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Historically, the 4-20 mA electronic control signal replaced the 3-15 psi, 20-100 kPa or 0.2-1.0 bar pneumatic control signals and its competitor 10-50 mA.
The ratio of contorl signal span to its 'live zero' is the same in all cases: 4:1
The ratio of span to the 'live zero' for 3-15 psi is:
span = 12 psi (15-3)
live zero = 3
ratio span:live zero = 12:3 = 4:1
The ratio of span to the 'live zero' for 20-100 kPa is:
span = 80 psi (100-20)
live zero = 20
ratio span:live zero = 80:20 = 4:1
The ratio of span to the 'live zero' for 0.2-1.0 bar is:
span = 0.8 bar (1.0-0.2)
live zero = 0.2
ratio span:live zero = 0.8:0.2 = 4:1
The ratio of span to the 'live zero' for 10-50 mA is:
span = 40 mA (50-10)
live zero = 10 mA
ratio span:live zero = 40:10 = 4:1
The ratio of the span of the active control signal to its live zero is identical in all cases.
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Answer / satish singh
because in this range the current value is continuity /or we can say that in thios range the value is linear
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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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The main reason for 4-20mA is that 2-wire field instruments need to use 3.5mA to power their own electronics in order to make a measurement and to control the regulated output.
4-20mA is also, as you pointed out, a range in which the energy in the circuit is low enough to qualify for Intrinsic Safety approval (when used installed properly with an approved DC power supply and I/S barrier).
The "live zero" is nice and convenient as a diagnostic for an open circuit fault, but it was not the main reason for 4-20mA; powering field instruments was the main reason.
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