3-15 PSI (Was: 4-20 mA standard?)

  • Thread starter Michael R. Batchelor
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Thread Starter

Michael R. Batchelor

> Decades ago there was much debate about this.
> An elevated zero is required to power the instrument at a zero signal output.
> 4 to 20mA won with 10 to 50mA a close second.
> 3-15psi??..... sorry.... I'm not that old.
> >
> > Does anybody know why must it be 4-20 mA. Is there any technical
> > background behind it? Why must it be 4-20..not other ranges? Same goes
> > why is it 3-15psi. Is there a standard for this?
> > Appreciate some answers.Thanks.

So, I guess this tells how much grey hair I have. 3-15 PSI gives two things. A "live" zero so you can see if the air is gone when the meters drop below the bottom of the range, and, just as
current is required to make the electronics work, a little bit of air bleed is required to make the pneumatics work.

6-30 PSI is a relatively uncommon air standard much like 10-50ma. It's around, but not seen very often.

The air bleed on the old pneumatic devices come from a thingy called the "flapper nozzle" valve. In all the devices there is small nozzle with a tiny hole that's always bleeding air. At the
business end of all those wonderful mechanical "link lever" arms is a little flap just in front of the nozzle that moves a few
thousands of an inch and changes the back pressure inside the tube where the air is bleeding out. Up close the back pressure is
high (15 PSI) and farther away the back pressure is low (3 PSI). A T connection off the nozzle tube transmits the back pressure to a bellows or bordon tube, depending on the instrument design,
and the mechanics to move either the pointer (the old name for the indicator) or the control mechanism is moved by the flexing of the bellows or bordon tube. Needless to say, what actually
moves depends on whether you have a transmitter, a controller, a receiver gage, a square root extractor, or whatnot.

If you've got clean dry air, a pretty stable process so you don't have to keep changing things, and you can keep the doors of the
case closed so the operators aren't turning the knobs all the time, then they'll run for years and years trouble free. Lose any one of these and you need a maintenance guy to look at it a lot.

However, just like with wrist watches, the fact that they were superbly crafted devices doesn't mean they make any economic sense to use today. I have a wonderful old wrist watch that I get out and wear to formal dressy occasions, but that's not what I wear inside a factory. Besides, a $20 electronic watch keeps better time.

Michael R. Batchelor - Industrial Informatics & Instrumentation, Inc.