Diaphragms pressure sensors

Investigate the operation of pressute-measuring devices such as manometers, Bourden tubes, and diaphragm pressure sensors. Write a brief tqmrt discussing your findings. 

Doping of the siHcon can have a large effect on the etch rate, and layers of different materials such as Si02 and Si N can have different etch rates. Eor pressure sensors, thin diaphragms of Si or related materials are etched into the wafer (see Pressure measurements). 

Many of the variations developed to make pressure sensors and accelerometers for a wide variety of appHcations have been reviewed (5). These sensors can be made in very large batches using photoHthographic techniques that keep unit manufacturing costs low and ensure part-to-part uniformity. A pressure differential across these thin diaphragms causes mechanical deformation that can be monitored in several ways piezoresistors implanted on the diaphragm are one way changes in electrical capacitance are another. 

DC Diaphragm controller P Vacuum pump M Measuring and switching device PS Pressure sensor V1 Pump valve V2 Gas inlet valve TH Throttle... 

As soon as the pressure in the process chamber drops below the set minimum switching point towards the end of the drying process, valve V2 opens and remains open. In this way the unthrottled cross-section of the diaphragm confroller is available again for rapid final drying. At the same time the final drying procedure can be monitored by means of the pressure sensor PS. 

DC Diaphragm controller PS Process pressure sensor RS Reference pressure sensor V1 Gas inlet valve V2 Pump valve... 

Fig. 15. Fiber-optic pressure sensor. As light passes into a cavity resonator formed by a glass substrate and a flexible diaphragm, it is reflected from both surfaces, forming an interference pattern that changes as the diaphragm flexes with pressure changes. (Yazbak, Foxboro, Massachusetts)...

Fig. 6.11. Capacitive pressure sensor with flexible diaphragm...

M. Hirita, S. Suwazono, and H. Tanigawa, Diaphragm thickness control in silicon pressure sensors using an anode oxidation etch-stop, J. Electrochem. Soc. 134, 2037, 1987. 

Fig. 5.1.17 Absolute pressure sensor for good media compatibility with pressure inlet port on the bottom of the silicon diaphragm and an on-chip vacuum reference volume, made possible by triplestack anodic bonding...

To improve the fuel vapor-pressure sensor accuracy, we reduced the mechanical strain conveyed from the package to the sensor device. Fig. 7.3.14 shows the structure of the fuel vapor-pressure sensor for 5 kPa. We chose a relatively thin silicon diaphragm, 14 pm, to achieve the sensitivity required for 5 kPa detection. That makes the sensor device more susceptible to the mechanical strain conveyed from the resin package. To solve that problem, we analyzed the effect of the mechanical strain from the package by FEM. 

High-pressure sensors differ mainly in the measuring principle used and the type of internal connection. Most sensor designs use a steel diaphragm to separate the pressure fluid from the sensor signal and environment. 

Figure 2.6 (a) An unbonded strain gage pressure sensor. The diaphragm is directly coupled by an armature to an unbonded strain-gage system. With increasing pressure, the strain on gage pair B and C is increased, while that on gage pair A and D is decreased. 

Fiber optic intravascular pressure sensors can be made in sizes comparable to piezoresistive ones, but at a lower cost. The fiber optic device determines the diaphragm displacement optically by measuring the varying reflection of light from the back of the deflecting diaphragm. These optical devices are inherently safer electrically, but lack a convenient way to measure relative pressure without an additional lumen either atmospherically vented or connected to a second pressure sensor. 

In using this type of sensor to measure blood pressure, it is necessary to couple the chamber containing the diaphragm to the blood or other fluids being measured. This is usually done using a small, flexible plastic tube known as a catheter that can have one end placed in an artery of the subject while the other is connected to the pressure sensor. This catheter is filled with a physiological saline solution so that the arterial blood pressure is coupled to the sensor diaphragm. This external blood pressure-measurement... 

The packaging of the pressure transducer is also a problem that needs to be addressed, especially when the transducer is in contact with blood for long periods of time. Not only must the package be biocompatible, but it also must allow the appropriate pressure to be transmitted from the biological fluid to the diaphragm. Thus, a material that is mechanically stable under corrosive and aqueous environments in the body is needed. Chronically implanted objects are usually coated with a fibrous capsule by the body as a part of the foreign body response, and this capsule can exert a force on the pressure sensor that will affect its baseline pressure. Thus, it is important to package pressure sensors with materials that will minimize this encapsulation. 

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