Answer / sai kiran.c

very goood my dear frnd ....actualy wat u said is
rite...itzz a usualy universal standard.
BUT ALSO ONE REASON STARTING FROM 4ma IS THAT THE DEVICE
WILL WRK IN OPTIMUM RESPONSE FRM THAT RANGE.
& MOSTLY AND ALSO IT NEVR STARTS FRM ZERO BCOZ IT DOESNT
MEAN THE DEVICE IS IN REST POSITION OR NOT.
WE CANT JUDGE THE DEVICE IS IN ON OR OFF.

Answer / j jones

Sai Kiran.c is correct is saying that we don't start from 0mA because we need a "Live Zero" this is so that if the control loop goes open circuit we have an indication of this.

why do we use 4-20mA, not 1 or 2mA - 20mA, well simply just as in a pneumatic system we use 5-15psi, (0.2-1bar) it gives us the most linear output, and above that, if we were to use 1mA, any induced voltages from eddie currents in cables running alongside could cause a mA to be produced by the resistance of the cable, so our readings would be affected, and if we amplify the signal, the amplification has to be greater as we have to amplify 1mA more than a 4mA signal

Answer / carl_ellis

Answer 1 is correct. The ISA SP50 standard established
4-20mA worldwide as the analog control signal decades ago.

But what the answer 1 and the others miss is that using 1 or
2 mA at the low end would not provide sufficient power to
power 2 wire loop powered instruments.

2 wire loop powered instruments use about 3.6mA at 24Vdc to
power the electronics which makes the measurement and
controls the output circuitry. That's about 85mW, not a lot
of power.

1mA or 2mA of current would provide only a fraction of 85mW
available at 4mA for powering the instrument itself, and
reduce the functionality of 2 wire loop powered instruments.

Carl Ellis
Measure First

Answer / larry thompson

As with all analog loop standards the range must be divisible by 4 so the technician can determine which quartile the signal is (25%, 50%, 75%, 100%) in. 4 mA was deemed the minimum current necessary to power most analog signal transmitters (it had previously been 10 to 50 mA)

Answer / suvankar haldar

because 4to20mA is a universal& Used mostly in all factories

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