40 • Appendix A: Introduction to 4-20 mA Current Loop Signals
AirLink 9850 Gas Monitor Operator’s Manual
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 AirLink 9850 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 9850 will maintain the proper current for the device, up to
its maximum voltage capability.
Calculations
I(4-20) = Current of loop, measured in mA
value = ppm (or %) of gas concentration
scale = full scale of sensor
Measuring Current
If the value measured is 0 mA, then: the loop wires are broken, the sensor assembly is not powered up,
the sensor assembly is malfunctioning, or the controller is malfunctioning. A digital multi-meter
(DMM), or current meter, may be used in conjunction with the controller and/or to test the 4-20 mA
current loop signal. To measure the current, place the meter probes in line with the current loop.
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
