4-20 mA transmitter to mismatched input

Thread Starter

parmstro

Joined Apr 29, 2022
2
My field devices are 4-20mA loop powered. If I connect them to a 0-20mA rtu input there will be 4 mA offset. Can I scale around that offset to compensate?
 

SamR

Joined Mar 19, 2019
4,292
Welcime to AAC!

You could but then you also have to look at cost to do so and reliability vs. buying the correct 4-20mA device...
 

eetech00

Joined Jun 8, 2013
3,206
My field devices are 4-20mA loop powered. If I connect them to a 0-20mA rtu input there will be 4 mA offset. Can I scale around that offset to compensate?
Depends on what information is provided by the loop. Is it a loop used for digital transmission? Or analog?
 

strantor

Joined Oct 3, 2010
5,971
My field devices are 4-20mA loop powered. If I connect them to a 0-20mA rtu input there will be 4 mA offset. Can I scale around that offset to compensate?
More info needed. Do you mean scale in software or in hardware? What is your software/hardware? Is this a PLC application? I do this in PLC all the time, it's a simple linear scaling operation where:
(Assuming 16-bit resolution)
X1 = 13,107
Y1 = 0%
X2 = 65,536
Y2 = 100%
IF X1 < 10,000 THEN [sensor disconnected alarm]
 

SamR

Joined Mar 19, 2019
4,292
Distributed Control Systems use 4-20mA field sensors and controllers. Some may use 0-10V but are not the majority. That is interfaced to controllers in racks tied to the computers. Scaling for 0-20mA would have to be done external to the control card racks and not in the computer software. Then it may also need to be certified for intrinsic safety and hardened for reliability. Not something that I or anyone tasked with supporting and servicing the equipment would likely recommend. One-offs in control systems with multiple people involved in keeping it reliably operating is not advised.
 
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strantor

Joined Oct 3, 2010
5,971
Distributed Control Systems use 4-20mA field sensors and controllers. [...] Scaling for 0-20mA would have to be done external to the control card racks and not in the computer software.
Why? And if this is the case, how do you perform calibration? There must be some provision for calibration (scaling) and this should also work to scale a 4-20mA sensor to a 0-20mA input (which is what TS described). I am quite used to seeing calibration and scaling performed in software.
 

SamR

Joined Mar 19, 2019
4,292
I could scale using an intermediary math function card but calibration of the input or output cards from 4mA to 0mA would be far out of range. Any field instrument that far out of range would be considered faulty to the DCS system. Typically, the primary calibration is done in the instrument shop of the field device being simulated through its range and calibrated properly. There is some calibration done in the DCS (at least for the Foxboro DCS) but it is mostly dependent on the field element being properly calibrated. Some tweaking of field calibration is done by the instrument tech guys in situ. In the control room the 4-20mA is linearly scaled to 0-100% of the range of the field input or output. In that way anything less the 4mA is seen as a field fault/cut wire/etc. Never worked with a 0-10V system as they are few and far between. 0-20 mA is typically used in standalone temp display instruments and not DCS. It's like mixing apples and oranges, two very different things. Any one-off kludge to input 0-20mA into the DCS would have the Instrument Techs, their Supervisor, and the plant Maintenance Engineer in a snit.
 
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strantor

Joined Oct 3, 2010
5,971
I could scale using an intermediary math function card but calibration of the input or output cards from 4mA to 0mA would be far out of range. Any field instrument that far out of range would be considered faulty to the DCS system. Typically, the primary calibration is done in the instrument shop of the field device being simulated through its range and calibrated properly. There is some calibration done in the DCS (at least for the Foxboro DCS) but it is mostly dependent on the field element being properly calibrated. Some tweaking of field calibration is done by the instrument tech guys in situ. In the control room the 4-20mA is linearly scaled to 0-100% of the range of the field input or output. In that way anything less the 4mA is seen as a field fault/cut wire/etc. Never worked with a 0-10V system as they are few and far between. 0-40 mA is typically used in standalone temp display instruments and not DCS. It's like mixing apples and oranges, two very different things. Any one-off kludge to input 0-40mA into the DCS would have the Instrument Techs, their Supervisor, and the plant Maintenance Engineer in a snit.
I think you are mistaking the OP. Or just referring to your own experience which isn't the same thing. TS has a sensor that is 4-20mA and an analog input that is 0-20mA. What you are describing is the other way around, and I have seen no indication that any DCS is involved.
 

SamR

Joined Mar 19, 2019
4,292
Ooops, I had it versa vice. When I see 4-20mA I think DCS since that is about the only use of 4-20mA field instruments that I know of. There are a lot of standalone 0-20mA temp sensors typically used for small package boilers that don't require the power of a DCS to run them.
 

strantor

Joined Oct 3, 2010
5,971
Ooops, I had it versa vice. When I see 4-20mA I think DCS since that is about the only use of 4-20mA field instruments that I know of. There are a lot of standalone 0-20mA temp sensors typically used for small package boilers that don't require the power of a DCS to run them.
4-20mA is ubiquitous across many industries. I have no experience with DCS yet I deal with 4-20mA sensors and inputs daily.
 

SamR

Joined Mar 19, 2019
4,292
Even before DCS we were using (still had a large base of in use) Foxboro Spec 200 4-20mA control panels and racks and even some pneumatic instrumentation control panels (replaced and upgraded that to DCS). We had upgraded most of the Spec 200 to DCS but there were still some here and there on the plant and even some hybrids where we kept existing Spec 200 and added DCS with the intention that all would eventually be migrated to DCS. Went from large floor to ceiling and long control panels to desktop monitors.
 
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