4 - 20 mA transducer question

[Bruce Durdle]

A couple of thoughts on 249 vs 250 ohms ...

The difference between them is 1 in 250, or 0.4 %. Before this error
becomes significant, you have to make sure the accuracy of the reading
is better than this - not just the accuracy of the transmitter or other
transducer, but also the effects of installation errors, the accuracy of
primary elements, and finally the accuracy of the receiving electronics
and the A/D conversion process. So if you're going to differentiate, the
resistors had better be at least 0.1% tolerance. And if you are using 2
resistors to get the "right" value, you need to be careful with the
tolerance as well. And don't forget about temperature ...


It's a bit like the early days of hi-fi - there was a large group of
people who would agonise for hours to get the frequency response of the
amplifier up from 28 kHz to 28.5 kHz, then connect it to a crappy pickup
and speakers with top frequency roll-off of 15 kHz.

4-20 mA / 1-5 V gives a good compromise between a low-level system with
bad noise problems, and a high-level system requiring a lot of lethal
voltages. Other combinations have been used - one major manufacturer
went 4-20 mA / 0.25-1.25V for a while, with a 62.5 ohm resistor, but
that system went the way of the Edsel.



And on the philosophical front ...
I think it was Newton who said "We are pygmies standing on the
shoulders of giants." If we don't make use of the giants, we are only
pygmies standing on the ground in the long grass - except of course for
the exceptional ones among us who have their own giant status.

Bruce.

DJ Bartlett wrote:

I believe that apologies are in order.

I'm a little embarassed that I didn't bother to watch the group a bit to see
what the general temperature of the discussions were like and get a gauge on
major players' personalities. I jumped right in feet first without checking
back messages in the thread (the server I am currently using keeps an
extremely limited backlog of messages).

I also stand corrected on the ISA bit. I appear to be guilty of pretty much
the same thing I was berating you for: using emotional pressure rather than
information to convince the audience.

Considering that you had already given a detailed answer and then some, and
that you obviously DO know what you are talking about (once I slowed down
enough to actually follow my assumptions up), a somewhat gruff answer
doesn't seem quite as out of place as it did at first.

However, it drives me crazy when I see something that looks like an attempt
to supress someone trying to use their own brain to solve a problem (even if
the approach is less-than-optimal), particularly if the question makes it
evident that the request is being put forth by someone who is relatively new
to a discipline.

I too, am reasonably new to the subject of applied electronics (thus missing
the ISA acronym - which goes far, in my not so humble opinion, toward
proving my point about the accessability of the information to the
uninitiated - I am NO stranger to researching appropriate standards in a
fair number of fields), though I have more than enough theoretical chops
from various areas to keep up with a whole lot more than most newbies.

I am not a young whippersnapper either. I have been opposing , for as long
as I can remember, the hard-core academic tendancy to paint pictures in
students' minds of some sort of absolute authority that can never say a
wrong thing, should never be questioned, and whose edicts apply to all
situations and always will.

This lesson is what caused so many of the great innovators to live their
entire lives practically peniless (or burned, hanged, jailed, eviscerated,
or simply overlooked long enough to ensure their ommission from the
textbooks of the next 200 years. At least one of them was made to
voluntarily drink hemlock (if the story is true), and much new thought had
been destroyed or perverted by religeous edict as heresy. I should probably
be thankful that we mostly just snipe and belittle each other nowadays for
not having always known "the truth," no matter that this truth is kept at
the bottom of a locked filing cabinet in a disused lavatory with a sign on
the door saying "beware of the leopard, and that this truth is really just
some people getting together and agreeing to try to convince everyone else
in the world to use "our new PLC's (they're great!)" instead of "brand X
collection of YUCKIE!!! discretes" even though NEITHER approach really
improves the nature of life on earth in any measurable way.

I defend error and experiment because it is hope for new understanding...
not of an authorized panel of approved high holy experts in the field
bringing understanding to the world in the form of universal compliance
(uhhh... acceptance), but of an individual who is expressing one of the most
innocent and benign urges that humans seem to have, ever... CURIOSITY (yes,
I know that they blow themselves and each other up a lot with the
by-products, but not as often or with as much efficiency or intent, for that
matter as the authorized experts seem to).

This should honestly be applauded and encouraged (the curiosity, not the
blowing up). It is the intent to innovate which is the innovation here, in
spite of the present lack of well-known understanding (easily corrected) and
the re-invent-the-wheel nature of the intent. Think back... didn't you also
learn by performing many of the classic experiments? All innovation MUST be
preceded by the intent to innovate, and the temerity to think that one can
through the use of one's own faculties arrive at something as akin to the
truth as an industry expert.

