How to tell the difference between 2 wire and 4 wire industrial instruments?

[Kart31]

How can I tell the if an industrial instrument (temperature transmitter) is a 2-wire or a 4-wire 4-20mA output? Given that I don't have the manual, can't find a manual but do have two wires and know it is a 4-20mA output?

 

[Reloadron]

A 4-20 mA is only a two wire system. Four wire comes into play with voltage with devices. For example a 4 wire kelvin resistance measurement.

Given that I don't have the manual, can't find a manual but do have two wires and know it is a 4-20mA output?

Depends on what the device is but if it's a 4 to 20 mA sensor of sorts yes, that's a safe assumption. Make sure if you use it you observe correct polarity and applied voltage. Below is an example of a thermocouple transmitter which outputs a 4 - 20 mA current loop shown with resistor to provide a voltage input to an ADC.

Ron

 

[BobW]

There are also 4 wire 4-20mA devices. These are generally instruments that have a lot more internal circuitry than can be powered off the loop. So they would have two extra terminals to accomodate a 24 VDC supply (or in some cases an AC mains supply). The output would still be 4-20mA. Temperature transmitters are simple enough that they should just be two wire. However, the temperature element itself, if separate from the transmitter, could be a thermocouple (2 wire), or an RTD (2, 3 or 4 wire).

 

[Reloadron]

There are also 4 wire 4-20mA devices. These are generally instruments that have a lot more internal circuitry than can be powered off the loop. So they would have two extra terminals to accomodate a 24 VDC supply (or in some cases an AC mains supply). The output would still be 4-20mA. Temperature transmitters are simple enough that they should just be two wire. However, the temperature element itself, if separate from the transmitter, could be a thermocouple (2 wire), or an RTD (2, 3 or 4 wire).

Good point and having used them I never gave much thought to the power being additional wires. Sort of slow on the uptake on my part. If it's not loop powered obviously it needs power.

Ron

 

[dr pepper]

Sometimes (not always) 2 wire is referred to as passive, and 4 active, pretty much similar to bobs post above.