AirLink 7032 Gas Monitor Operator’s Manual
Appendix B: Introduction to 4-20 mA Current Loop Signals
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Appendix B: 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 AirLink 7032
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 AirLink 7032 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
Table 4: 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
