Are 4-Wire Transmitters Better?

Whichever you use, there are only 2 wires going between the instrument and the PLC/DCS.

The difference is that a 2 wire transmitter gets it's power for the 4-20ma (or whatever you use) from the PLC/DCS; while a 4 wire transmitter has it's own power supply to drive the 4-20ma loop.

Theoretically, one wire on each side of the sensor is used to supply current to it, and one wire on each side of the sensor is used to send the voltage reading to the measuring instrument. Since there's current in the two wires supplying current, part of the voltage drop is in the wire itself, meaning that the longer the wire, the higher the voltage drop and greater the error. However, since there's no (very, very little) current in the voltage sense wires, there's no voltage drop in the wire, and all voltage drop occurs across the sensor.

The main difference is - as mentioned - that 4 wire transmitters have sepparated power source. But I'd add that its most commom to see 4-wired sensors, which differs from a transmitter as known here in its output signal, which would be in tension, not in current. They are always 4 wired. Distance? Yeah, surelly. But I never experienced a practical case where both couldnt be used with proper wire installation, good power sources and noise protection practices. I run test cells everyday with hundreds of meters of wires to 2-wired transmitters. They do just fine, never had a calibration problem. When you need some reading really far from your measuring point, in a big distributed data acquisition setup, you should go digital. Access virtually anywhere. Of course, speed constraints should be observed.

First, what specific transmitters you want to discuss? By then we may help you better to your specifics. Regards

less noise using a 4-wire transmitter compared to a 2-wire transmitter in the same situation

The 2 wire transmitter is usually called "loop powered" for obvious reasons. If the final or receiving unit is looking for purely a (say) mA signal, but does not supply (say) 24VDC supply AND had part of the measuring circuit grounded, then a separate 24VDC power supply is needed, and can not be grounded. This is the situation where 24VDC supply is on 2 wires and signal on the other two. Some older PLC's require this configuration. My pref is 2 wire unless stated not to.

Rok

hi Ankur,

temperature ,pressure etc conventional transmiters are preferred for 2 wire system (type-2)

magnetic flowmeter ,massflowmeter requires 4wire (2 for power 2 for signal )

(type-4) here distance is more important concern with the sensing element

Refer to ISA S50.01 it categorizes transmitters into two wire,three wire or four wiretypes .

Adil

I think you have enough answers regarding the difference.

Basically each has it's place in a plant design. The 2 wire system gets power from the Analogue input channel or recorder etc. In some instances the voltage available in the circuit to the transmitter may be insufficient for the transmitter to operate with a particular IO brand. In addition you may have a series loop with a recorder and controller. In this case voltage drop across each device in the circuit may cause problems. Here a 4 wire TX would probably be better.

Two wire is easier to implement and look after in the long run.

It is always better to feed the instruments from something other than the PLC's power supply.

However using analog 2 wire is better!

• cost saving (if you use 4 wires, you have to pay twice for copper)
• space saving (you don't need to pull heavy/big cable, you can use small cable tray)
• and low maintenance (easier to troubleshoot)

But for some case, 2 wire is not enough to deliver the power (0,48watt max).

You cannot use 2 wire for transmitter/equipment which requires much power, that is why another 2-wire use to power your device.

Dear Guest,

One BA (bad answer) for you!

Use, at the reception site, a 1 million Watt 24 V dc power supply, connect its minus output to the PLCs analog to digital converter. One wire from your transmitters goes to +24Vdc. The other one goes to your sensing resistor on the ADC module. Thus you can have more than 2 million (each of 0.480 W) transmitters.

The main difference between 2-wire and 4-wire transmitters is their source of power.

2-wire devices are refered to as "Loop Powered" with power originating from the control system, typically 24 VDC. A single pair of wires is used to create a current loop that delivers transmitter power over the 4-20 mA signal wiring.

A 4-wire device has a separate 4-20 mA current loop for the signal and a second pair of wires connecting the power source to the transmitter. Thus the need for 4 wires.

Most modern transmitter electronics and sensors are available as 2-wire devices that connect to standard 4-20mA inputs of a DCS/PLC, due to highly efficient microprocessor and sensing technologies. Some sensing technololgies still require more power to operate mechanical actuators or sensors, such as coriolis flow meters, Magnetic Flow sensors, or analyzers. For these devices a separate constant power feed is used that is not limited by the control signal current of 4-20 mA.

Ultimately, you should choose the right device/technology for your process sensing needs, considering the installed cost of the device and the benefits of the different solutions. In some cases, a 4-wire solution may be the only option.

Finally, the issue of signal wire distance. This is not complicated. A 4-20 mA current loop generates voltage drops across resistances in the loop. These include the sensing load (250 Ohms), the transmitter and the signal wires. The transmitter controls the current signal by varying its resistance in the loop. A minimum voltage is required at the 2-wire tranmitter so that it has sufficient power to function, usually 10 V at 20 mA. Therefore the voltage drop across the wires is limited and is determined by the wire length and gauge. A 4-wire device gets it's power separately. This allows for some additional distance on the signal wiring over a 2-wire device. However, a typical 18 gauge wire has 6.385 Ohms/1000 ft. At 20 mA, that has a voltage drop of 0.128 Volts per 500 ft of signal pair. With a typical headroom of 5 to 7 Volts, 18 guage wire can allow up to over 20,000 ft for a 2-wire device, approaching 5 miles, and still have enough power for the field device. So distance is typically not the issue here.