Electro-pneumatic transducers convert current or voltage input signals to proportional output pressures. They are used as process control elements in valves, pneumatic relays, and flow regulators for applications such as spray and damper control. There are three basic types of electro-pneumatic transducers: voltage-to-pressure (E/P), current-to-pressure (I/P), and digital-to-pressure (D/P). Voltage-to-pressure devices offer finite control ranges such as 0 – 5 V or 0 – 10 V. Current-to-pressure devices provide current loop control and signal ranges such as 4 – 20 mA or 0 – 20 mA. Digital-to-pressure devices use either serial or parallel controls. Commonly used serial controls include RS232 and RS485. Examples of parallel protocols include general-purpose interface bus (GPIB) and IEEE 488. Specialized inputs such as transistor-transistor logic (TTL) are also available.
There are several performance specifications and features for electro-pneumatic transducers. Performance specifications include output pressure, maximum air volume, maximum supply pressure, and operating temperature. Some devices can accommodate connections with national pipe thread (NPT) measurements of 1/8”, 1/4”, 3/8”, 1/2”, 3/4”, 1”, or 2”. Other devices fit pipes with unified course threads (UNC) or unified national fine (UNF) standards. Electro-pneumatic transducers with British standard pipe thread (BSPT) and metric threads are also available. In terms of features, electro-pneumatic transducers often provide analog or digital gauges for pressure readings. Outdoor rated devices include seals and other materials that can withstand a variety of weather conditions. Explosion proof, corrosion resistant, and sanitary devices are also available.
Body materials for electro-pneumatic transducers include aluminum, brass, bronze, cast iron, steel, stainless steel, zinc, and plastic. Aluminum, a metallic element with good electrical and thermal conductivity, is resistant to oxidation. Brass provides good strength and conductivity, excellent corrosion resistance and high temperature ductility, and low magnetic permeability. Bronze is a porous material that can be impregnated with oil, graphite, or polytetrafluoroethylene (PTFE). The term “cast iron” refers not to a single material, but to a family of materials whose major constituent is iron, with important trace amounts of carbon and silicon. Steel, a commercial iron that contains carbon, is malleable under suitable conditions. Stainless steel is chemical and corrosion resistant and can have relatively high temperature ratings. Zinc and zinc alloys are also corrosion resistant. Plastic body materials for electro-pneumatic transducers include acetal, nylon, and polycarbonate.
Electro-pneumatic transducers are used in applications that require the operation of valve actuators, pneumatic valve positioners, damper and louver actuators, final control elements, relays, air cylinders, web tensioners, clutches, and brakes. They are used widely in the petrochemical, energy management, textile, paper, paper converting, and food and drug industries. They are also used in heating, ventilation, and air conditioning (HVAC) systems.