Answer / ravinder

Because Voltage drops as resistance of cable conductor
increases but current will be same at any point of the
loop.4mA is called Live Zero,shows liveliness of the loop
when process variable is zero.

Answer / jaldeep

For signal transmission we are using 4 to 20 mA, because of
following reasons.
1. If you are using 0 to 10 for long distance then
signal will be dropped due to cable resistance you will not
get correct feedback
2. 4 to 20 mA will give correct feedback even for
long distance. It will not drop according to cable
resistance. Only you have to keep loop impedance as per
your field instrument.
3. Open loop, you can easily detect in 4 to 20 mA.
You can program if mA is less than 4 then you can define it
as open loop.
4. For 0 to 10 v input if cable is open then it will
be difficult to identify that cable is open or input is 0
volt.

Answer / himanshu

beacause total loop resistence is 250ohms.
so 24VDC supplied loop 1v corresponds to 4mA
and 5v corresponds to 20mA.
HART PC communicates to 1-5VDC only.
As explained in above answer 4mA is LIVE ZERo.
for 20mA reason is above 20mA current can be hazareous for
fieled.(fire).like HC and Refineries
thats why 20mA signal is used.

Answer / ecxlsync

current signal is used because as explained
it will not change even the loop resistance is changed
due to transmission lines resistance. it is stated in the KCL that current entering in the circuit will be equal to the
current leaving no matter if the circuit resistance is changed.

Voltage signal is greatly affected by the circuit resistance.
long distance transmission has large wire resistance that
will cause large voltage drop.

Answer / kalpesh

Because if we use 0-10V signal and if there is a break in
cable loop then it will show 0V which is the low level of
the measured signal but in reality it is a signal failure
from transmitter.

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