Resistor type sensors

Device structures adopted for resistor type sensors in practice, (a) Sintered block, (b) thin alumina tube-coated layer, (c) screen printed thick film, (d) small bead inserted with coil and needle electrodes, (e) small bead inserted with a single coil (heater and electrode), (f) practical sensor element assembling sensor device, metal cap and filter. 

Contact potential-conscious sensor design Gas response of a resistor type sensor seems to be promoted significantly by contact potential if a properly designed composite gas sensing layer is used. 

The resistive-type sensors measure strain-related mechanical properties such as strain, force, acceleration, and pressure-utilizing piezoresistive properties of sensitive film resistors [2,14,38,39]. These sensors are also incorporated as biosensors for detecting biomolecules. The electrode configuration is the same as in electrochemical transducers, but it measures the change of resistance when biomolecules interact with the biologically sensitive elements [2]. The piezoresistive microcantilever sensors are also extensively studied for biosensing. The piezoresistive sensors. 

The gas-sensitive material in thick-film gas sensors has a sintered layer area of a few square millimeters, and about 30 im in thickness, on a ceramic substrate (Fig. 13.546). In other types of gas sensors, it is on the outer surface of a thin tube or as a sintered button (Fig. 13.54l and c). The sensor is heated tet the operating temperature of 300-500 °C by means of a resistor. 

Several types of palladium-based hydrogen sensors have been reported in the literature. The most notable ones are based on Pd thin-film resistors, FETs, Pd nanowires, Pd nanoparticle networks, Pd nanoclusters, and Pd nanotubes as shown in Table 15.2. 

However the carbon specific heat was not as low as the crystalline materials employed later. Also important, the resistor material exhibits an excess low-frequency noise. Nowadays, two types of resistance sensors are used to realize LTD NTD Ge sensors and TES. 

The electronic circuit of the safety sensor consists of a p-type silicon electrode, an LED, a resistor, two 3 V lithium batteries, and a platinum wire as a counter electrode, connected in series, as shown in the right part of Fig. 10.7. These components are assembled in a pen-like housing, optimized to measure even thin layers of liquid on a flat surface, as shown in the left part of Fig. 10.7. This configuration is advantageous if a puddle, observed for example under a wet bench or other equipment, is to be analyzed. 

Thermistor basedflow-through calorimetric sensors. Enzyme thermistors make the most widely developed type of heat measurement-based sensors. The thermistors are normally used as temperature transducers in these devices. Thermistors are resistors with a very high negative temperature coefficient of resistance. They are ceramic semiconductors made by sintering mixtures of metal (manganese, nickel, cobalt, copper, iron) oxides. Like the two previous groups, thermistor sensors do not comply strictly with the definition of "sensor" as they do not consist of transducers surrounded by an immobilized enzyme rather, they use a thermistor at the end of a small... 

The essential part of the thermal chemical sensor is the device that allows fast, sensitive, and precise detection of the temperature a thermometer. There are many thermometers available here we mention only three types, in the descending order of their sensitivity. They are thermocouples, platinum resistors, and thermistors. 

As was shown in Figure 3.159, cryogenic temperatures can be detected by integrated circuit diodes types K, T, and E thermocouples (TCs) class A and B resistance temperature detectors (RTDs) acoustic and ultrasonic thermometers germanium and carbon resistors and paramagnetic salts. As TCs and RTDs will be discussed in separate subsections, here the focus will be on the other sensors. 

Perovskite-type titanates (e.g. SrTiCL, BaTiOi) constitute an important class of electroceramic materials and are, for example, used in PTC (positive temperature coefficient) resistors, capacitors, varistors or sensors [324-326]. A degradation process that... 

There are two types of conductometric procedures commonly used. Firstly, a Wheatstone Bridge circuit can be set up, whereby the ratio of the resistance of unknown seawater to standard seawater balances the ratio of a fixed resistor to a variable resistor. The system uses alternating current to minimise electrode fouling. Alternatively, the conductivity can be measured by magnetic induction, in which case the sensor consists of a plastic tube containing sample seawater that links two transformers. An oscillator establishes a current in one transformer that induces current flow within the tube, the magnitude of which depends upon the salinity of the sample. This in turn induces a current in the second transformer, which can then be measured. This design has been exploited for in situ conductivity measurements. 

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 tf-type Fe2 Ty03 and type Ni1 LyO. Thick film resistors are also made from transition-metal oxide solid solutions. Glass-bonded By 2 Pb2yRu207 having the pyrochlore [12174-36-6] structure is typical. 

Barium titanate (BaTiOj), a perovskite-type electro-ceramic material, has been extensively studied and utilized due to its dielectric and ferroelectric properties. The wide applications of barium titanates include multiplayer capacitors in electronic circuits, nonlinear resistors, thermal switches, passive memory storage devices, and transducers. In addition, barium titanate can be used for chemical sensors due to its surface sensivity to gas adsorption. 

The sensitivity of a piezoresistive pressure sensor depends on the piezoresistive coefficient. Silicon crystal face selection and gage layout on the crystal face are important because of the anisotropy of the piezoresistive effect. Silicon (100) and (110) are often used with P-type diffused resistors to achieve a desired sensitivity. The next consideration is the thermal stress effect originating from the silicon crystal face. Fig. 7.3.5 shows the stress-distribution maps for silicon (100) and silicon (110) by the finite element method (FEM). 

The calibration of the characteristic curve is done by a current divider with a trimming resistor in the sensor connector, for example. This sensor type has been in mass production since the mid 1990s, with production volumes of millions. 

The fundamental requirement for the e-noses is to produce a pattern of discem-ibly different responses for different samples. In other words, each sensor composing the array is not required to be highly specific, but it should respond to a broad range of analytes. Moreover, the correlation between responses of different sensors into the array system should be avoided because it decreases the information content of the pattern produced by the array [37], The use of arrays of gas sensors to quantify the concentration of gasses in mixtures was first proposed in the 1980s, and since then different types of sensors, such as amperometric, piezoelectric, metal-oxide resistors, and MOSFET sensors, have been employed in this technology [38 1],... 

The word thermistor is a portmanteau of thermal and resistor. Therefore a thermistor is a type of resistor whose resistance varies significantly with temperature. Thermistors are used as temperature sensors. Thermistors differ from RTDs in that the material used in a thermistor is generally a ceramic or polymer, while RTDs use pure metals. The temperature response is also different RTDs are useful over larger temperature ranges, while thermistors typically achieve a higher precision within a limited temperature range (usually 90 to 130 °C). 

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