Diaphragms high-pressure sensors

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. 

Flg. 7.6. Electronic pressure transducer. This sensing unit is based on the measurement of the capacitance between the diaphragm D and sensors S. Px is the pressure being measured and PH is a reference pressure which may be a high vacuum. 

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... 

AH these devices are fiUed with sHicone oil and have low gradient, corrosion-resistant barrier diaphragms on both the high and low pressure sides of the sensor. 

Figure 6.196 is an illustration of a DP cell which operates by varying the distance between the plates of two adjacent capacitors (see also Fig. 6.11)(24). The high and low pressure signals from the sensor are applied to ceramic diaphragms to which one plate of each capacitor is attached. The subsequent change in the separation of the capacitor plates produces a variation in capacitance which is detected by incorporating the cell into a capacitance bridge, as described in Section 6.5.3 (Fig. 6.316).

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. 

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