Answer / vijay ranipa

if we use 0 to 20ma insted of 4 to 20 ma then we can not
find actual position zero reference output, for example if
we have a motorized valve, it's full close at that time we
get 0ma (0%)reference and at fully open position we get
20ma(100%)reference, in this condition if power supply of is
off then we also get 0ma reference,so here in this kind of
condition we can't get actual result like that gate is
actually close or any other problems as per explanation of
above..

Answer / chiru mudra...

There r 2 reasons for this.....

1) All micro controllers, plc, dcs etc., are operated with 1
to 5 volts supply.( Ex: 1v=low, 5v=high).
At the input & output of these devices we use 250 ohm
resistor for converting this 4-20mA to 1-5volts.
ie; 4mA*250 ohms = 1Volt
20mA*250ohms = 5Volts.......

2) If we use this range from 1mA to some range there is a
problem with that.
which is, in industries we use bunch of cables together from
field to control room and vice verse. Due to this there is
some signal distortion in the other cables due to magnetic
induction (electro magnetic induction effect in the other
cables). ie, 1-2 mA current will generate in the unactivated
cables which may cause the mall fn in the total industry.
Ex: 1mA = 0% valve open
2mA = 10% valve open
3mA = 20% valve open.... etc.

Answer / vijay

we have field instruments like voltage,current..etc.but
there is an advantage of using 4-20mA,If we are using the
voltage sensors,u know the voltage is resistance
dependence,and resistace of coductor varies as the distance
and temperature,varies.so u may not get the accurate signal
if u prefer for the voltage sensors.using current sensors u
don't find this problems

Answer / ajith294

The above answer is correct. we have to need the dead zero,
so we are using 4 to 20 mA instead of 1 to 16 ma

Answer / diya.

4-20mA is use in instrumentation feild, bcoz when we consider 1-16mA there is difficult to find error in that...

Answer / anand

also the signal is linear in 4-20mA range which is not below
4mA or above 20mA.

Answer / adarsh choudhary

We used 4 to 20 mA for signal because it is a least affected electrical noise & harmonic signal. Also it is suitable for long transmittance of signal without restricted of resistance of signal wire.

Answer / bhagwan

We use 4-20 mA out put because this signal goes to the field (from control room to the field,) where it is then converted from 3 psi to 15 psi as output to control the flow , tempersture, pressure etc.
In the control room, the same signal is converted from 1-5 volts to read measure flow, temperatures,pressure and at the same time , it is sent back to out put card
to convert 4-20mA. to 3-15 p.s.I. to actuate control Valve.
As the distance beween Control Room & the field is very far, so the loses for transfering 4-20mA. are nill.

Why shield is not connected at both end.

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