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Can someone explain the whole 4 to 20mA thing to me? I understand that it's 4mA at zero % of your process, so you can tell if you have an open sensor or if you are reading your minimum reading, but why is it a 16 mA span? Been in the industry for a while but I never knew why it's 4 to 20.

There is a long tradition based on characteristics of the original pneumatic
transmitters of having a span which is 4 x the zero suppression.


On October 16, 2007, Rafa wrote:
> Can someone explain the whole 4 to 20mA thing to me? I understand that it's
4mA at zero % of your process, so you can tell if you have an open sensor or
if you are reading your minimum reading, but why is it a 16 mA span? Been in
the industry for a while but I never knew why it's 4 to 20. <

( Complete thread: http://www.control.com/thread/1026240195 )

Dear,

Usually we use 0 to 20 MA or 4 to 20 MA for measurements & control in Process control. If we use 4-20 mA following are the advantages:

1. The device will ready from 4 mA onwards and possibility of device historisis etc. will be reduced.

2. It is the standard for ensuring Intrinsic Safety in plant area. This 20 mA signal is not enough to trigger a spark and therefore we can limit fire hazards in plant area.

You just search with Google, you will get more details.

Regards
BCG, Amtech Automation, Blr.

As there is a leakage current so 0 to 3 MA, so we consider as 4-20MA. But in some cases we will find some transmitters 0-20MA which are more costly than 4-20Ma.

See this thread from awhile back...

http://www.control.com/thread/1026235722#1026235739

Issac

The 4-20 mA is typically sent through a 250 ohm load. The logic being read is therefore 1 - 5 volts (0 volt on loss of signal) at the control system. The old 10-50 mA standard was sent through a 100 ohm terminating resistor also resulting in 1 - 5 volts at the controller. Why does the control system prefer a 5 volt max? - perhaps it has something to do with transistor logic operating at 0 and 5 volts - I'm not sure.

Basically, milliamps are transmitted in common wire, so it can be measured. 0 miliamps is the absence of current, so instead of a null signal it means an error. So 0 mamps will not be 0 eu.

Somewhere you might see eu, with is engineering unit. Anything might be codified as between 4 and 20 milliamps, and generally translated to a linear scale, i.e. a tank level with empty water will be 4 mamps, and 20 would be full... So if you read in the wire about 12 mamps, depending on the device you might guess it's 50% full. Here my eu are percent, but all the things can tell with liters now. 0 mamps = 0 liters, 20 mamps = 5000 liters, so 12 mamps = 2500 liters, got the idea?
Now this is for analog values.

Hope this helps,

We have discussed this here before. The best theory for the 20ma value seems to be that 20ma used to be a teletypewriter signal level. Teletypewriters were at one time used extensively and electronic (and other) components intended for them were probably readily available. I have seen contemporary references indicating that teletypewriter technology was used in remote signalling of instrumentation readings.

The 4ma value probably comes from being a nice round 20% of full scale. It would likely have been the minimum current that someone felt comfortable using, combined with components being readily available. It is also possible though that this is another value inherited from teletypwriter systems.

A lot of designs orginate this way. You pick some value because it works and you can get parts for it easily. A good example of this that I can think of is that we used to use 25Hz electric power where I am (Ontario, Canada). This number came about more or less by accident. The first large power plant at Niagara Falls was originally planned to produce compressed air (which at one time was commonly produced by central plants and distributed by pipeline). After the turbines were ordered but before the contract for the compressors was let, the owners decided to build it as an electric generating plant. The turbine speed was already fixed, so the choice of frequencies for the generator was limited to however many poles were used. Of the possible choices, 25Hz was picked as being the highest frequency that they felt comfortable synchronising manually. Subsequent plants were built to be compatible with this.

Michael,

I used to live in Port Colburne at the time of 25 Hz power (lights flashed like a strobe).

The use of 4 mA also has a history of supplying the quiescent current (4 mA) to run the instrument using two wires and produce the analog signal (0 to 16 plus the 4 quiescent current mA) using the same two wires (what a novel idea using two wires instead of four). Prior to that I imagine Honeywell and Foxboro required a larger quiescent current and came up with the 10 to 50 mA circuit.

Dennis

Foxboro at least put out a 10-50 mA transmitter built around their standard mechanics as for a pneumatic transmitter of the time, but with the feedback bellows replaced with a magnetic assembly. I guess 50 mA was needed to develop the equivalent force...

Bruce

For the historians out there, at one time, the ISA S50.1 standard that defined the 4-20mA standard defined 10-50mA as an alternative. Later, that alternative was dropped.

