OTIS GEN2 OI-7032, Руководство по эксплуатации

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4-20mA Current Loop Introduction
This appendix in only an introduction. The information should serve as a brief overview of 4-20mA, and
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 value is linear.
The 4mA allows the receiving monitor/PLC to distinguish between a zero signal, a broken wire, or a
dead instrument. Benefits of 4-20mA 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 apposed to a voltage). Only one current level can be present at any time; each device which
operates via 4-20mA must to wired directly to the monitoring device.
Calculations
I
 4 − 20 
=


16 ⋅ value

scale

 4
I
 4 − 20 
: current of loop, measured in mA
value : PPM or %, of gas concentration
scale : full scale of sensor (see below for usual ranges)
Table – Gas Sensor Details
Actual ranges may vary with our product. If unsure, confirm with the actual gas sensor assembly
distributor, Otis Instruments sales representative, or call the main Otis Instruments office for more
details.
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Target Gas Range 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 0-4 % -20 to 40C
NH3 = Ammonia 0-100 ppm -40 to 40C
CO = Carbon Monoxide 0-999 ppm -20 to 50C
F2 = Florine 0-1 ppm -10 to 40C
HF = Hydrogen Fluoride 0-10 ppm -10 to 40C
H2S-2 = Hydrogen Sulphide 0-100 ppm -40 to 50C
(extended temp)