History of 3-15 psi, 10-15 mA, and 4-20 mA signalsS Bharadwaj ReddySeptember 2, 2016June 7, 2018 Before 4-20 mA the signals were 3-15 psi and 10-50 mA. These signals all have a “live zero” (3 psi, 10 mA, and 4 mA) rather than a “dead zero”