Current Loop Monitoring
Current Loop Monitoring
(OP)
I have several circuits comprised of a transducer providing a 4-20 ma signal to an electronic gauge. I'd like to capture that signal in a data acquisition system (DAS).
I have the design vendor looking for gauges with a data output, but in the meantime I thought I would inquire here.
My first thought was to have the transducer feed the DAS and the DAS feed the gauge, but I can't do that for safety/regulatory reasons.
I'm wary of sticking things in current loops, my second thought was sticking a .1 ohm resistance in the loop and using a high impedance channel DAS to measure the drop and extrapolating the value, but it seem to me that there's probably an off the shelf solution.
PLCs sample data, how is this handled? I don't like the idea of a second transducer to only feed the DAS, because they will eventually disagree with each other.
I have the design vendor looking for gauges with a data output, but in the meantime I thought I would inquire here.
My first thought was to have the transducer feed the DAS and the DAS feed the gauge, but I can't do that for safety/regulatory reasons.
I'm wary of sticking things in current loops, my second thought was sticking a .1 ohm resistance in the loop and using a high impedance channel DAS to measure the drop and extrapolating the value, but it seem to me that there's probably an off the shelf solution.
PLCs sample data, how is this handled? I don't like the idea of a second transducer to only feed the DAS, because they will eventually disagree with each other.





RE: Current Loop Monitoring
RE: Current Loop Monitoring
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If we learn from our mistakes I'm getting a great education!
RE: Current Loop Monitoring
Most transducers with 4-20mA output will drive up to 500 ohms of loop resistance, that resistance being the sum of the resistances of the input resistances of the devices and some small amount of resistance for the copper wiring.
RE: Current Loop Monitoring
RE: Current Loop Monitoring
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If we learn from our mistakes I'm getting a great education!
RE: Current Loop Monitoring
I want the DAQ to be able to die (power off) and the gauge still works.
The former will occur if they use the same signal and tolerances. I suspect the latter will only occur on a vendor by vendor basis. I will explore with them next week.
Thanks for the info.
RE: Current Loop Monitoring
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Current Loop Monitoring
I like the idea of a gauge with a data output.
RE: Current Loop Monitoring
What happens is over time the operators notice and learn that two sensors have say a 1 degree F offset with each other. That's okay! When one day it is a 3F offset they know something is failing.
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Current Loop Monitoring
Not that operators are six year olds, but they work in absolutes in the real world. When the company man is standing over their shoulder questioning their every move, having one set of indicators make everyone happy. If they have a gauge that says they have 11,622 feet in the hole and another one that says 11,620 feet it provides a significant conundrum. Sure I have standby indicators but they requires the operator to dismount and their contradiction is not blatant.
When the company man is happy, the operators are happy. When the operators are happy, I'm less concerned. Sometimes it's easy to fall in love with the design of the hammer and forget that to an operator it's only there to drive nails.
Thanks for the reply.
RE: Current Loop Monitoring
However, there is the digital comm option. For example Precision Digital's (PD) Trident model is a panel meter whose front end takes 4-20mA and displays it as a digital value. A Modbus slave option is available with an RS-485 port.
A different model meter has Modbus only with an RS-485 port (no analog input), and acts as a limited Modbus master, which polls the PD slave and gets the value via Modbus. The displayed values have to agree, within the timing of the Modbus cycle, because the value coming over Modbus is a digital value in hex, not an analog value.
But I'm not sure what the precision is (3 1/2 or 4 /12 digit), nor what the accuracy spec is, and I'm nodding off after a day at the plant, too tired for web search.
If PD makes a Modbus meter, there's probably other brands as well.
Dan
RE: Current Loop Monitoring
http://www
RE: Current Loop Monitoring
It appears there are three options to do this:
1. Use a low resistance resistor in series with circuit and measure voltage drop.
2. Wire SCE in series with circuit and measure current
3. Use a signal splitter.
Each option has its pros and cons, I'll run it by my DAS vendor for their recommendation.
Regarding the potential problem of different values displayed on two indicators, I'm going to use an analog instrument and a digital display. There's enough ambiguity inherent in the analog to assuage any concern.
RE: Current Loop Monitoring
Nice solution.
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Current Loop Monitoring
Eric R.
RE: Current Loop Monitoring
I'll check with the vendor tomorrow regarding the nature of the meter.
Thank you!
RE: Current Loop Monitoring
I'm not sure if I got it right but I think you want to sample and log the mA signalb. I did something similar using one of the small Modicon PLCs with limited No of analog inputs. I connected 2 diodes in series with each loop. The normal voltage drop is 1.2 V. Using a 0.2to1V analog input module and 2 8 input relay output modules sequenced to sample each pair of diodes in turn I was able to read 16 loops with 1 analog input in about 5 seconds.
The primary loop was never broken as the diodes maintained the circuit while not sampling.
Regards
Roy
RE: Current Loop Monitoring
I tried to attach a sketch to explain my last post. Send me your address I will send it back
Regards
Roy
rmatson@noram-eng.com