Analog Problem

[Unregistered]

Wow , you guys like going round in circles over a small problem !!
Could have all been solved in about 10 minutes !
Assuming the meter reading is OK at 0.004 mA , then we can assume that this is zero - you could stick the test probes in your ear and get the same or more .
Unlikely the device would pass more if reversed in cct
Disconnect the thing completely would show that the input has broken wire or funny scaling .
Simulating the input would prove the PLC side of things , you do have a current loop simulator don't you ?
A couple of PP3's and a meter and you could prove the device . There , 10 minutes - not necessary to take the cooker apart , just considered fault finding

 

[Craig Spinharney]

"That low of a reading I would pretty much ignore, technically as was stated any reading below 4ma will/should give a 0 reading, and .004 is pretty much 0. "


Not necessarily. If properly connected, a 0 ma. signal will register a value of -25% of full scale span on many instruments including some PLC's analog cards. In this case a -50 psi could be the result in thise systems.

 

[rsdoran]

"That low of a reading I would pretty much ignore, technically as was stated any reading below 4ma will/should give a 0 reading, and .004 is pretty much 0. "


Not necessarily. If properly connected, a 0 ma. signal will register a value of -25% of full scale span on many instruments including some PLC's analog cards. In this case a -50 psi could be the result in thise systems.

I dont think so.

 

[Craig Spinharney]

rsdoran,

"I dont think so."

Sorry, but 5 years in the instrument business and 3 yrs as I & C tech says otherwise.

Think about it. I have a 4-20 ma. loop whereby the signal represents pH readings from 0 to 14. I feed this into a Red Lion, Newport, Cincinnati Electrosystems, etc. indicator.
With a 4 ma signal I get a pH reading of 0...12ma. gives a reading of 7...and 20ma. results in a pH of 14.
When loop power is either removed or drops to 0 ma. what will the indicator read? Unless it blinks wildy to warn of an undervoltage condition, it had better not read 0 as this suggests that the current is 4ma. So, instead it reads out -3.5 (-25% of span less the offset).
This is the whole concept of a "live" zero 4-20ma instead of 0-10ma. or 0-20ma....or 1-5 volts instead of 0-4 volts, or 0-5 volts.
I've calibrated far too many of these to think it's just my 'magination...runnin' away with me.

Craig

 

[rsdoran]

Sorry, but 5 years in the instrument business and 3 yrs as I & C tech says otherwise.

Say what??? How about I call that and add 20 years?

First and foremost any device that is designed to output 4-20ma does just that and nothing else, there will be no zero ma output from the device execept in wire break or malfunction situation.
Using a 4-20ma device with a plc will use the range 4-20ma, anything outside that is WRONG...PROBLEM>>>ALARM.

You stated and I quote:

Not necessarily. If properly connected

Connections have nothing to do with it, its how you HANDLE what happens when its outside the 4-20ma range. YOU have to configure whatever device to TELL you when it is out of range...it is not automatic.
Pertaining to what has been stated the reading can be anything you WANT...ie FULL SCALE to indicate wire break, or -xx or whatever YOU decide or the system allows.
What you stated is NOT arbitrarily so.

AS I said...I dont think so.

 

[Unregistered]

Nor do I , Craig . Take the trouble to read - the card is a 4-20mA card , therefore such a small signal would result in broken wire , either zero or FSD .
If someone has used a 0-20mA card , with a 4-20mA device , then what you say is correct , otherwise no - less than 4mA is broken wire . You as such an experienced instrument tech will know why the industry drifted towards 4-20mA loops . In your application where safety is so important , I would hope you have devices that can detect broken wire .
You state that many devices would detect 0mA as -25% : only if poorly scaled - think about it or range is now -25%--> 100% , 125% , pretty terrible resolution on a 12 bit card ?

I wouldn't tend to quote time at a particular skill - I have had the pleasure or working with a couple of so called instrument techs who turned I&C , with similar experiance to you - in their case , they were totally useless - one of them thought that a PT100 read zero ohms at 100C - a superconductor , and he called himself a tech - I called him something different .
Unfortunately many people hide behind age or "experience" rather than just saying they are good at something or not - personally I would say that I am a good if slow industrial and domestic electrician , a pretty good PLC programmer on most platforms , good instrument tech , good mechanical engineer , good commissioning engineer , a pretty hopeless electronic tech (limits being ohms and aways , simple circuits and basic theory ) - Thats me , time served doesn't come into to it , though for what it is worth , I have quite a bit of experience.

