Why better to use 2wire then 4wire and why better to use current then voltage ?

When you have PT100 Sensors connected in 4-wire, then u transfer a measurement to the PLC, when u use 2-wire, the PT100 works like a resistor. 2-wire is better to use, why ?

And second: 0 to 20mA is better then using 0 to 10V, why ?

tnx

Combo said:

And second: 0 to 20mA is better then using 0 to 10V, why ?

tnx

Click to expand...

0 to 20 mA won,t pickup noise signal rather than 0 to 10V

when you use 0-10V, the voltage can drop if the signal cable is very long, this doesn't happen with current. Best to use is 4-20mA, because then you can detect wether a wire is broken or not: if there's no problem the "0 signal" is 4mA, but if there is a problem the "0 signal" is 0mA.

I don't know which is best; 2 or 4 wire PT100.

4 to 20mA is even better, because you can detect broken wires.

In 2-wire mode you simply measure the resistance of the complete circuit: PT100 plus the wiring, connections et cetera. This circuit usually forms a voltagedivider with a fixed resistor inside the measuring system, e.g. an analog inputcard.

PT100 4-wire is better than 2-wire because the effect of the wires is less. It also works in a different way: you send a current through the PT100 using one pair of wires and you measure the voltage drop over the PT100 over the other pair. For the voltage measurement you can use a high impedance measuring circuit, which influences the measuring results in a far smaller way. In some circuits the current sent through the PT100 is fixed, but sometimes it's a modulated signal. By filtering out the desired frequency in the measuring circuit you can eliminate noise.


Kind regards,

A 4-wire RTD instrument compensates for resistance of the connecting leads, therefore there will be no loss in accuracy over long distances.

Likewise 0-20mA suffers no loss of accuracy over long distances - volts will drop proportional to the resistance of the cable, milliamps will stay the same regardless of the cable resistance (as long as the instrument sending the 0-20mA can provide sufficient voltage to drive the required current)

Jeroen has beaten me giving the answer regarding 0-20mA and 4-20mA.

Welcome Jeroen, it's good to see someone from the neighbourhood.

Kind regards,

use 4 wire RTD to make compensation & accurate measurement of temprature at the measuring end

I Hope 3 wire RTD is used for the applied voltage compensation at the measuring end

4 wire RTD used for Applied voltage compensation & Measured voltage compensation

Not exactly

It is a common misconception that wire impedance causes a significant error in voltage signal. Please refer to the attached calculations. The effect of very significant wire length and resistance is negligible because it is very small compared to the resistance of the voltage input on the PLC.

The advantage of current analog signals is that immunity to EMI/RFI induced voltage is better. The voltage results in very little change in the signal current, so the noise doesn't affect the measurement as much. In addition, 4-20 mA allows two wire devices, where voltage output transmitters are generally 3-wire or 4-wire devices.

I always use 3 wire RTDs. 2 wires measure the resistance of the RTD plus the cable in the run. The measuring device then measures the resistance of the 3rd wire and the other wire attached at the common point at the RTD and subtracts that value leaving the exact resistance of the RTD and so an accurate measurement is made.

All wires in the system have to be exactly the same size and length. If you run a 4 wire cable and tie 2 wires together, the resistance of that cable will be less and you will introduce error.

More info

http://www.rtdcompany.com/technical_info/selecting_rtd2.cfm

these guys have a lot of info on the website http://www.rtdcompany.com

One more question.

Is there a difference in accuration between 4-20mA or 0-10V?

assuming the cable length is very short (lets say about 5m)

Leonardo said:

One more question.

Is there a difference in accuration between 4-20mA or 0-10V?

assuming the cable length is very short (lets say about 5m)

Click to expand...

Wouldn't the difference be in resolution ?
0-10 V similar to 0-20 mA.
A min value gives 0 mA and a max value gives 20 mA. So you have a range of 20 mA to display your measured value. When using 4 - 20 mA you "sacrifice" 4 mA for the ability to detect a broken wire effectively giving you a range of 16 mA instead of 20 mA.