Why 4~20mA?

This question (or at least the 4mA part of it) was answered a couple of weeks back, search for it.
Del

What value would you like to use?

4-20 mA signal is used in instrumentation since it is a current signal not a voltage signal. Voltages of 0-10 v or 24 V dc drop off for the longer distances so we prefer the 4-20 mA signal.

Another advantage is that whenever a field wiring is broken down 24 V dc will still be there but 4-20 mA signal will be vanished because of breaking.

i understand the reson behind using current loop and also reason for using 4mA but why high signal is 20mA?? WHY NOT ANY OTHER VALUE?

Just off the top of my head, 20-4=16, which is handy for analog to digital conversion. No idea if that's the original purpose. Could simply be that the original circuitry designed for the function happened to have those values. Lots of things become "standards" for similar reasons.

There are all sorts of possible reasons, it might be more instructive if you looked at some proposed alternative values and worked out the voltages and resistance values required to see if they are sensible and feasible.
Maybe the original use determined the value, like most things it's pretty arbitary, but if the first use was for some specific thermistor or other sensor and the voltage source was maybe a 12v battery then that might give the answer.
Del

Hi, All

in our plant we still use 10mA to 50mA, and few signal use 4mA to 20mA. subsequently will upgrade all the field instrument into 4mA to 20mA.

Regard's

wew! this thread is somehow tracing the ancestor of 4-20mA standard looop current.

i came across this topic since im also searching why 4-20mA is used in the industry as standard signal for instrumentation.

http://findarticles.com/p/articles/mi_qa3739/is_199909/ai_n8864411/