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2, 3, or 4 wire transmitter/field device

2, 3, or 4 wire transmitter/field device

2, 3, or 4 wire transmitter/field device


What is the criteria for choosing 2 wire, 3 wire, or 4 wire configuration for transmitter/field device ?

Your comments please

Thank you

RE: 2, 3, or 4 wire transmitter/field device

I'd think 2-wire is the first go-to because it has the least expensive wiring cost and the AO of every 2-wire device I've run across so far is isolated from earth ground (avoids ground loop problems)

There are market and end user considerations.

If the end use is a DCS, the first choice is 2-wire loop-powered because most DCS AI's expect 2-wire, so DC loop power is hard wired to one of the AI terminals making it an active input. If the field device is 3-wire or 4-wire, then an additional isolator is needed, passive on the DCS side, active on the field side (can't run a active-output field device to an active input, the power supplies will buck each other).

The power requirements for Intrinsic Safety are so low that an I/S requirement will probably be a choice of a 2-wire device.

For the rest of the market, pick 2-wire if it's available for the technology. Some technologies just take too much power to run with the 3.5mA available with 2-wire loop power. Coriolis and thermal dispersion flow meters are typically 4 wire. Gas analyzers are typically 4 wire.

If it's oddball and non-industrial, like a scientific analyzer, it'll probably be 4-wire, non-isolated voltage output and that's what you get.

There are rare occasions with really long wiring distances where the DC power supply's output for 2-wire devices can be increased to overcome the loop resistance of the field wiring. A 4-wire device with an active output probably does not provide adjustment of its the internal power supply and would need an interposing isolator that could handle more DC voltage.

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