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2, 3, or 4 wire transmitter/field device
- Thread starter helpyou
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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.
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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