Help with 4-20mA Analog input module

First I would like to thank you for taking the time to read.

I am using an AB CompactLogix L43-C with an IF4/A Analog input module and a Dwyer differential pressure transmitter. We are using this system to monitor velocity of air through a duct as part of our spark detection/fire suppression system. I have the IF4 set for 4-20mA input with a 50Hz noise filter and the percentage based output setting. My Dwyer unit is set to send a 20mA signal at 19,279 CFM, and a 4mA signal at 0 CFM. My question is this:

Will the analog input module (with these settings) automatically assign the value 0(%) to a 4mA signal, or do I need to program an algorithm to make a 4mA signal from the Dwyer unit produce a value of 0%?

What output will the analog input module send to the PLC if the mA signal going into it is 0?


Thank you all so much,

Monte

If the input is set for 4-20mA, then your 4mA will be 0.

No it won't do this automatically.

You'll have to configure the IF4 for 4-20mA signal, then you'll have to write logic to "scale" the raw data value from the 4-20mA signal into the engineering units you wish to use.

Search for "analog scaling", it's really just y=mx+b. You'll need the manual for the IF4 to see what the raw data is at 4mA and at 20mA so you can to the math.

If your data format is set for Percent Range, yes it absolutely will.

http://literature.rockwellautomation.com/idc/groups/literature/documents/um/1769-um002_-en-p.pdf
Page 3-11 has all you need. I hope it helps.

So then let me see if I'm understanding what people are generally saying:

That Paully's5.0 doesn't understand my question, as I detailed that my analog module IS set for 4-20 mA, so when my module gets a 4mA signal the percent reading for the associated data tag will be 0, automatically.

Am I correct in this understanding? This appears to be the case judging by the numbers I am seeing, but the dag values fluctuate at a different rate than the LCD screen on my Dwyer unit so it is difficult to be sure.

Thanks to you all for your speedy reply!

Monte

SolarNinja said:

So then let me see if I'm understanding what people are generally saying:

That Paully's5.0 doesn't understand my question, as I detailed that my analog module IS set for 4-20 mA, so when my module gets a 4mA signal the percent reading for the associated data tag will be 0, automatically.

Am I correct in this understanding? This appears to be the case judging by the numbers I am seeing, but the dag values fluctuate at a different rate than the LCD screen on my Dwyer unit so it is difficult to be sure.

Thanks to you all for your speedy reply!

Monte

Click to expand...

Yes, you are correct. I use this all the time (for example, level probes).

Whatever you end up doing, don't forget to set an alarm when if the input goes low, like below 3.0mA or so

How will I know if my 4-20mA signal falls to 3.99mA or below? Won't the Analog output data show 0?

I will have an alarm sound WELL before it gets to that point, if it falls below 8mA with the "run" bit on that would indicate really bad air flow.

SolarNinja said:

How will I know if my 4-20mA signal falls to 3.99mA or below?

Click to expand...

Did you download the manual provided by @AustralIan? On page 3-3 are described the Under & OverRange bits.

I was just shooting from the hip. Yes I have all the documentation. I will review it in detail.

Thanks!