OTIS OI-6000K NXP Series, Operation Manual

Page: 56 / 68

APPENDICES
51 OI-6000K-X-X-X-NXP-X OPS_GUIDE_REV 1.0
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 vo ltage). 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