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Analog Scaling Issues 2

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TEXASDNA

TEXASDNA

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Sep 3, 2010
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I have this situation:

I have a seametrics ex215b flow meter with a seametrics ft420 totalizer with 4-20ma output.

I have interfaced this with a red lion signal conditioner to change the 4-20ma to 0-20ma.

I then have it wired into a Netbiter EC220 analog input which I set the dip switches for 0-20ma.

Now for the hard part, what vaules do I set for scaling?

where 0.00ma = 0 gallons per minute

and 20.00ma = 1400 gallons per minute

my question is what would my scaling and offset be?

min. flow in this 14" pipe is 134gpm, this is the rate at which the influent water begins to be measured... and the maximum is 1,400 gpm.

but I just cant seem to figure it out...here are some pictures of what I have going on.
 
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Seems to me that the 4-20 to 0-20 mA took care of the offset. And the scaling is exactly what you say - 1400 gpm at 20 mA. That would mean 70 gpm/mA or, if you want it the other way round, 0,0143 mA/gpm.

I don't really understand your question - or why you ask. Or did I miss something?

Gunnar Englund
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
 
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Since you have a totalizer that outputs 4-20 mA and a PLC card that takes 4-20 mA (presumeably because you say you had to change dip switches to change it to 0-20mA) then why don't you ditch the conditioner and wire it directly?

Or if you need the conditioner and the PLC input must be 0-20mA then the scaling is -20%*1.25.

 
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4mA ....should be scaled to 0
20mA.....should be scaled to full scale
signals below 4mA have electrical noise and is unreliable....this is an industry standard hence 4 to 20 mA.....?????
 
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Well carl, not quite. The 4 mA is to supply electronics in the transducer.

We used 0 - 20 mA for many years without (m)any noise issues. Using the 'bottom' 4 mA to supply the electronics makes wiring easier and also makes wire break detection possible.

Gunnar Englund
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
 
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well there is an interesting point of view....4mA to supply the electronics...and never any noise problems at 0 mA...I suggest you do a google on the merits of the 4 to 20 mA signal
not that i'm suggesting that you are wrong...you might well be measuring something that is course in nature....but I would not pass any equipment off that used 0-20mA as a transducer signal for some type of measurement device....maybe you are talking about a signal at component level of the electronics ....but please make could you make some reference to a source other than "this is the way we do it"....your never to old to learn
 
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Young and knowitall?

I'm waiting for some more regulars to give their view...

Gunnar Englund
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
 
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It is not about any non-linearity in a transistor. All 4-20 and 0-20 mA devices use heavy feedback and opamps so the 0-20 mA is just as problem-free as the 4-20 mA.

The only thing is that you seldom see any 0-20 mA devices today because industry has standardized on 4-20 mA because of the benefits I mentioned before.

There is also the Hart protocol that doesn't work with 0-20 mA because it loses the 'carrier' when signal gets close to zero mA.

What you read about 4-20 mA and immunity to noise as compared to 0-10 V is valid. But it is just as valid for 0-20 mA. No difference in that respect between 0-20 and 4-20 mA.

We are seeing a modern myth being formed by an audience without perspective and technical/historical knowledge and that thinks that googling is the ultimate source of knowledge. It takes a bit more than googling - it also takes understanding of the underlying principles.

BTW, we honor correct grammar and spelling in these fora. And we use capital letters when needed.

Gunnar Englund
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
 
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The international standard for industrial signal compatibility is ANSI/ISA-50.00.01-1975 (R2002), titled:
"Compatibility of Analog Signals for Electronic Industrial Process Instruments", known colloquially as "ISA 50".

642qt5.jpg


relevant portion:
3.2 Transmitters
The standard output signal shall be of a constant current nature having a range of 4 mA to 20mA, dc (a 16 mA span).
3.3 Receivers
3.3.1 The standard current input signal shall be 4 mA to 20 mA, dc (a 16 mA span).

The committee's reasons, cited back in 1975, for 4-20ma were those cited by Skogsgurra. 4-20mA was already in widespread use throughout the process industries when the standard was issued.

There is some 0-20mA use in Europe, but there must, literally, be 10'a of millions of 4-20mA loop circuits in use throughout the world.
 
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Yes it’s nice to see a reference.....I have worked on equipment that is particularly noisy and signals that sit near the 0 mark I find very noisy and unreliable …especially when it comes to inverters and cable runs of several meters…also know as ground noise… I would suggest people give the ANSI reference a Google as I’m sure it would clarify some of the points.

As for the comment “BTW, we honor correct grammar and spelling in these fora. And we use capital letters when needed.”
I’m sure that your grammar is no reflection of your technical ability…..
 
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It may reflect the fact that I write from Sweden and that I live in and was born in Sweden. My French and German is just as bad as my English. And I am afraid my Swedish is even worse.

Sorry if my bad English has caused you any inconvenience.

Gunnar Englund
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
 
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There may be a few other skills, too...

But, this is not about who is more skilled. It is about, as you say, helping each other. And I think that I helped you out of a misunderstanding - or didn't I?

Gunnar Englund
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
 
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Yes …all I was trying to say and possibly not very clearly is that the 4mA should be reflected as zero in the software ….so no flow would be anything less than 4mA….
but yes you have given me some food for though...thanks
 
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OK. I forgot to say welcome to Eng-Tips. I noticed that you are quite new in here. I think that you should read the site rules. The boss tries to run a tight ship. And that may show sometimes. Hope you were not scared off.

Gunnar Englund
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
 
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OK. That is google or Babelfish?

'Se' is see, that's right.

'dig' is a form of you that means you are spoken to or told to do something. The neutral form is 'du'.

'omkring' means around you in a panoramic sense.

So, the short sentence means 'look around you' and that is why my answer may have confused you.

The way we say 'see you around' is 'Vi ses' which means 'We will see each other (sometime)'

Gunnar Englund
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
 
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