First appearing in the 1950’s with the advent of electrical and electronic controls, the 4-20mA signal standard reigns as one of the most popular mediums for signal transmission and electronic control in industrial environments nearly 60 years later. Prior to the widespread adoption of electrical and electronic controls, buildings often used pneumatic control systems. Large and powerful compressors drove 3psi to 15psi pneumatic signals throughout a plant and these pneumatic lines connected to pneumatically controlled valves and pneumatically controlling valves in order to drive proportional controls and actuators throughout the building, all powered from compressed air. Air pressure at 3psi served as the “live-zero” and 15psi represented 100%. In this way, the more modern 4-20mA signal standard emulated the earlier 3-15psi pneumatic controls. Any pressure below 3psi was considered “dead zero” and an alarm condition. Some installations still use pneumatic control today. Modern I/P converters (current-to-pressure transducers) are available to convert the 4-20mA control loops to common pneumatic ranges, such as 3-15psi, 1- 18psi, 3-27psi, and 6-30psi. In two-wire 4-20mA control loops, we use 2-wire transmitters to convert various process signals representing flow, speed, position, level, temperature, pressure, strain, pH, etc., to 4-20mA DC for the purpose of transmitting the signal over some distance with little or no loss of signal. This paper reviews the operation of this transmission standard and its advantages, in particular as it relates to two-wire transmitters and the associated 4-20mA current loop.
Acromag (2014) Introduction to the two-wire transmitter and the 4-20mA Current Loop