T3A Transmitter • 55
Appendix A: Introduction to 4-20 mA Current Loop Signals
This appendix is only an introduction. The information should serve as a brief overview of 4-
20 mA current loop signal ranges and should not be considered a complete reference for
proper implementation or use.
Industry standards pertaining to 4-20 mA current loop signals and other aspects of electronics
are assumed to be known by the technician. For proper connection to a controller or
Programmable Logic Controller (PLC), refer to the manufacturer’s specific manual or
instructions for that device.
Overview
When using 4-20 mA wired output signal devices, the 4-20 mA defines the current loop
analog signal range, with 4 mA representing the lowest end of the range and 20 mA the
highest. The relationship between the current loop and the gas value is linear. In addition, the
T3A uses values below 4 mA to indicate special status conditions, as shown below:
The 4 mA allows the receiving controller/PLC to distinguish between a zero signal, a broken
wire, or an unresponsive instrument. Benefits of 4-20 mA convention are that it is: an industry
standard, low-cost to implement, can reject some forms of electrical noise, and the signal does
not change value around the “loop” (as opposed to voltage). The key advantage of the current
loop is that the accuracy of the signal is not affected by a potential voltage drop in the
interconnected wiring. Even with significant resistance in the line, the current loop T3A will
maintain the proper current for the device, up to its maximum voltage capability.
Only one current level can be present at any time. Each device that operates via a 4-20 mA
current loop signal must be wired directly to the controller. Units that are wired in a daisy
chain configuration for the 4-20 mA current loop signal will not properly transmit data
communications to the controller.
Calculations
I(4-20) = Current of loop, measured in mA
value = ppm (or %) of gas concentration
scale = full scale of sensor
4-20 mA Ranges
Current
Detector Status
2.5 mA
Sensor Fault
3 mA
Sensor in Menu Mode
3.5 mA
Sensor in Calibration
Mode