These bonded foil strain gauges (BFSG) have been the industry standard for years and are continually used because of their quick 1000 Hz response times to changes in pressure as well as their large -268 ☌ to +273 ☌ operating temperature. In the bonded strain gauge pressure sensor, a metal foil strain gauge is actually glued or bonded to the surface where strain is being measured. Strain gauge pressure transducers come in several different varieties: the bonded strain gauge, the sputtered strain gauge and the semiconductor strain gauge.įigure 5: Cross section of a typical strain gauge pressure sensor. Using a full-bridge configuration greatly increases the sensitivity of the circuit to changes in strain, providing more accurate measurements. This configuration is called full-bridge. In some applications, such as load cells, four strain gauges may be used in a Wheatstone bridge configuration, meaning that each resistive leg of the circuit is active. The output voltage of the bridge in Figure 7, V o, is equal to: The general Wheatstone bridge, illustrated in Figure 7, consists of four resistive arms with an excitation voltage, V EX, that is applied across the bridge. The most common circuit configuration is called a Wheatstone bridge. Because the changes in strain, and therefore resistance, are extremely small, you have to use additional circuitry to amplify the changes in resistance. When a change in pressure causes the diaphragm to deflect, a corresponding change in resistance is induced on the strain gauge. The most widely used strain gauge is the bonded metallic strain gauge as shown in Figure 6.

All strain-based sensors make use of a strain gauge and a diaphragm as seen in Figure 5. These sensors are used for high- and low-pressure applications, and can measure absolute, gauge or differential pressure. Of all the pressure sensors, strain-based sensors are the most common, offering solutions that meet varying accuracy, size, ruggedness and cost constraints. The three most universal types of pressure transducers of this form are the strain gauge, variable capacitance and piezoelectric. The sensor then converts this displacement into an electrical output such as voltage or current. Often pressure can be converted to some intermediate form, such as displacement. Also, the abbreviation psid (pounds per square inch differential) is often used to describe differential pressure.īecause of the great variety of conditions, ranges and materials for which pressure must be measured, there are many different types of pressure sensor designs. Similar to absolute pressure, the abbreviation psig (pounds per square inch gauge) is often used to describe gauge pressure.ĭifferential pressure is similar to gauge pressure, but instead of measuring relative to ambient atmospheric pressure, differential measurements are taken with respect to a specific reference pressure as shown in Figure 4. Gauge pressure is measured relative to ambient atmospheric pressure as shown in Figure 3. Often the abbreviation psia (pounds per square inch absolute) is used to describe absolute pressure. Absolute pressure measurement is measured relative to a vacuum as showing in Figure 2. There are three types of pressure measurements: absolute, gauge and differential. Head pressure is a function solely on the height of the liquid and weight density of the liquid being measured as shown in Figure 1.Ī pressure measurement can further be described by the type of measurement being performed. Head pressure measures the static pressure of a liquid in a tank or a pipe. For example, the pressure inside a balloon or at the bottom of a water tank would change as air is let out of the balloon or as water is drained out of the tank. Such a pressure measurement is known as a dynamic pressure measurement. Often the motion of a fluid changes the force applied to its surroundings. Examples of static pressure include the pressure of the air inside a balloon or water inside a tank. The pressure in cases where no motion is occurring is referred to as static pressure. The SI unit for pressure is the Pascal (N/m 2), but other common units of pressure include pounds per square inch (psi), atmospheres (atm), bars, inches of mercury (in Hg), and millimetres of mercury (mm Hg).Ī pressure measurement can be described as either static or dynamic. For example, pressure is a function of force and area. Pressure is defined as force per unit area that a fluid exerts on its surroundings.

Pressure is one of the fundamental quantities measured in processing and manufacturing systems.

1670 psi to atm