The 10-50mA alternative was there because the committee could not reach consensus without it. Foxboro, then the largest field instrument supplier, could not deliver 4-20mA instruments because their technology was based on magnetic
amplifiers, which required the base 10mA to work. Later, they developed transistor circuits with sufficient accuracy and stability to replace the
expensive mag-amps, and would work on 4-20mA.

It took SP50 more than 11 years to arrive at the compromise of the 10-50mA alternative.

Dick Caro
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Wow, this brings back some memories. I can't even remember the last time I saw a mag-amp. I'm sure it will come to me in a few days, but honestly I think the last one I saw was in the military.

Now I'll have visions of saturated cores in my sleep tonight.

Michael
--

Michael Batchelor
www.IndustrialInformatics.com

Industrial Informatics, Inc.
3281 Associate Dr.
N. Charleston, SC 29418

I've been working with a large UV ink polymerization system that uses saturable reactors to control lamp current. We had a
crisis where the DC current control was toast. I hacked together a quick variable DC supply that got them up again. :^) I have "owned" the thing ever since :^( Nobody else there had any idea how that worked. Kinda early "Solid State".

Regards
cww

This may lost to history. I often wonder about these kinds of things. My guess would be that the electronics of the time made that range a reasonable one, but I don't know that for a fact.

Turns out this question has been asked on these forums before:
http://www.control.com/thread/1026148494
Best answer in there is from Walt Boyes:
"The basic reason is that it provides enough power to run many instruments, yet not enough power to make intrinsic safety impossible, and allows a failure alarm at 0 mA; while easily converting to 1-5 VDC using a simple dropping resistor."

He also says "check the archives," but that's easier said than done. The first guy to respond to your post said to "check Google," but that didn't turn up much for me. As an example, here's an all-to-brief mention of the 4-20mA history:
http://www.bapihvac.com/CatalogPDFs/I_App_Notes/Understanding_Curr ent_Loops.pdf

Are there any 60+ year old engineers out there who remember?

As a side note, consider the goofy Asynchronous Transfer Mode (ATM) network standard. ATM uses packets that are 53 bytes long. Why? Some of the companies working on the standard wanted 32, some wante 64. The lawyers decided on 48 without grasping the whole "power of 2" concept and then a 5 byte header was added.

-James Ingraham
Sage Automation, Inc.

Back in the 60s before 4-20 the instruments used to be 10-50mA which gives 1-5 volts across a 100 Ohm resistor.

4-20 over 250 Ohms also gives 1-5 volts.
The 10 mA or current 4 mA provides the power for the transmitter to operate.

Hope this helps
Roy

roy_matson@yahoo.ca

Sure thing.

In the beginning, there was pneumatics. These air devices were designed around 3-15 psi of pressure. Diagphrams on values used this range to stroke the stems on the valves.

Then came current to pressure transducers. A certain voltage would result in the air pressure output to drive these valves.

Then the age of the DCS came about. Electronic devices of voltage to current converters and current to voltage converters operated on a range of 10 volts. But the standard input by DCS systems became 1 to 5 volts. Therefore if you take a current input and wish to deliver this voltage range to the input of the DCD, you need to drop the wires across a 250 ohm resistor.

I think Foxborough had a system that was 1-to volts but it is the exception.

bob

Why is it 4-20 mA?

Because the ANSI/ISA 50.1 standard issued in 1972 calls for these values. For a while, before 1972, Foxboro held firm on the 10-50 mA option of the earlier edition of 50.1, because their magnetic amplifiers needed a minimum of 10 mA for power. This reveals why the standard calls for instrument zero to be a 4 mA - that is the minimum power to run the instrument. 20 mA is NOT just 16 mA more than 4 mA, it is a ratio of 5:1 times the minimum current. The 5:1 ratio is not scientific, it is tradition: 3-15 psi air, 1-5 volts, etc. Also, 20 mA happens to be the current necessary to operate Bell 202 modems then commonly used in early current loop communications, and lots of parts and power supplies were available for such circuits.

All this is now ancient folklore with the age of digital communications. However, remembering the past is good to help us not repeat some of those mistakes.

Dick Caro
===========================================
Richard H. Caro, Certified Automation Professional, CEO, CMC Associates,
2 Beth Circle, Acton, MA 01720
Tel: +1.978.635.9449 Mobile: +.978.764.4728
Fax: +1.978.246.1270
E-mail: RCaro@CMC.us
Web: http://www.CMC.us
===========================================