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Answer Posted / marmikjani
Why we use 4-20mA signal?? Not start from 0-03mA? & 20mA??
· We can not use 0-20 mA signal cos then it becomes very difficult to distinguish between "zero" signal and "instrument fail" signal.
· Where as if we use 4-20 mA. its very easy like when we get 4mA means the signal is at low level and if it is below 4mA it shows us that the instrument is not working or it is failed
•That may have been one of those arbitrary decisions that is sometimes made when a standard is developed. In addition to providing a non-zero minimum to allow detecting signal loss, the minimum signal voltage provides for a margin between the minimum signal and any noise that might be present. Perhaps someone determined that 4ma was large enough to help with noise and thought that the numbers worked out conveniently. A 16 volt difference between min and max, difference = 4 X min, range of 20% to 100%, perhaps seemed easier to work with than 17 volts, 5.67 X min and 15% to 100% or 18V, 9 X & 10% to 100%
•This 20 mA signal is not enough to trigger a spark and therefore we can limit fire hazards in plant area
•As there is a leakage current so 0 to 3 MA, so we consider as 4-20MA. But in some cases we will find some transmitters 0-20MA which are more costly than 4-20mA
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