Complete these sentences using Unless or If.

1. there is no shear in the fluid, the pressure at any point can be shown to be independent of the orientation of the imaginary surface under consideration.

2. silicon has large piezoresistive effect, it does not become the most commonly used material for strain gages.

3. pressure is not defined as a vector quantity, it is nondirectional.

4. the isolating diaphragms are made of special metal alloys, it can enable them to handle corrosive fluids.

5. the oil is chosen to set a predictable dielectric constant for the capacitor gaps, it can not provide adequate damping to reduce shock and vibration effects

Complete the following table.

Read and translate Text B.

TEXT B

Many highly accurate (better than 0.1%) pressure sensors in use today have been developed using the capacitive detection approach. Capacitive pressure sensors can be designed to cover an extremely wide pressure range. A metal or silicon diaphragm serves as the pressure-sensing element and constitutes one electrode of a capacitor. The other electrode, which is stationary, is typically formed by a deposited metal layer on a ceramic or glass substrate. An applied pressure deflects the diaphragm, which in turn changes the gap spacing and the capacitance. In the differential capacitor design, the sensing diaphragm is located in between two stationary electrodes. An applied pressure will cause one capacitance to increase and the other one to decrease, thus resulting in twice the signal while canceling many undesirable common mode effects. The isolating diaphragms are made of special metal alloys that enable them to handle corrosive fluids. The oil is chosen to set a predictable dielectric constant for the capacitor gaps while providing adequate damping to reduce shock and vibration effects.

Piezoresistive sensors (also known as strain-gage sensors) are the most common type of pressure sensor in use today. Piezoresistive effect refers to a change in the electric resistance of a material when stresses or strains are applied. Piezoresistive materials can be used to realize strain gages that, when incorporated into diaphragms, are well suited for sensing the induced strains as the diaphragm is deflected by an applied pressure. The sensitivity of a strain gage is expressed by its gage factor, which is defined as the fractional change in resistance, or the extension per unit length. It is essential to distinguish between two different cases in which: (1) the strain is parallel to the direction of the current flow (along which the resistance change is to be monitored); and (2) the strain is perpendicular to the direction of the current flow. The gage factors associated with these two cases are known as the longitudinal gage factor and the transverse gage factor, respectively. The two gage factors are generally different in magnitude and often opposite in sign. Typical longitudinal gage factors are ~2 for many useful metals, 10 to 35 for polycrystalline silicon (polysilicon), and 50 to 150 for single-crystalline silicon. Because of its large piezoresistive effect, silicon has become the most commonly used material for strain gages. There are several ways to incorporate strain gages into pressure-sensing diaphragms. For example, strain gages can be directly bonded onto a metal diaphragm. However, hysteresis and creep of the bonding agent are potential issues. Alternatively, the strain gage material can be deposited as a thin film on the diaphragm. The adhesion results from strong molecular forces that will not creep, and no additional bonding agent is required. Today, the majority of piezoresistive pressure sensors are realized by integrating the strain gages into the silicon diaphragm using integrated circuit fabrication technology. Silicon micromachined pressure sensors refer to a class of pressure sensors that employ integrated circuit batch processing techniques to realize a thinned-out diaphragm sensing element on a silicon chip. Strain gages made of silicon diffused resistors are typically integrated on the diaphragm to convert the pressure induced diaphragm deflection into an electric resistance change. Over the past 20 years, silicon micromachined pressure sensors have gradually replaced their mechanical counterparts and have captured over 80% of the pressure sensor market. There are several unique advantages that silicon offers. Silicon is an ideal mechanical material that does not display any hysteresis or yield and is elastic up to the fracture limit. It is stronger than steel in yield strength. The piezoresistive effect in single-crystalline silicon is almost 2 orders of magnitude larger than that of metal strain gages. Silicon has been widely used in integrated circuit manufacturing for which reliable batch fabrication technology and high-precision dimension control techniques have been well developed. A typical silicon wafer yields hundreds of identical pressure sensor chips at very low cost. Further, the necessary signal conditioning circuitry can be integrated on the same sensor chip no more than a few millimeters in size. All these are key factors that contributed to the success of silicon micromachined pressure sensors.

Date: 2016-04-22; view: 1041