Pressure transducer.

[ptonsparky]

Why would I want a 4 wire transducer vs a 2 wire? 4-20ma.

 

[rcwilson]

A 4-wire transmitter provides the power to run the 4-20 mA loop. A 2-wire needs a DC power supply in the loop. If you only have one transmitter and a readout, a 4-wire saves the cost of a 24VDC power supply.

Most PLC, VFD or DCS systems' 4-20mA input cards can supply the DC power and a 2- wire transducer makes sense. Mixing 2-wire and 4-wire on the same card can be done, but you need to pay attention to the detailed wiring to prevent interconnecting two power supplies or creating ground loop problems.

Some 4-wire transmitters have better accuracy and repsonse characteristics than a 2-wire version.

 

[petersonra]

Why would I want a 4 wire transducer vs a 2 wire? 4-20ma.

I don't think in general it makes any real difference.

Sometimes in a particular situation it might make more sense to go one way or the other.

Most of the time it is going to be more cost effective to use 2 wire pressure transmitters though.

 

[GeorgeB]

Why would I want a 4 wire transducer vs a 2 wire? 4-20ma.

SOME 4-20 transducers require one connection to 0V, sensing in the positive lead. SOME (IMO, poor implementations, but I'm in electrohydraulics) inputs require the 4-20 be sourced to a 0V input. Interconnecting lets the magic smoke out.

An isolator is relatively inexpensive to resolve this. I've never seen a 4 wire transducer that wouldn't work without requiring the isolator.

 

[petersonra]

SOME 4-20 transducers require one connection to 0V, sensing in the positive lead. SOME (IMO, poor implementations, but I'm in electrohydraulics) inputs require the 4-20 be sourced to a 0V input. Interconnecting lets the magic smoke out.

An isolator is relatively inexpensive to resolve this. I've never seen a 4 wire transducer that wouldn't work without requiring the isolator.

2 wire is inherently isolated.

IME 3 wire rarely is. 4 wire as often than not is isolated. But, these days I avoid both. I can't recall the last time I used anything but a 2 wire PT.

The isolators are not free and neither is the labor to wire them in. Take up panel space too.

I can often find a way to hook up a 4 wire output that is not isolated without requiring an isolator, but a lot of times people want to put two 4 wire devices that are not isolated in the same loop and that will provide some entertainment.