Pressure transducers are just one of the many types of transducers used to detect and measure process variables such as temperature, flow, level, composition and transmit a continuous analog output signal. These output signals can be voltage, current or proportional pneumatic and are used for indication, control, recording and data acquisition systems. Pressure transducer varieties are available to measure static pressure, differential pressure, vacuum pressure and derived variables such as liquid head and partial pressures of vapors and gases.
      Operation
Pressure transducers measure a pressure such as water pressure in a tank, and convert the detected pressure to a proportional output signal that can be transmitted to a remote location. For example, if a measured pressure range for a water supply system is 0-100 psi (pounds per square inch), and the output range of the transducer is 0-5 volts DC proportional to that range, then an output signal of 2.5 volts, or 50 percent full range would signify a pressure value of 50 psi. The calibrated voltmeter indicator would read 50 psi, not 2.5 volts. Early transducers used bellows, bourdon tubes (wound flattened tubing), and diaphragms in motion balance systems such as variable resistors, capacitance and slidewires. Most transducers today use high-accuracy strain gage, piezoelectric or capacitance solid-state sensing principles.
      Purpose
Pressure transducers were developed for several reasons. First, they allowed operators of processes with many remote pressure variables to monitor them accurately, safely and economically from a central location by eliminating potentially dangerous tubing runs. Secondly, the transducer concept allows universal indicating meters to display any type of process variable. Finally, the variables would be viewable in other locations merely by extending a pair of wires.
      Types of Pressure Measured
Pressure transducers are available with the most common, gauge pressure, pounds per square inch gauge (psig) which is positive pressure compared to atmospheric; absolute pressure such as pounds per square inch absolute, (psia); differential pressure, which compares two different pressures and transmits one proportional output; and vacuum pressure, which is a negative pressure compared to atmospheric. Very small pressures and differential pressures can be expressed as inches of water, (inches H2O) where 27.71 inches of water equals one psi. All the above units can also be expressed in Metrics.
      Industrial Applications
Ruggedized pressure transducers for control systems in every process industry are called transmitters. Miniature pressure transducers are widely used in transportation -- with oil pressure transducers for engines and transmissions in automobiles and a wide assortment of pressure transducers for every functional aspect of aviation including propulsion, hydraulics, and navigation. Cumbersome medical instruments have given way to miniature medical grade transducers for blood pressure and other readings.
      Domestic Applications
Domestic appliances and devices also incorporate pressure transducers--to sense washer water levels, detect plugged filters and vacuum cleaner bags. There are digital tire pressure gauges. Even that digital weather station may well measure barometric pressure with an electronic pressure transducer.