4 wire rtd with one wire not connected

[Yarno]

Does anyone know what this type of meassurement does? It's in my electrical engineering class and i can't make anyhting out of it..
It's called a floating point four wire measurement.

 

[waross]

Some circuits use three wires.
The third wire is used to cancel out conductor resistance.
Some circuits use four wires for the same thing, cancelling loop conductor resistance.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[IRstuff]

4-wire Kelvin connection is the gold standard for eliminating resistive parasitics from the measurement. 2-wire basically sucks for low-resistance measurements, since contact resistance is often on the order of 0.5 ohm. 3-wire is kind of a compromise, often because someone got lazy in running the 4th wire, or couldn't because of an existing layout.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

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[itsmoked]

Read what I wrote in this thread:

thread240-165067

It explains how three wire RTDs are used. Three wire are THE most common version.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Yarno]

Ah so it's just the same as a three wire measurement?
And what's up with the name? why do they call it a 'floating point 4wire measurement'?
Why don't they just leave the fourth wire and make it a real 3 wire rtd?

 

[waross]

The monitoring circuits are different between three wire schemes and four wire schemes.
A four wire RTD gives the flexibility to use either monitoring scheme.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[IRstuff]

A 4-wire configuration is the most precise/accurate, particularly if the RTD or whatever resistor isn't soldered in place, i.e., a switching network or clips are used; the 4th wire allows the parasitic resistances on both sides of the resistor to be eliminated from the measurement

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

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[danw2]

I suspect that if you re-draw your diagram you'll find that the unlabeled R's and R3 form a classic bridge circuit for a 3-wire RTD; and Rd1, Rd2 and Rd4 being the lead resistance.

Compare to the 3-wire RTD circuit from Wikipedia (of all places):