field instruments output

History, convention, standardization, physics, the technology of the times. In ancient times (my lifetime), you had 2 choices, you either had 2 voltage/current supplies, or you offset the signal (lifted it above ground - because you CAN'T get to 0 Volts with 1 supply), because all silicon/germanium devices have a voltage drop. Two power supplies cost.

In order to be (economically) competitive (only 1 supply), it soon became a "standard". At least in the U.S., I think the European standard was different back in those days, but don't quote me (I lie a lot).

Once it reaches that stage, it becomes convention, and then a quasi-law.

Since 99.99% of the equipment on the market understands how to deal with that standard, why build an instument that won't talk the same language?

4 - 20mA has certainly been used in the UK all my working life (i.e. the last 30-odd years), and was designed into gear that was already just about obsolete when I started.

... Just been researching a bit.

There was also a 10-50mA standard at one time, but this was dropped in favour of 4-20mA after years of argument.

One suggested answer to "why 20mA?" is that it's an old standard for Teletype data loop transmission.

Simple:

Take a standard high tolerance resistor of 250 Ohm

4 mA x 250 Ohm = 1 Volt

20 mA x 250 Ohm = 5 Volt

Early times the analogue measurements were in Volt because it is easier and more accurate to measure a voltage over a high resistor. But greater risk for disturbanties due to the very low current. So we went for current signals with a resistor at the input (cfr Allen Bradley). With the electronics we have today (op-amps, foto transmitters, ...) the resistor at the input disappeared, but in some PLC systems or analogue signal systems you can still find that 1 - 5 Volt system.

what the reason for this type of Division

The simple and logical answer is that it's become the standard.

How did our forefathers come up with the inch? Perhaps several kings, emperors, tribal or clan leaders, came up with separate decrees that the length from the tip of their thumbs to the first joint is an inch.

Of course, everyone's thumb is different. However, if King#1 defeats King#2 in battle, his thumb becomes the rule (just a little pun there ). Soon other kings bow down to a superior king and adopt his thumb as well. What happens is that the superior king's inch becomes the accepted standard. There were some cases, however, where people just adopt someone else's standard rather than make their own.

So why 4-20mA and not 5-25mA?

I'm not sure why. Part of me thinks that it's because even numbers are easier to work with than odd numbers. So why not 20-100mA? Part of me thinks that it's because it's cheaper to operate something at low current than high current.

Dear Sir,

I have been in Instrumentation line, by chance, educated in the discipline of telecommunication back in 1965, started work in a factory joint venture with Japanese firm, CHIYODA and Singapore EDB and Four Seas group company.

My first job, that is with Electrical and Instruments. They have FUJI DENKI instruments, which was with Siemens of Germany and their standard signal was 10 to 50 mA. Then, there were some out of standard signals like the Boiler Fuel Flow transmitter and the Drum level control loop. It was 11mA to 50mA.

Subsequently, organizations such as ISA and IEE gathered together and agreed for a world standard of 4 to 20 mA current output. Why not 0 mA, you may ask, but is it possible to get 0 mA for a 2-wire system?

Other digital communication system started such as CAN bus, and HART protocol later part of the decades.

tijit

Starting above 0mA is useful for a number of reasons. I admit I don't know them all, but here's a couple of thoughts:

0mA indicates an open circuit (fault condition)

When converted to 1-5V with the 250R resistor, the 1V is above a diode "drop" (Vf)

with a very noisy signal, if 0mA was included in the range then noise could take the signal negative - being lost in a single-supply system, and possibly rectified and producing a false positive signal in a dual supply.

PLEASE don't any of you clever blokes shoot me down in flames - these are just ideas on a lazy Sunday morning (while I'm putting off working on the User Manual I should be writing)!

Your thoughts are quite correct John.

One of the reasons for the 4mA lower range value is to help the people determine if the signal cable has been cut or the wires have been disconnected somehow. Another reason is to provide a safe way to fail.

If, for example, you have a valve that operates at 4-20mA (4mA=full close, 20mA=full open) and you need to have it closed when the signal is lost, once the wire is cut, the current goes down to 0mA. Now 0mA is so much less than 4mA that the valve will be forced to the closed position for sure (of course, assuming that there are no other problems with the valve).

All good points. My explanation has always been that voltage drops with line length, but current does not, and that zero is represented as some value above zero to indicate that the line is live and zero, rather than cut or disconnected.

I never thought why it has the range of 4-20 mA. I'm thinking that it may have something to do with intrinsic safety. At 24 VDC and 20 mA max., intrinsic safety is met, even with a few inductive and capacitive components.

Tad

There are some more standards in Control field; In VFDs [Inverters] around the
1ma Constant-Current-Source. 0-10V, [-10 - 0 - +10V]
A motor-Driven Multi-turn Potentiometer is used mostly.
Stepped Potential-Dividers used for different Drive-Speeds.