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Anonymous Poster

Field nstruments 2 wire or 4 wire ?

10/24/2008 4:25 AM

Technical comparison between 2 wire and 4 wire field instruments.Selection criteria of 2 wire or 4 wire instruments.

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Guru
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#1

Re: Field instruments 2 wire or 4 wire ?

10/24/2008 7:38 AM

if you would get more precision and less tolerant. use 4 wire.

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Guru
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#2

Re: Field nstruments 2 wire or 4 wire ?

10/24/2008 7:41 AM

Please be more specific. What type instrument are you referring to? Are you asking about input or output?

Don't be shy about giving us more information. I could give you an answer that is technically correct but doesn't exactly answer your particular question due to misinterpretation.

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#3
In reply to #2

Re: Field nstruments 2 wire or 4 wire ?

10/24/2008 7:59 AM

Specifically i am talking about pressure transmitter & RTD.

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Guru
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#5
In reply to #3

Re: Field nstruments 2 wire or 4 wire ?

10/25/2008 1:05 AM

Okay, that isn't specific enough but to give you an answer (actually several):

  • An RTD is a Resistance Temperature Device. The sensor itself usually comes with two wires. When connecting it to a transmitter:
    • you can use two wires. It's not recommended practice but still possible.
    • you can use three wires (one wire on one lead, two wires on the other lead). This is better for accuracy.
    • you can use four wires (two wires on each lead). This is best for accuracy though more expensive.
  • If you're talking about transmitters, two wires mean that the transmitter sends its signal and obtains its power from the two wires. Four wires mean that two wires carry the signal and the other two wires carry the power for the transmitter. Which is better?
    • If you have dozens and dozens of transmitters, two wire instruments will save you money on installation. Your termination cabinets will be smaller with less terminal blocks than if you use four wires.
    • With regards to accuracy, there is no advantage to using two wires or four wires.
    • Sometimes, an instrument needs more power than the 4 milliamps that a two wire system can give at minimum. In these cases, four wires are the only way to go.
    • With regards to troubleshooting, I personally find it easier to find a problem with two wires than four wires. There's just two wires, right?

Personally, if it were up to me, I'd use two wires than four.

regards,

Vulcan

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Guru
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#4

Re: Field nstruments 2 wire or 4 wire ?

10/24/2008 12:45 PM

For me an instrument should be of least number of wires but not at the cost of

Precision, repeatability, reproducibility, and reliability.

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Anonymous Poster
#6

Re: Field nstruments 2 wire or 4 wire ?

07/23/2010 7:53 AM

2 Wire is better option than 4 Wire if wiring is not long........Cost also reduces

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