Why we are using 4-20 mA as Analog signal transmission
other than 3 to 19 or 2 to 18 or 6 to 22 like wise ? What
is the reason behind using 4 & 20 mA? Can anyone give
proper technical reply ?
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Answer Posted / t.naveen babu
The reasons for the 4-20 mA standard (and, before it, the
10-50 mA standard) have a lot to do with the original 3-15
psi/20-100 kPa air
signal. The origin of the suppressed or "live" zero in the
pneumatic system was two-fold - if you want to reduce a
passive pneumatic signal quickly, you need to maintain some
differential between the pressure and the "sink" - in this
case, atmospheric air. The basic characteristic of a
flapper-nozzle assembly also has a bit to do with it -
the "linear" range of the flapper pressure is about 3 - 15
psi for a 20 psi supply. Next question - why a 20psi
supply? Anyone else remember 6-30 psi equipment for
final actuators needing a bit more grunt??
The original electronic instruments were direct imitations
of the pneumatic systems with bellows replaced by magnet-
coil assemblies. The first I came across were used in a
geothermal power project (1958 vintage)where the pressure
at the power station was controlled by manipulating valves
in the steam field 1 1/2 miles or so away. The flapper-
nozzle was replaced by a
contact moving between positive and negative terminals - I
guess a bity of mains hum in the amplifier kept it
jittering. In the mid-70's Foxboro produced a range of
pressure transmitters using the magnetic principle which
used the same basic mechanism as their pneumatic equipment -
but the size of the coils made the assemblies a lot
bigger ...
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