 

[BoxerBrats]

well I am heading back down to southern california tomorrow to remedy this when I find problem i will let y'all know

 

[Craig Spinharney]

rsdoran
"Say what??? How about I call that and add 20 years?"

Do that and I'll include the fellow tech I worked with and he'll match you on year. He agreed with my assertion with the qualifications I posted.

"...there will be no zero ma output from the device execept in wire break or malfunction situation."
Exactly the point I was making.

"Using a 4-20ma device with a plc will use the range 4-20ma, anything outside that is WRONG...PROBLEM>>>ALARM."

Please reread and noticed I qualified my statements with respect to the equipment used. i.e., indicators, recorders, etc.
Same goes for "unregistered"...go back and reread my comments.

"YOU have to configure whatever device to TELL you when it is out of range...it is not automatic."

Actually, it is in some instruments...or better stated, you may NOT have any control over what the instrument does/indicates
below 4ma.

"What you stated is NOT arbitrarily so."
That's why I qualified it.

'AS I said...I dont think so."
Which until you reread my original comment, is an unqualified thought.

Unregistered

"otherwise no - less than 4mA is broken wire "

Not always. Transmitter failures are often not the result of anything loop power/wiring related.

"I wouldn't tend to quote time at a particular skill.."

I do. Anyone can say they know instruments...but banjos may be the extent of their knowledge.

 

[darrenj]

"That low of a reading I would pretty much ignore, technically as was stated any reading below 4ma will/should give a 0 reading, and .004 is pretty much 0. "


Not necessarily. If properly connected, a 0 ma. signal will register a value of -25% of full scale span on many instruments including some PLC's analog cards. In this case a -50 psi could be the result in thise systems.

i did go back and read your post's...the part that has you in trouble now is the P.L.C reference when you buy a plc card you spec 4-20 ma or 0 -20 ma... 0ma=fault on a 4-20ma card..no exeptions...BTW re experience please read "unregistered" post..while i don't agree with most things he says i agree with this..

 

[Craig Spinharney]

darrenj,

"the part that has you in trouble now is the P.L.C reference when you buy a plc ..."

I never disputed this. I know how the PLC analog cards work. My point was that not ALL instruments in a process loop necessarily register a 0 reading...or an error, when the loop power is either lost or something in the loop such as a transmitter causes the loop to drop below 4ma.

I stand by my response to unregistered. To his comment,
"You state that many devices would detect 0mA as -25% : only if poorly scaled ..."
Not necessarily. Again, it depends on the device in question.


Craig

 

[Tark]

BoxerBrats - Please find the answer to your problem and let us know what it is so we can put this thread to rest

 

[rsdoran]

I am curious to see what the problem is too.

Its my experience that when using any 4-20ma device, especially with a plc, it will be scaled to use those values only. Any value outside that range should trigger an alarm/error to indicate invalid or out of range input. In some cases FSD, full scale deflection is used.

In the case of a plc I would think it would be unlikely to have a negative % value displayed if out of range or a loss of signal.

I will assume that it could be done with devices like a Red Lion indicator etc but again WHY would you? These devices have alarm setups, if the device is out or range or has loss of signal why wouldnt it display Error XX or Alarm X or something definitive to indicate the condition? People may see the number but not notice a minus/negative sign.

Maybe that is how some do it but personally I would not think it an efficient way to use the options available with the device(s).

My experience with most 4-20ma devices usually NEVER use full scale...ie an air system will be 120-130psi max but scaled to 200psi, scales may use 0-5000 pounds but the avg weight applied is 2500-3000 pounds, temp could scaled to a 1000 degrees but average is 350. Using Temperature as an example this would allow you to set an OVERTEMP setpoint and alarm prior to it reaching full scale.

Maybe its me but with the options a plc/hmi or digital indicator offers now I would prefer having an alarm message displayed when an out of range situation occurs. I believe it would/could definitely expedite troubleshooting issues.

I was a little too short on my previous replies and did the one thing I totally dislike, I sounded condescending, it was not my intention.

 

[BoxerBrats]

Life is good

ok problem fixed.....

The pressure transducer went bad in the process wiped out the first 2 analog inputs of the card.

was kind of odd, removed wire from input and still said the BCD was at 4095, so I moved to second input that read at 4095. So spent hours going over my code but so i moved to the 3rd input life was great.

so all fixed and working great.
Do want to say thanks for ecveryones input

 

[Unregistered]

OK , Card stuffed as can happen , good to see the answer was here in the thread