4-20mA Current Loop Introduction
This is just a brief o er iew of 4-20mA—this o er iew should not be considered a complete reference
for proper implementation or use. Prior knowledge of industry standards pertaining to 4-20mA
specifically, and other aspects of electronics, are assumed to be known by the technician. For proper
connection to a monitor or PLC, refer to the manufacturer's specific manual or instructions for that
particular piece of hardware.
Overview
4-20mA ("four to twenty”), is an analog electrical transmission standard used by Otis Instruments for
some of its ambient gas sensors and monitors. The signal is a current loop where 4mA represents zero
percent signal, and 20mA represents 100 percent signal (full scale of the sensor assembly). The
relationship between the current loop and the gas alue is linear.
The 4mA allows the recei ing monitor/PLC to distinguish between a zero signal, a broken wire, or a
dead instrument. Benefits of 4-20mA con ention are that it is: an industry standard, low-cost to
implement, can reject some forms of electrical noise, and the signal does not change alue around the
“loop” (as apposed to a oltage). Only one current le el can be present at any time; each de ice which
operates ia 4-20mA must to wired directly to the monitoring de ice.
Calculations
I4−20=
16⋅value
scale
4
: current of loop, measured in mA
alue : PPM or %, of gas concentration
scale : full scale of sensor (see below for usual ranges)
Table – Gas Sensor Details
-more-
8
Tar et Gas Ran e Temp.
H2S = Hydrogen Sulphide 0-100 ppm -20 to 50C
O2 = Oxygen 0-25 % -30 to 55C
SO2 = Sulfur Dioxide 0-20 ppm -20 to 50C
CL2 = Chlorine 0-10 ppm -20 to 50C
H2 = Hydrogen 0- % -20 to 0C
NH3 = Ammonia 0-100 ppm - 0 to 0C
CO = Carbon Monoxide 0-999 ppm -20 to 50C
F2 = Florine 0-1 ppm -10 to 0C
HF = Hydrogen Fluoride 0-10 ppm -10 to 0C
H2S-2 = Hydrogen Sulphide 0-100 ppm - 0 to 50C
(extended temp)
