Resistive sensor systems

A factor which previously limited installation of automatic corrosion monitoring systems was the cost of cabling between sensors and control room instrumentation-this was particularly relevant to the electrical resistance (ER) systems. Developments to overcome this have included transmitter units at the probe location providing the standard 4-20 mA output (allowing use of standard cable) for onward transmission to data systems or the use of radio linkage which has been successfully used for other process-plant instrumentation. 

Tin oxide Resistance Neodym Systems 0-20,000 ppm Yes Uses Figaro sensors, but selectively ... 

The output of the sensor system delivers a digital signal- The sensor is supplied with 5VDC via a coded RAST 2.5 plug connection. The sensor consists of water resistant material and is therefore especially suitable for the use in household appliances Like dishwashers or washing machines. 

A cross-sectional schematic of a monolithic gas sensor system featuring a microhotplate is shown in Fig. 2.2. Its fabrication relies on an industrial CMOS-process with subsequent micromachining steps. Diverse thin-film layers, which can be used for electrical insulation and passivation, are available in the CMOS-process. They are denoted dielectric layers and include several silicon-oxide layers such as the thermal field oxide, the contact oxide and the intermetal oxide as well as a silicon-nitride layer that serves as passivation. All these materials exhibit a characteristically low thermal conductivity, so that a membrane, which consists of only the dielectric layers, provides excellent thermal insulation between the bulk-silicon chip and a heated area. The heated area features a resistive heater, a temperature sensor, and the electrodes that contact the deposited sensitive metal oxide. An additional temperature sensor is integrated close to the circuitry on the bulk chip to monitor the overall chip temperature. The membrane is released by etching away the silicon underneath the dielectric layers. Depending on the micromachining procedure, it is possible to leave a silicon island underneath the heated area. Such an island can serve as a heat spreader and also mechanically stabihzes the membrane. The fabrication process will be explained in more detail in Chap 4. 

The microsystems may also serve potential applications in material science and in the growing field of nanotechnology. Microhotplates can be used for material processing, and, at the same time, for the monitoring of material properties such as the electrical resistance [10]. Moreover, the microsystems can be applied to determine thermal properties of new materials such as the melting point, especially when only small quantities of material are available [145], so that monolithic microhotplate-based devices are not only powerful sensor systems for a broad range of applications, but also new research tools for sensor science and nanotechnology. 

Passive types of leak detection such as observation wells and collection sumps where product is collected and analyzed directly should work effectively with methanol. Active leak detection systems that rely on thermal conductivity and electrical resistivity sensors will not work with methanol because its properties are so different from gasoline. Another type of active leak detection system that will work with methanol or any other type of fuel relies on changes in impedance in a sensor wire as it becomes wetted with the fuel [4.5]. These leak detection systems also have the advantage that they can pinpoint the location of the leak along the length of the sensor wire. 

For each application envisaged in this chapter, tests were performed to evaluate the behaviour of the sensor system in its performance for sweat and urine detection. For this purpose, stainless-steel yarns were incorporated in baby nappies for detection of urine. Figure 10.6 shows the results of some of these tests. It can be seen clearly that the detection of urine is marked by a large change in resistance between the electrodes of the sensor system. In addition, the value of R after urine detection is an indication of how much urine has been formed. However, the latter is not really of scientific relevance, and is therefore not considered further here. 

Temperature and pressure measurements during freeze-drying are difficult tasks. Thermal elements (Th) and temperature-depended electrical resistance (RTD) systems measure only their own temperature and that of their surroundings only if they are in very close contact with them. Furthermore, they heat themselves and their surroundings by the current flow through the sensors. Also, they influence the crystallization of the product in their surroundings ... 

In contrast, rapid growth of integrated, smart sensor systems in real and industrial application has not been fulfilled till now. The industry used well-established mechanical and electromechanical sensors in the past, especially in aircraft and automotive industries which resisted changes to electronic systems mainly for reasons of safety. 

