PT100 3 wire, how exactly does it work?

[hno090877]

Dear Users,

I have been trying to figure out the math behind the PT100 3-wire connection. All available material states that the lead resistance cancels itself out. But I am having trouble setting up the formulas that show this.

I have a system with a PT100 connected via 3-wires (5mtr). I use 3VDC as excitationvoltage and my bottomresistor is 200R. I see a significant change in measured temperature if I connect the PT100 device directly to my PCB without the wires.

So, if anyone could so kind as to help me understand the math involved, especially the leadcancelling, I would be most thankfull.

Thanking you in advance.

Henrik

 

[IRstuff]

I don't think it "cancels" on its own. However, if you assume that ALL 3 leads have the same characteristics, you can measure the IR drop in the one leg and use that value to "correct" the measurement for the other leg.

If you want true cancellation, you need to use a 4-wire configuration.

TTFN

 

[itsmoked]

You excite the RTD thru the single lead (red) and one of the double leads (white). You measure the voltage from the second white lead back to the first excitation white lead... This will be a negative value voltage. It represents the voltage drop in a lead due to its resistance and the excitation current flowing thru it.

Next you measure the the voltage between the non-excitation (white) lead and the red lead. This represents the RTD's temperature resistance AND the red lead voltage drop due to the excitation current.

You add the first white lead measurement voltage (which was negative) to this last measurement and it cancels any contribution to the RTD resistance measurement contributed to the red lead because of excitation current.

 

[ScottyUK]

You only assume that the two current carrying leads have the same volt-drop. The third sense wire characteristics don't matter.

The third wire senses the volt-drop of one of the current carrying wires, and the calculation bases its assumption that the other, unsensed volt-drop is equal to the first. Once the voltage across the RTD is known, Ohm's Law takes over.


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[cbarn24050]

Hi, your error occurs because your "bottom" resistor is 200 ohms insted of 100 ohms, assuming that you have it wired correctly.