Answer / eswar

we are using 4 ma instead of 0 ma because the in order to
differentiate 0 input and fault condition(open circuit cable
or transmitter).
we are using maximum of 20ma because current above range can
cause fire or spark from short circuit conditions due to
weak installations.

Answer / abdus sattar mollah

initially,0-20ma of current is used as an transmitting
standard unless a objection was met.It was that:0 ma of
current used to indicate no signal/minimum signal from a
signal conditioner but if there is a faulty condition(such
as loop break),then also it will be show 0 ma of
current.Operator may think that there is no signal but
actually a fault has occured.So he cant distinguish between
them unless the plant is damaged.This is called TRUE ZERO.

So,after a reserach a new standard was proposed i.e 4-20 ma
of current.There was minimum interference in this
range,good quality transmittance,low power consumptions.So
this satndard is used.

Answer / deepesh

1) As stated earlier, 4mA is used to avoid the conflict
between live zero and dead zero.

2)In 4 mA to 20 mA range, current to pressure converter
gives linear output.

3)This range is intrinsically safe.

4)Low power consumptions

Answer / v.r.m.kiran

We are using 4 to 20ma because
1. Distinguish between process Zero & Instrument Zero.
2. It is easy to convert 4 to 20ma to 1 to 5v(Digital
Signal) by using a 250 Ohms Resistor in series.
V = IR
V=4*250=1000 MV = 1V, 8*250=2000 MV = 2V, 12*250=3000 MV =
3V, 16*250=4000 MV = 4V, 20*250=5000 MV = 5V.
3. Now we are using digital Indicators for display. Olden
days there will be analog display meter in the transmitter
for indication(0 to 100)%. When we apply 4ma or while
checking the transmitter online pointer should indicate
zero then we say that instrument is working correctly but
it comes bellow zero we have to check our instrument & have
to calibrate.

Answer / nagarajan audaiyarpatti

In the fault or open condition we cannot find the 0mA as 0
sothat the 4 mA consider as a zero and 20mA as a span ,in
this condition we can maintain 250mohms in the loop,sothat
we use usually 4 to 20 mA instead of 0 to 20mA in the
transmitter for measuring all the parameter such as
pressure ,temperature ,level,flow.

Answer / sachin bijole

there is no required above current of 4 mA at zero % range b'cause of faulty condition we think the current have found 0 signal.

Answer / rahul

why only 4-20 mA why not 8 to 30ma

Answer / hari

In industry we using 4 to 20 MA b'cuz drop of voltage will
less ...the signal will transmit long distance ..so sme
kind of drop voltage will there ..

Answer / mr gauge

We use 4mA instead of zero because 4mA is a minimum current
required to power up the transmitter, positioner etc. How do
we get the device turn on with no current.

i want to control the temp of sand(welding flux) between the range of 150deg C to 170deg C. sugges me sum temp control method without PID controller.

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