Gauge pressure sensors

Gauge pressure sensor Absolute pressure sensor Differential pressure sensor... 

FIGURE 2.5 Structure of an unbonded strain gauge pressure sensor. (Reproduced from Neuman M.R. 1993. In R.C. Dorf (Ed.), The Electrical Engineering Handbook, Boca Raton, FL, CRC Press. With permission.)... 

What is actually measured in an ionization gauge pressure sensor How is this actual measurement related to pressure ... 

Pressmes can be measured in environments from the near vacuum of space to more than 1,400 megapascals (MPa) and from steady state to frequencies greater than 100,000 cycles per second. Sensors that measure with respect to zero pressure or absolute vacuum are called absolute pressure sensors, whereas those that measure with respect to some other reference pressure are called gauge pressure sensors. Vented gauge sensors have the reference side open to the atmosphere so that the pressure reading is with respect to atmospheric pressure. Sealed gauges report pressure with respect to a constant reference pressure. 

Absolute pressure is pressure measured relative to a perfect vacuum, an absolute 2ero of pressure (2). Like the absolute 2ero of temperature, perfect vacuum is never reali2ed in a real world system but provides a convenient reference for pressure measurement. The acceptance of strain gauge technology in the fabrication of pressure sensors is resulting in the increased use of absolute pressure measurement in the CPI (see Sensors). The pressure reference... 

The pressure sensors discussed herein are reasonably accurate for measurement into the low vacuum region. Upon some modification, these sensors can be extended into the lower end of the range, to approximately 1.3 x 10 Pa (10 torr). Vacuum gauges ate required for accurate measurement of lower pressures. 

To obtain the benefits of this seal, it is necessary to install a gauge indicating the actual seal chamber pressure. Sensors and transmitters can be used to monitor and act on a pressure change. One of the two seals can fail without product loss or fugitive emissions. This seal would be connected to a pumping unit seen later in this chapter. 

Pressure. Pressure so defined is sometimes called absolute pressure. The differential pressure is the difference between two absolute pressures. The most common types of pressure-measuring sensors are silicon pressure sensors, mechanical strain gauges, and electromechanical transducers. 

Pressure loss coefficient, 13 261 Pressure measurement, 11 783 20 644-665. See also Vacuum measurement electronic sensors, 20 651-657 mechanical gauges, 20 646-651 smart pressure transmitters, 20 663-665 terms related to, 20 644-646 Pressure measurement devices. See also Pressure meters Pressure sensors location of, 20 682 types of, 20 681-682 Pressure meters, 20 651 Pressure microfiltration/ultrafiltration,... 

The instruments consist of the actual sensor (gauge head, sensor) and the control unit required to operate it. The pressure scales or digital displays are usually based on nitrogen pressures if the true pressure pj of a gas (or vapor) has to be determined, the indicated pressure p, must be multiplied by a factor that is characteristic for this gas. These factors differ, depending on the type of instrument, and are either given in tabular form as factors independent of pressure (see Table 3.2) or, if they depend on the pressure, must be determined on the basis of a diagram (see Fig. 3.11). 

Calibrate and document all pressure gauges and sensors, both monitoring and controlling. 

Further devices use strain-gauge transducers as so-called dead-end instruments or sensors mounted on a pipe with internal flow. The principle of this arrangement is the elastic deformation of a metallic cylinder, measured with the strain gauges. Pressures up to approx. 15 kbar can be measured [11]. 

Fig. 6.15. Basis of strain gauge pressure transducer (a) thin-film strain-gauge used as pressure sensor (b) deformation of conductor under tensile stress...

H Pressure Sensor I Bores for Heating Wire J Air Cylinder K Air Cylinder Piston L Back Pressure Regulator M Compressed Air Cylinder P Pressure Gauge R Regulator Unit VB Bleed Valve VN Needle Valve VP Purging Valve... 

The aim is to eliminate entrance effects as much as possible and any influence on the flow of the pressure tap holes into the channels. This was achieved by integrating on the same silicon chip the microchannel, the pressure taps and the pressure sensors. The fabrication process and the operating mode are described in [28]. The pressure sensors are constituted cf a membrane which is deformed under the fluid pressure and on which is deposited a thin film strain gauge. This strain gauge forms a Wheatstone bridge whose the membrane deformation modifies the electrical resistances. 

Polysilicon and NiCr are currently used for high volume production of high pressure sensors. Because the origin of the change in resistance as a function of the applied strain is different for the two materials, we will analyze the physical contributions to the gauge factor and then discuss these materials in more detail. The Wheatstone bridge, which is commonly used to detect small changes in resistance, is also discussed. 

Three layers have to be patterned to fabricate strain gauges the sensing layer, the contact layer, and the passivation layer. Usually, the highest precision is required when patterning the sensing layer. Minimum line widths are about 30 pm. Examples of layouts for high-pressure sensor elements are shown in Fig. 5.4.11. 

Air and gas pressure (pressure sensors, pressure gauges, U-tube manometer)... 

There are no special requirements for fhe insfrumenfa-tion used to measure the temperature and pressure of the compressed air. Orifice gauges and hof wire anemometers measure flow rate in a relatively precise manner. As regards to steam, the instrumentation needs to resist not only pressures but also high temperature. In the case of manometers and pressure sensors it is advisable to use cooling separators filled wifh silicon oil. It is appropriate to use orifice gauges to measure flow rate because high temperature does not affect them. 

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