Why Is 4-20mA Still Widely Used for Sensors Like Honeywell ST3000?

I am new to 4-20mA area and i have some sensor i need to interface and convert these into digital.. then mention a sensor part number or sensor name.. never post without datasheet ot part number.. i am really curies to know why they choose 4-20mA as a standard and i read its a very old protocol then why its so widely used and popular... any feedback will be much appreciated...

Bart
Why a current loop? As the name implies current must flow in a loop and as described in Kirchoff's Laws, the exact current that is sent out must be returned. If you use a wire pair to do this you can a) transmit over a very long length of wire (subject to votage drop due to line resistance as I will mention in a minute) and any common mode noise appears on both wires and hence has a high noise immunity.
Why 4-20mA? Well I have seen 3 variations of current loop: 0-20mA (not to be confused with to 20mA digital current loop used in teleprinters), 10-50mA (which I have only seen in books and never in reality) and 4-20mA. There are 2 reasons for the 4mA offset: firstly it allows open circuit detection and secondly it allows the remote circuit to be powered from the 4mA offset. I have been in the business for 40+ years and the actual reason predates that.
There is another advantage- provided the 4-20mA receivers have differential inputs or are floating, you can place several receivers in series. For instance you could have a meter in series with a valve.
The world is not perfect and this holds true for current sources. There will be a voltage drop across the current source, let's say V2 volts. There is a supply of V1 volts (typically 24VDC in industrial systems) so you only have V1-V2 volts available for the current loop. There are volt drops (often current dependent) at the inputs of each receiver, and there is a volt drop over the length of the cable pair and these votages must some to less than V1-V2, in the worst case.
I wrote an article on the subject including circuits that you can use to realise the current loop drivers and receivers. It is in the August 2010 issue of Circuit Cellar (issue #241)
https://www.cc-webshop.com/241-August-2010-The-4-to-20-mA-current-Loop-SA-2010-241-028.htm

Hi Bart-Aubrey is absolutely correct on everything he said, but I'd like to add one little bit. In order to convert the 4-20 mA signal to something meaningful you must apply two correction factors to it.  These are usually called zero and span (or offset and gain, depending on who you listen to).Zero tells your equipment how much to subtract (offset) from the 4 milliamp reading to get an actual value of zero, and span tells it how much amplification (gain) must be applied to the 20 milliamp reading to get a value of 100% of whatever is being measured.-Rick

Hi sorry for my improper forum post question it was very helpful though I was able to know about 2wire 4-20mA current loop receivers But where should we use 3 wire and 4 wire 4-20mA current loop receiver what are its importance 

HiI am really sorry for my improper post question
Well with help of your replies i am able to understand the about 4-20mA current loop protocol and also understand the working of 2 WIRE current loop receiverBut in what way i should use 3 wire and 4 wire current loop receiver what is the importance these receivers? 

The primary benefit of the 3 and 4 wire 4-20mA approach is that the transmitter can draw more than 4mA, thus allowing more sophisticated circuitry including micro controllers  for signal processing. The power is supplied through one of the wires and the signal is on a pair. On the 4 wire the ground is separate and the 3 wire the ground is commoned with the signal return. As with all systems you need to be careful about earth loops with these configurations.
One of the important aspects of the current requirement is with the circuitry needed for galvanic isolation.

The circuitry for 3/4 wire systems is much easier to understand and debug. Again I refer you to my article I mentioned in my first response