Pressure, absolute strain gauge

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... 

Strain-gauge pressure transducers are manufactured in many forms for measuring gauge, absolute, and differential pressures and vacuum. Full-scale ranges from 25.4 mm of water to 10,134 MPa are available. Strain gauges bonded direc tly to a diaphragm pressure-sensitive element usually have an extremely fast response time and are suitable for high-frequency dynamic-pressure measurements. 

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. 

Manometer h. The pressure manometer is connected to the stainless-steel capillary by a capillary T-tube containing a capillary stopcock, X, which can be opened to admit or remove gas from the bulb. During measurements this stopcock is kept closed and the quantity of gas in the bulb and dead space is thereby fixed. The pressure manometer should have a small internal volume and be capable of measuring absolute pressures with a resolution of 1 Torr or better. As discussed in Chapter XVIII, either capacitance or strain-gauge manometers can be used. The latter are generally less expensive, and high-precision versions of these gauges are available. 

The pressure gauge used in this experiment should be a direct-reading gauge with a relatively small and constant internal volume. Reproducibility is more important than absolute accuracy since relative measurements are made on air (the standard gas used for calibration) and the other gases. The critical feature is that the same initial P and final P inlet pressures are used in all runs. Thus, one can use capacitance, reluctance, or strain-gauge manometers (see p. 596-597). The latter manometers are the least expensive and are adequate if models with the best resolution are chosen. 

The specimen is placed onto the wooden platform over the pressure pin and a series of known metal block weights are placed onto its surface. The strain gauge device detects the pressure transmitted through the specimen at each known pressure in increments created by the metal blocks. The amount of pressure absorbed and dissipated within the textile stmcture and the actual pressure felt immediately below the specimen ie the patient s leg is determined. The transmitted pressure throu the thickness of the specimen is the absolute pressure exerted on the patient s leg. 

Strain gauge or piezo-resistive pressure transducers can be bought as versions for absolute and gauge pressure. One disadvantage is the high temperature drift of the zero signal which constrains the measurement range to two pressure decades. 

A seen from Figure a3, the difference in pressure is expressed by change in strain gauge resistance, which is later on amplified, filtered and compensated for temperature and atmospheric pressure, resulting in an absolute pressure value. After the linearization stage within the sensor circuitry the pressure value is manifested an output voltage (or electrical current) with a linear transfer function. 

---