Why Use Standard o/p 4ma to 20ma

I think its a same like in past Pneumatic instrumets signal(3 to15 psi)and in electronics 4 to 20 mA.For ensure to check the instrument healthness checking.If instrument is not working can be easily point out some thing with instrument.

Correct for the fault indication.

There are other considerations as well, regarding the integrity of the transmitted/received signal.

4ma to 20 ma are genrally used for bi-directional measurements. (Eg). Power export or Import. When there is no power flow, there will be an output of 12 ma.For full scale export it will reach 20 ma and for full scale import it will come down to 4ma.

For unidirectional measurement also 4ma-12 ma transducer is preferred.

The reason is suppose it reads zero at telimetery, there will be a doubt, whether this is due to fault in the transducer or defect in the telemeter.

The measurement of 12ma or 4ma during zero input condition will clear the healthiness of transducer and narrow down the process of identifying the defect.

As stated above and on many reference web sites...

The main reasons why a current signal (like 4-20mA) is superior to a voltage signal:

> A current signal is less sensitive to noise because induced voltages on the wiring loop are ignored.
> The voltage drop due to (long) wire lead resistance is also ignored.
> 4mA as a "zero" sensor value is also a diagnostic state that indicates the sensor is still active and the wiring is intact with no open or shorted wire leads.

Hope that helps.

Because it's a standard. If the output were anything else, then a smaller number of pieces of equipment would be compatible with it, which would curtail sales somewhat....