why 4 to 20 ma is a standard in instrumentation?

Answer 1

4 - 20 ma is a good standard since you can use a common 250 ohm resistor for 1 - 5 volt conversion ...

in other words ... if you place a 250 ohm resistor into a
4-20 ma signal loop ... you obtain by V = I R ... the follwoing

V = 4 ma * 250 ohm = 1000 mV = 1 V
V = 20 ma * 250 ohm = 1000 mV = 5 V

also - this range of amperage is good for long loops ... i.e. 1000 ft, and works well in a common plant setting. yet, keeps the overall currnet very low so that shocking and sparking hazards are minimized.

you will find as the current gets lower - for instance 0.1 ma, line losses and noise will interfere with the signal over longer distances. and if the current is much higher, 1000 mA (i.e. 1 Amp) then shcoking hazards and stray currents (grounding problems) become more common.

thus somewhere along the way --- the ISA or whomever ---decided that 4 -20 mA is a good comprimise and that serrendipitously ... someone likely had a bunch of extra 250 ohm precision resistors to get rid of ... well the last statement is a joke - but it makes sense that a standard off the shelf resistor like a 250 ohm precision puppy would be desired in loop calibration activites.

hope this helps

Answer 2

4-20ma is resistant to electrical noise as compared to either 0-5V or 0-10V standards. Also 4 to 20 has what is known as a live zero. 4ma equals the zero reference, this allows you to detect a broken wire if the current drops to zero. (A short is detected by exceeding 22ma)
Also since zero is 4ma it is possible to have the loop provide power to the signal transmitters.

EDIT:
If you had a 0 to 20ma control loop you would not be able to self power the transmitters because once you reached zero the current flow would be zero and the device would shutdown....

Answer 3

Never really thought about that one. I'll have to guess it has to do with the output semiconductors used. Most of the controllers I use at work are designed with that 4-20ma output range. Have a great day! ;-)=