4-20mA under range

[Hisma 2]

Hi guys This is actually a general question and not directly associated with AB products... though, we do use Control Logix PLCs so perhaps you guys can chime in. Is there an accepted standard for what constitutes an instrument that uses 4-20mA feedback as under range? The reason I ask is because the way my current employer determines if an instrument is out of range is a bit perplexing. They scale the analog signal in the hardware, which makes sense, but they calculate everything else (high limit, low limit, over range, under range, etc) using an analog processing routine. So, when the AI is brought into the logic, it comes in as a REAL number already scaled for it's desired range. Then, it goes into an analog processing routine which then dertermines it's limits for over range, under range, etc.... The problem (in my eyes it's a problem) this causes is that we calculate an under range as 3% below it's SCALED value, not it's raw value based on 4-20mA. What this means is that you can have 2 identical RTDs, for instance, that are scaled differently. RTD 1 would be considered under range at 3.8mA, and RTD2 would be considered under range at 3.5mA. Does anyone else think this is not the correct way of handling this? Is the solution to do either all scaling and setpoints in the card, or all scaling and setpoints in the logic? Are we technically going against the standard tolerance for what deems an instrument under range? I'd love to hear some of your thoughts. I personally think we should be looking at the raw value for determining the over/under range, but perhaps I am missing something.

[Hisma 2]

hmm, after some quick research I see the card we use is the 1756-IF16, using an input range of 0ma-20ma. In the rockwell documentation, it says under range is detected for this card if it goes below 0mA. But, when you see the scaling for the card, the low signal is 4.0ma and the high signal is 20.0ma. So, it seems almost as though there would be no way to use a raw value to properly detect over range and under range, since the card is set up for 0-20ma input range. Is there some way around this? or is the only other way to use a card that is meant for an 4-20ma input range?

[DanW 41]

I can't answer for specifics of A-B analog inputs, but there is a general specification that might address what you're tackling - NAMUR. NAMUR is a German association that publishes recommended practices for instrumentation. Many field instruments (optionally) use NAMUR settings for fail-safe conditions, I say optionally, because the factory settings are typically not NAMUR, but NAMUR fail-safe levels can be selected if the option is available. Here's a graph of the NAMUR NE-43 range for 4-20mA. I suspect that some systems (PLC/DCS) allow for an input selection for NAMUR which allows detection of these fail safe levels, but doesn' t affect the scaling of the PV signal in the 4-20mA range.

[Hisma 2]

I appreciate this response, because it gives me some traction on the existence of a standard across all instruments using 4-20mA range. In my eyes, if we are not being notified an instrument (in this case, a watts transducer which is very important) is out of range until 3.52mA, we can potentially create unnecessary hazards when operating our equipment. So that did answer one of my questions. Now the big question is still, how can I get all of our instruments to fit within that standard, when we use a 1756-IF16 with 0-20ma input range...

[DanW 41]

Not all devices comply with NAMUR, but most have some form of fault detection and alarming outside of the "normal under/over operating" range. which as NAMUR considers 3.8mA to 20.5mA. But you have to check each field device type to see what the fault range is. Your meter might use 3.85mA and 20.8mA According to the Publication 1756-6.5.9, ControlLogix Analog I/O Modules (Cat. No. 1756-IF16, IF6I, IF8, IR6I, IT6I, OF4, F6CI, OF6VI, OF8) http://www.pacpro.co...ix_If16_of8.pdf your AI card has a range of 0-21mA, so you scale 4-20 mA and recognize that negative ranges less than (the eng unit equivalent of ) 3.80mA or positive ranges greater than (the eng unit equivalent of ) 20.5mA (using NAMUR values) are alarm states and alarm accordingly. Edited 26 Jun 2013 by DanW

[Hisma 2]

This would work, because if you scaled the engineering units as a percentage of 0-100, then every analog would have the same scale, and thus slope, so you could easily have standard alarming that would be identical across all devices. However, we scaled our points with their ACTUAL range... that is... 0-500psi, 0-200*C, etc. So, they're all different, which means 3% of one would be a different value than 3% of another.

[Hisma 2]

I think you are on to something though... there's nothing stopping us from scaling 4-20mA as 0-100% in the card, and then doing all the individual scaling in the logic. hell, we already set the high and low limits in the logic, nothing stopping us from scaling the analog signal itself, similar to using an SCP in an SLC. I'd like to see how some of you in here would approach this though.