OTIS GEN II OI-6000, Operation Manual

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APPENDICES
54 OI-6000-X-X-X-X-O OPS_GUIDE_REV 3.1
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 monitor 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, Otis devices use values below 4 mA to indicate special status conditions, as shown below:
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
The 4 mA allows the receiving monitor/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 transmitter 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 monitoring device. 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 monitoring device.
CALCULATIONS
I
(4-20)
= Current of loop, measured in mA
value = ppm (or %) of gas concentration
scale = full scale of sensor