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Answer Posted / carl_ellis
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.
Carl Ellis
Measure First
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