The sensor system was applied to the quantification of solvent-resistance of BPA-HQ-RS polycarbonate copolymers in chloroform, THF, and MEK. In a typical experiment, the 6 x 4 sensor array was periodically immersed into the wells containing copolymers in respective solvents. The analytically useful frequency changes were measured upon sensor removal from the solvent and solvent evaporation. Comparison of solvent-resistance of copolymers in the solvents is presented in Fig. 19.6 where data were obtained from measurements of sensor signal change after 46-48 min from the beginning of the experiment. Copolymer solvent-resistance is represented as the absolute sensor frequency shift per mass of polymer in each well... 

The integrated system, including transducer and enzyme reactor, provides improved reliability and stability in multianalyte determinations, as compared with discrete thermal sensor systems. In addition, application of micromachining and IC technologies is of benefit for the manufacture of uniform, cheap thermal transducers with flexible shape, size, and resistance, as well as delicate microstructure on the chips. The good thermal insulation of the transducers from the flow stream eliminates interference from the reactants on the transducers, and the intrinsic stability of the transducers obviates the need for frequent recalibration of the sensors. 

Sensor systems are composed of the sensor, the transmitter, and the associated signal processing. The sensor measures certain quantities (e.g., voltage, current, or resistance) associated with devices in contact with the process such that the measured quantities correlate strongly with the actual controlled variable value. There are two general classifications for sensors continuous measurements and discrete measurements. Continuous measurements are, as the term implies, generally continuously available, whereas discrete measurements update at discrete times. Pressure, temperature, level, and flow sensors typically yield continuous measurements, whereas certain composition analyzers (e.g., gas chromatographs) provide discrete measurements. 

Determining the time constant of the sensor is usually more difficult than estimating the repeatability. To determine the time constant of the sensor system, one needs to know the actual process measurement. Consider a temperature measurement. A measurement of the actual process temperature is required to estimate the time constant of a sensor. Instead, the thermal resistance, which causes the excessive thermal lag of the temperature sensor, can be evaluated. The location of the thermowell should be checked to ensure that it extends far enough into the line that the fluid velocity past the thermowell is sufficient the possibility of buildup of insulating material on the outside of the thermowell should be assessed, and the thermal contact between the end of the temperature probe and the thermowell walls should be evaluated. In this manner, an indirect estimate of the responsiveness of the temperature sensor can be developed. The velocity of a sample in the line, which delivers a sample from a process line to a GC, can indicate the transport delay associated with the sample system. A low velocity in the sample line from the process stream to a GC can result in excessive transport delay, which can greatly reduce controller effectiveness. 

Compact chemical sensors can be broadly classified as being based on electronic or optical readout mechanisms [28]. The electronic sensor types would include resistive, capacitive, surface acoustic wave (SAW), electrochemical, and mass (e.g., quartz crystal microbalance (QCM) and microelectromechanical systems (MEMSs)). Chemical specificity of most sensors relies critically on the materials designed either as part of the sensor readout itself (e.g., semiconducting metal oxides, nanoparticle films, or polymers in resistive sensors) or on a chemically sensitive coating (e.g., polymers used in MEMS, QCM, and SAW sensors). This review will focus on the mechanism of sensing in conductivity based chemical sensors that contain a semiconducting thin film of a phthalocyanine or metal phthalocyanine sensing layer. 

Both thick- and thin-film versions of a solid state, resistive hydrogen sensor were designed and fabricated at ORNL [69, 70], Both versions of the sensors (25 mm x 25 mm x 0.6 mm) are small enough to be incorporated into hand-held leak detectors or distributed sensor systems for safety monitoring throughout large areas. 

Since biological systems are extremely sensitive to changes in temperature and cells typically have sharp optima for growth and product formation (often different values), temperature sensors are the most common probes in biotechnology. Thermistors, platinum resistance sensors, and thermocouples are all used to provide an on-line signal thermometer bulbs are also employed but cannot be used for automated monitoring. 

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