Many people in the HVAC/R industry remember the days of pneumatic control; some buildings in fact still use pneumatic control systems. In these systems, ratio controllers, PID controllers, temperature sensors and actuators are powered by compressed air. Three to fifteen pounds per square inch is the modulation standard, 3 psi for a live zero and 15 psi for 100%. Any pressure below 3 psi is a dead zero and an alarm condition. In the 1950’s electric and electronic controls made their debut. The new 4-20 mA current signaling emulated the 3-15 psi pneumatic signal. Current signaling quickly became the preferred method because wires are easier to install and maintain than pneumatic pressure lines and energy requirements are a lot lower – you no longer needed a 20 to 50 horsepower compressor for instance. Also, electronics allowed for more complicated control algorithms.
Building Automation Products Inc (2006) Understanding Current Loops