Strain high-pressure sensors

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. 

These measures allow reduction in the effect of mechanical strain on the sensing element, resulting in a pressure sensor with a high accuracy of 2% FSD at 5 kPa. 

As a general feature of most osmometers, the membrane is clamped into a stainless steel thermostated chamber (the measuring cell and the pressure measuring system of modem osmometers are built into a high-grade electronically stabilized thermostat) and serves as barrier between the pure solvent and the polymer solution sides of the chamber. The solvent side (bottom) is in juxtaposition with a pressure sensor, e.g., the diaphragm of a capacitance strain gauge or a piezo-chip. The solvent transport is measured across the... 

Dynamic osmometers reach equilibrium pressures in 10 to 30 minutes and indicate osmotic pressure automatically. Several types are available. Some commonly used models employ sensors to measure solvent flow through the membrane and adjust a counteracting pressure to maintain zero net flow. A commercially available automatic osmometer operates on the null-point principle. In this high-speed membrane osmometer schematically represented in Fig. 4.4, the movement of an air bubble inside the capillary immediately below the solvent cell indicates the solvent flow to the solution cell. Such movement is immediately detected by a photocell, which in turn is coupled to a servomechanism. If any movement of the air bubble is detected by a photocell, the servomechanism is stimulated to move the solvent reservoir upward or downward in order to adjust the hydrostatic pressure such that the solvent flow is completely arrested. The pressure head of the reservoir gives the osmotic head. Some osmometers also use strain gauges on flexible diaphragms to measure the osmotic pressure directly. 

In spile of the case of construction, robustness, and high sensitivity, the lack of a truly static response of a ferroelectric polymer pressure transducer represents a serious limitation to widespread use of this type of device. Several techniques can be conceived in order to overcome this drawback [21] (such as the parallel use of a strain gauge sensor and the use of this transducer to calibrate at intervals the signal detected by the PVDF sensor), but excessive complexity and costs are usually added to the originally simple and cheap design of the piezoelectric polymer catheter-tip sensor. 

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