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P29
Piezoelectric pressure transducers offer many advantages over other measurement technologies. A significant advantage is their ruggedness which makes them suitable for harsh environments [7].In combination with the currently seen temperature capabilities of up to 700°C, the wide frequency range, and good sensitivity behavior, they are well suited for high-temperature dynamic pressure measurements in gas turbines. They also qualify as low power devices with insensitivity to electromagnetic influences and radiation. Disadvantages include the unsuitability for static pressure measurements.
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P30
Piezoresistive pressure transducers use the piezoresistive materials to measure pressure. When a piezoresistive material is deformed or force is applied to it its electric resistance changes depending on the force or deflection applied. Unlike piezoelectric transducers, piezoresistive transducers do not generate charge when deflected but change their impedance. A commonly used material for piezoresistive pressure transducers is silicon (Si). [8]
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P31
To build a piezoresistive transducer, several thin wafers of silicon are embedded between protective surfaces, which are connected to a Wheatstone bridge. A small amount of current is run through the sensor and the connected Wheatstone bridge measures the resistance change within the sensor when it deforms. Usually, the piezoresistive elements are connected to a diaphragm, which deflects when pressure is applied to it. Figure 2.4 shows two different forms of piezoresistive pressure transducers.
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P33
Piezoresistive pressure transducers are known for their linear behavior reacting to changes in the measurand. However, they are sensitive to temperature shifts and compensation for temperature errors has to be done. The advantages of piezoresistive transducers are the small feasible sizes, the simple resistance measurement needed, their ability to perform static and dynamic pressure measurements, and their low price. The disadvantages are the high electrical noise, temperature sensitivity, and comparably high power consumption. Nowadays, uncooled piezoresistive transducers are rated up to 500°C operating temperature.
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P34
Each of the formerly mentioned technologies has its advantages and disadvantages compared to the other technologies. Therefore, the different transducers can have tailored applications in which they excel and other cases where they are not suitable. A short overview of the sensing mechanisms and their respective advantages and disadvantages can be found in Table 2.1 below.
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P36
Sensing Mechanism | Advantages | Disadvantages |
Capacitive | Low power, low noise, hermetic package | Complex measurement circuitry |
Optical | EMI insensitive, hermetic package | needs specialized signal processing |
Piezoelectric | Self generating charge – transducer conumes no power, hermetic package | no static pressure measurements |
Piezoresistive | Simple resistance measurement, inherently shielded structures, hermetic package | High electrical noise, temperature dependency, high power consumption |
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P37
Outline: This chapter should contain an overview of different applications dynamic pressure transducers are used for within gas turbines. This could be grouped by the different turbomachinery sections and the applications within those. Applications can be described with the preferred transducer mounting, employed transducer technologies, signal processing and other relevant aspects. |
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P38
Outline: This chapter is intended to summarize existing standards on pressure transducer datasheet content and to provide room for further improvements and adaptations regarding not yet covered transducer technologies. The three main documents which are used for the creation of this chapter are ISA-S37.1-1975 [1], ISA-S37.10-1982 [2], and ISA-S37.3-1982 [3]. Any other standards which are found and used during the development of this topic need to be stated and referenced to increase the overview of existing standards. An exemplary calibration or validation procedure for all specifications that are mentioned here that need some sort of testing needs to be stated in the next chapter. |
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P39
• Cable, non-integral 1 If a non-integral cable is supplied with the transducer, the type, length, connector types, and maximum operating temperature of this cable shall be specified. Applicable for the following transducer technologies: All • Case sealing 1 If case sealing is employed, the mechanism and materials for sealing shall be described. The same requirement applies to the electrical connector. The resistance of the sealing materials to common and corrosive fluids shall be stated. Applicable for the following transducer technologies: All • Connection, pressure 1 The pressure connection shall be indicated on the outline drawing (see also Dimensions). For threaded cases of fittings, indicate the nominal size, thread pitch, thread series, and thread class. For a flush-mounted transducer, indicate whether flange mounting, cemented installation, or other specified means is employed. Applicable for the following transducer technologies: All • Connector, electrical 1 The connector on the transducer shall be described. If the transducer is supplied with an integral cable, the connector at the end of the cable shall be described. The mating connector for the above connector shall also be described or designated. Applicable for the following transducer technologies: All • Dimensions An outline drawing of the transducer shall show its complete configuration with dimensions given in millimeters [2]. The tolerances shall also be given for all relevant dimensions. The dimensions can additionally also be given in inches. Applicable for the following transducer technologies: All • Identification The transducer model, the manufacturer, the serial number, the measuring range, the operating temperature range, and, if applicable, hazardous environment certification markings shall be permanently inscribed on the outside of the transducer. Applicable for the following transducer technologies: All • Materials, housing 1 The case materials exposed to the environment shall be specified. • Material, transduction element 1 The type of sensing material employed as the transduction element shall be identified. (A proprietary name is acceptable.) The sensing mode of this element shall also be specified. Applicable for the following transducer technologies: All • Fluid limitations 1 If specific corrosive fluids are associated with a particular transducer application, the compatibility of the transducer and its accessories with such specified fluids shall be stated. Applicable for the following transducer technologies: All • Mounting and mounting dimensions Unless the pressure connection serves as the transducer mounting, the outline drawing shall indicate the method of mounting with dimensions in millimeters [2]. Dimensions in inches can be given additionally. Applicable for the following transducer technologies: All • Mounting force or torque 1 Mounting force or torque shall be specified. When pressure connection is not integral with mount, pressure-connection torque shall also be specified. • Weight 1 The weight of the transducer shall be specified in grams. If the transducer includes an integral cable, its weight also shall be stated. |
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P40
• Dead volume 1 For non-flush-mounted transducers, the dead volume may be given in cubic millimeters. Applicable for the following transducer technologies: All • Material, pressure sensing 1 The diaphragm material and thickness may be specified, if a diaphragm is employed. Applicable for the following transducer technologies: All • Vibration isolation 1 If the transduction element is mechanically isolated in some way from the transducer mounting points (to reduce vibration sensitivity) this may be described. Applicable for the following transducer technologies: All • Volume change due to full scale pressure 2 The change in volume of the sensing element due to application of full scale pressure shall begiven in cubic millimeters. Applicable for the following transducer technologies: All |
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