Back in the '40s and '50s before electronics were invented they used a range of 3-15 psi - 3 rather than 0 because it would take a very long time for a system to vent down to 0%. The first electronic transmitters were in fact electromechanical and used the force balance effect of a current in a magnetic field instead of the force developed by air pressure. Apart from the feedback sensing mechanism the transmitters were basically the same construction.

Then someone noticed that by the way we can use 0 mA as a fault indication - oh and 10 mA is enough to power quite a bit of stuff so we don't need a separate power circuit - And lo! The 4-20 mA loop powered circuit was born.

Bruce

 

In response to Doug,
I beg to differ for a zero based system (2 wire) where would the strain gauge or other measuring device get it's excitation voltage from?
The 4 mA provides enough power for the electronics to operate. A typical transmitter drops 11 Volts 11 x 0.004 = 44 mW to operate the electronics, display etc.

True it also provides open circuit detection but that is only secondary, many of the early PLC and DCS systems would not recognize anything less than 4mA as a fault. e.g. a Modicon 584 read 4 mA and less as 0 counts, 20 mA and above as 4095 counts.

In from the field!
Roy

 

>In the beginning, there was pneumatics.<

....Then the age of the DCS came about.
..But the standard input by DCS systems became 1 to 5 volts.
Not really - 4-20mA, 250 Ohms and 1-5V were there long before the arrival of the DCS.

 

In the beginning was pneumatics...that's true.

Then there was the electronic standalone loop controller...

...THEN the age of DCS.

And the 4-20 mADC standard was designed to:

1. produce a "fault detection" at 0 mA

2. be isolated from ground

3. be able to drive a minimum of 1000 ohms (so you could put four devices, at least, on the loop: sensor, controller, recorder and final control element).

4. provide a widely rangeable signal equivalent to or better in resolution than pneumatics.

Walt Boyes
Editor in Chief
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Walt, I can't agree with your point No 3, 1000 Ohms would need a lot more than 24 VDC given that a typical transmitter drops over 11 volts. Besides the final control element is in a different loop (current loop) controller to final control element. I also maintain that the 4 mA main purpose is to power the transmitter, otherwise why not 1 mA or less?

Roy

 

Unfortunately, that's what the standard says. I only paraphrased it.

Walt

Walt Boyes
Editor in Chief
Control and Controlglobal.com
www.controlglobal.com
Mailto:[email protected]
Read my blog SoundOFF!! At www.controlglobal.com/soundoff

 

Roy,

4/20mA loops aren't only to do with Transmitters, you have I/Ps and Indicators as well which typically need a lot less than a transmitter.

1000 ohms just sounds like a nice round number to me though.

Steve

 

<p>Which version of the standard are you referring to, Mr. Boyes, that give 1000 ohms?

<p>From my copy of ANSI/ISA-50.00.01-1975 (R2002); Formerly ANSI/ISA-50.1-1975 (R1992)

<p>3.3.3 The standard current output signal shall be of a constant current nature having a range of 4 mA to 20 mA, DC, and shall deliver rated current into any external load between zero and 600 ohms, minimum.

<p>Table 1 states the load resistances at the minimum supply voltages for 3 classes, L, H, U:
Table 1 (I fear the table will not reproduce on this forum, they never do.)
<pre>
Class L Load resistance, 300 ohms, 23 V min
Class H Load resistance, 800 ohms, 32.7V min
Class U Load resistance, 300 to 800, 23 to 32.7V min
</pre>
<p>Dan

 

All companies are using 4-20 mA as standard, because suppose if we are using 0-16 ma means, for 2.5 psi it can't show a negative value for this offset, only we are using 4-20 ma as standard.

 

Nice to see so many people arguing a good point. I didn't notice anyone say why 3-15 psig was used for pneumatic instruments?

The reason was that the backpressure produced by the flapper, (often 'baffle' in the U.S.) nozzle arrangement was most linear over that range.

Not much use to this discussion I suppose, but as time passes these bits of 'history' don't tend to get passed on to the younger generation.

 

Thanks Mr. Britman for keeping the history alive.

Regards,
C Potdar