Table of Contents
In general, the design of pressure sensors employed for measurement of pressure higher than one atmosphere differs from those employed for pressure less than one atmosphere. In this section the basic operating principles of many types of pressure sensors used for the higher pressures are considered. You should be aware that this is not a rigid separation, because you will find many of these same principles employed in the lower (vacuum) pressure measurements.
Most pressure sensors used in process control require the transduction of pressure information into a physical displacement. Measurement of pressure requires techniques for producing the displacement and means for converting such displacement into a proportional electrical signal. This is not true, however, in the very low pressure region (p < 0.001 atm), where many purely electronic means of pressure measurement may be used.
One common element used to convert pressure information into a physical displacement is the diaphragm shown in Figure 5.30. If a pressure p1 exists on one side of the diaphragm and p2 on the other, then a net force is exerted given by
where A = diaphragm area in m2
p1, p2 = pressure in N/m2
A diaphragm is like a spring, and therefore extends or contracts until a Hooke's law force is developed that balances the pressure difference force. This is shown in Figure 5.30 for p1 greater than p2. A bellows, shown in Figure 5.31, is another device much like the diaphragm that converts a pressure differential into a physical displacement, except that here the displacement is much more a straight-line expansion.
FIGURE 5.31 A bellows is another popular method of converting pressure to displacement. In this case an LVDT converts the displacement to an electrical signal.
FIGURE 5.32 The Bourdon tube is a common transducer for converting pressure to displacement.
Figure 5.32 also shows how an LVDT can be connected to the bellows so that pressure measurement is converted directly from displacement to a voltage. In addition, the displacement and pressure are nearly linearly related, and because the LVDT voltage is linear with displacement, the voltage and pressure are also linearly related.
A common pressure-to-displacement conversion is accomplished by a specially constructed tube, shown in Figure 5.32. If a section of tubing is partially flattened and coiled as shown, then the application of pressure inside the tube causes the tube to uncoil. This then provides a displacement that is proportional to pressure.
Many techniques are used to convert the displacements generated in the previous examples into electronic signals. The simplest technique is to use a mechanical linkage connected to a potentiometer. In this fashion, pressure is related to a resistance change. Other methods of conversion employ strain gauges directly on a diaphragm. LVDTs and other inductive devices are used to convert bellows or Bourdon tube motions into proportional electrical signals.
Often, pressure measurement is accomplished using a diaphragm in a special feedback configuration, shown in Figure 5.33. The feedback system keeps the diaphragm from moving, using an induction motor. The error signal in the feedback system provides an electrical measurement of the pressure.
Solid-State Pressure Sensors
Integrated circuit manufacturers have developed composite pressure sensors that are particularly easy to use. These devices commonly employ a semiconductor diaphragm onto which a semiconductor strain gauge and temperature-compensation sensor have been grown. Appropriate signal conditioning is included in integrated circuit form, providing a dc voltage or current linearly proportional to pressure over a specified range.
These devices are available for absolute-, gauge-, and differential-pressure measurement. They are simple to use, often needing only three connections: a dc supply, ground, and signal output. Of course, the connection to the measurement environment is made through a fitting or welded-pipe connection.
FIGURE 5.33 A differential pressure (DP) cell measures pressure difference with a diaphragm. A feedback system minimizes actual diaphragm motion.
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