You started with the same level of knowledge. We must all progress from
zero-understanding to whatever level of expertise we wind up achieving. We
shouldn't try to make others miserable for taking the steps. Wouldn't you
have agreed when you were in the same place?

Actually, I am definitely going to give you the benefit of the doubt in
light of your totally unhostile (and far less defensive than I was
expecting) response. I expect that it is your intent to help people come to
deeper understanding of this subject while gaining some of your own. I'll
bet you didn't expect the philosophy homework as assigned by a student?

One more time, I apologize for the roughness of my response, but I am not
embarassed by my underlying opinion and I'm going to let that stand without
retraction.

Thanks hugely for the informative bits, and since I'm as able as anyone else
not involved in the chain of events leading up to 249Ohm resisters being
used, and am curious about the real answer, if it's ever forthcoming, I have
a tentative guess at a possible cause...

It seems highly likely that the not-quite perfectly linear behavior of
semiconductors to actual real-world application might cause one to arrive at
the use of a 249Ohm resister and an trimmer for fine-adjustment a slightly
better choice along some lines of thinking than the assumption of a
"theoretically ideal" 250Ohms and a trimmer for fine-adjustment, although, I
can think of about 10 reasons off the top of my head why it doesn't really
matter, either way (all guesses).

The 250 is more popular, surely (my guess), due to the nice round, neat
package of different base-10 multiples (another guess). But some people are
fiercely adherant to certain approaches, even if met with a good argument to
consider alternatives (an opinion based on plenty of experience).

Alternatively: maybe some production foul-up resulted in a warehouse full
the the eyeballs with millions of precision 249Ohm resisters which some
regional marketing vice president was ordered to unload at minimum loss "or
else" and he wheeled and dealed until enough purchasing people were
convinced that it didn't matter at the price being quoted that a certain
batch of units 10 years down the road came out of the woodwork in ones and
twos whenever someone or other got to scraping the bottom of the barrel for
250s and found only the discount precision 249s...

feasable?

maybe the 249s were intended to adjust for the ~1.1CR time constant with a
nice round result... I'll check my math Monday.
I seem to have a mental block re: simple multiplication/division today.

I really don't want to shoot option 2 out of the water, owing to some really
entertaining mental imagery it calls up.

The suggestions illustrate 3 examples of how unspoken standards can be
understandably, intentionally avoided in violation of common assumptions.

1). Narrow scope of possible approaches - often indirectly/directly caused
by less-than-applicable-but-inflexible regulations
2). Murphy's Law - a little social engineering can beat a similar amount of
electrical engineering dollar for dollar (at least long enough for everyone
to get out of the area before anyone likely to want to kill notices the
problem).
3). Man likes nice neat, easy-to-remember black/white universal truths that
can be tallied in multiples of 10 (?? this is the part that cracks ME up)and
grudgingly with lots of complaining, pi, e and whatever specific
relationship between values that is represented by "electronic pi" without
undue effort. Nature, stubbornly refuses to cooperate in any way that
actually makes anything easier to intuitively understand, possible, or even
capable of being remembered or calculated with 100% accuracy, no matter how
much effort man pours into insisting that anything less significant than the
1st decimal is too much of a hassle to really matter, even in cases of being
raised to a very large irrational exponent. Funny thing is... he's right...
and proves it regularly, but he LOVES to play devil's advocate with his
neighbors...

Rant off,

I am pleased and gratified by everyone's responses (even the one that
defended walter by verbally attacking me, and the one poking fun at my not
using caps - I am frankly more distressed by not catching my numerous typos
before sending, I drop caps when I prioritize speed and point being made
over grammatical redundancies like capitalization (I use most appropriate
punctuation unless intentionally diverging from MHA recommendation to most
efficiently make an implied statement or to lighten the mood of my writing
slightly). I rarely use spelling/grammer checkers unless I'm in definite
doubt (hee hee - JUMBO shrimp), but I am very good at recalling those sorts
of things with a little direct attention and I don't want to ever get too
dependant on the limited ability of a machine to simultaneously pass
intentional dramatic nuance and fix 100% of grammatical and spelling errors
with enough discernment to satisfy me).

You guys actually all seem pretty okay, in my book.

Again, Walter, thank you for posting the summary of the standards document
in question. my next post will be direct responses to your other e-mail's
questions (some of which are included in this one).

Lastly, consider yourselves lucky. I removed a good deal of my original line
of thought re: philosophy of standards development and human individuality
and imagination as deserving further development, but not as a direct
negative commentary on anyone's behavior or lines of thinking. I'd be happy
to share it once/if ever I clean it up and finish the line of thought, if
anyone's at all interested in that sort of thing.

DJ