Temperature resistors

PTC resistors could be classified as critical temperature resistors because, in the case of the most widely used type, the positive coefficient is associated with the ferroelectric Curie point. 

A cross section of the sensor element shows the heating zone where the temperature is controlled to be about 180 K above ambient temperature. Without air flow the temperature is identical upstream and downstream of the diaphragm. With air flow the upstream side of the diaphragm is cooled. Temperature resistors upstream and downstream of the diaphragm detect the resulting temperature difference, which is used as an airflow signal... 

Two other types of variable resistors use external conditions and not human input to control the electron flow of a circuit. The thermistor is a variable resistor that is regulated by temperature (Figure 15-13). Sometimes we call the thermistor a temperature resistor. When the temperature rises, electrons are able to flow with less opposition, so the resistance of the thermistor goes down. When the temperature gets colder, the resistance of the thermistor increases. Thermistors are used in electric thermometers and also in new electronic home thermostats. Thermistors are typically manufactured from semiconductors and metals such as manganese, copper, or nickel. 

The calorimetric type of gas flow sensors analyze the temperature distribution built up in the environment around a central heater element [2]. A photograph of a novel calorimetric sensor design is shown in Fig. 1, while Fig. 2 depicts the general idea behind the concept of calorimetiic-type gas flow sensors. Figure 2 shows a single chaimel with two sensing elements, the temperature resistors, attached upstream and downstream of the direction of flow. These elements cOTifirm the direction of flow that can be in two possible directions. 

Upstream temperature resistor Downstream temperature resistor... 

Figure 7.10 Sublimation rate of graphite. Calculated Experimental. Source Adapted from Larsen HL, Graphite elements for high temperature resistor furnaces, Union Carbide Reprint Part 2, industriai...

Table 9.7. Upper temperature limits in degrees Celsius for selected high-temperature resistors in various furnace atmospheres ...

Several kinds of conduction mechanisms are operative in ceramic thermistors, resistors, varistors, and chemical sensors. Negative temperature coefficient (NTC) thermistors make use of the semiconducting properties of heavily doped transition metal oxides such as n-ty e Ti O andp-ty e... 

H. T. Sa whill and co-workers, "Low Temperature Co-Firable Ceramics with Co-Fired Resistors," International Society of Hybrid Microelectronics Proceedings, 1986, pp. 473—480. 

The most widely used and best known resistance furnaces are iadirect-heat resistance furnaces or electric resistor furnaces. They are categorized by a combination of four factors batch or continuous protective atmosphere or air atmosphere method of heat transfer and operating temperature. The primary method of heat transfer ia an electric furnace is usually a function of the operating temperature range. The three methods of heat transfer are radiation, convection, and conduction. Radiation and convection apply to all of the furnaces described. Conductive heat transfer is limited to special types of furnaces. 

From 760 to 960°C, circulating fans, normally without baffles, are used to improve temperature uniformity and overall heat transfer by adding some convection heat transfer. They create a directional movement of the air or atmosphere but not the positive flow past the heating elements to the work as in a convection furnace. Heating elements ate commonly chrome—nickel alloys in the forms described previously. Sheathed elements are limited to the very low end of the temperature range, whereas at the upper end silicon carbide resistors may be used. In this temperature range the selection of heating element materials, based on the combination of temperature and atmosphere, becomes critical (1). 

Chrome—nickel alloy heating elements that commonly ate used in low temperature furnaces are not suitable above the very low end of the range. Elements commonly used as resistors are either silicon carbide, carbon, or high temperature metals, eg, molybdenum and tungsten. The latter impose stringent limitations on the atmosphere that must be maintained around the heating elements to prevent rapid element failure (3), or the furnace should be designed to allow easy, periodic replacement. 

Conduction furnaces utilize a Hquid at the operating temperature to transfer the heat from the heating elements to the work being processed. Some furnaces have a pot filled with a low melting metal, eg, lead, or a salt mixture, eg, sodium chloride and potassium chloride, with a radiation-type furnace surrounding the pot. Although final heat transfer to the work is by conduction from the hot lead or salt to the work, the initial transfer of heat from the resistors to the pot is by radiation. 

Nonferrous alloys account for only about 2 wt % of the total chromium used ia the United States. Nonetheless, some of these appHcations are unique and constitute a vital role for chromium. Eor example, ia high temperature materials, chromium ia amounts of 15—30 wt % confers corrosion and oxidation resistance on the nickel-base and cobalt-base superaHoys used ia jet engines the familiar electrical resistance heating elements are made of Ni-Cr alloy and a variety of Ee-Ni and Ni-based alloys used ia a diverse array of appHcations, especially for nuclear reactors, depend on chromium for oxidation and corrosion resistance. Evaporated, amorphous, thin-film resistors based on Ni-Cr with A1 additions have the advantageous property of a near-2ero temperature coefficient of resistance (58). 

Polymer thick films also perform conductor, resistor, and dielectric functions, but here the polymeric resias remain an iategral part after cuting. Owiag to the relatively low (120—165°C) processiag temperatures, both plastic and ceramic substrates can be used, lea ding to overall low costs ia materials and fabrication. A common conductive composition for flexible membrane switches ia touch keyboards uses fine silver particles ia a thermoplastic or thermoset polymeric biader. 

Thermistors Thermistors are nonlinear temperature-dependent resistors, and normally only the materials with negative temperature... 

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. 

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