost infinite variety, and for base metal thermocouples the component materials are very cheap. Properties of various types of working thermocouple are shown in Table 3 additional properties are given in Reference 5. 

Typical positive temperature coefficient (PTC) device behavior for a doped polycrystaHine BaTiO thermistor is presented in Figure 4. At temperatures below - 100° C and above - 200° C the material shows the expected negative resistivity vs temperature associated with semiconductors as expressed by ... 

For a large number of applications involving ceramic materials, electrical conduction behavior is dorninant. In certain oxides, borides (see Boron compounds), nitrides (qv), and carbides (qv), metallic or fast ionic conduction may occur, making these materials useful in thick-film pastes, in fuel cell apphcations (see Fuel cells), or as electrodes for use over a wide temperature range. Superconductivity is also found in special ceramic oxides, and these materials are undergoing intensive research. Other classes of ceramic materials may behave as semiconductors (qv). These materials are used in many specialized apphcations including resistance heating elements and in devices such as rectifiers, photocells, varistors, and thermistors. 

Thermal Methods Level-measuring systems may be based on the difference in thermal characteristics oetween the fluids, such as temperature or thermal conductivity. A fixed-point level sensor based on the difference in thermal conductivity between two fluids consists of an electrically heated thermistor inserted into the vessel. The temperature of the thermistor and consequently its electrical resistance increase as the thermal conductivity of the fluid in which it is immersed decreases. Since the thermal conductivity of liquids is markedly higher than that of vapors, such a device can be used as a point level detector for liquid-vapor interface. 

In addition to the mercuiy-in-glass thermometer, other temperature-sensing elements may be used for psychrometers. These include resistance thermometers, thermocouples, bimetal thermometers, and thermistors. 

Temperature detectors embedded in the motor winding give close, accurate indication of motor temperature. Both conventional resistance temperature detec tors (RTD) and special thermistors (highly temperature-sensitive nonlinear resistors) are used. With appropriate auxiliaries these devices can indicate or record motor temperature, alarm, and/or shut down the motor. 

Resistance meastirement of all w indings and auxiliary devices (heaters and thermistors, etc.), when the machine is cold (at room temperature)... 

This is a later introduction in the sensing of temperature compared to the more conventional types of temperature devices available in an embedded temperature detector (ETD), such as a thermocouple or a resistance temperature detector (RTD) described below. Thermistors can be one of the following types ... 

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