What's the difference between 4-wire and 2 wire resistance measurements w/ 9219?

[jdude]

It appears that the NI-9219 provides a small current in either 4-wire or 2-wire resistance measurement mode.  What is the difference between the two methods?  Just accuracy?

Also, if I am measuring the resistance of a solenoid (when it is not excited) using either method above, do I need some type of shunt resistor or do I just litteraly just wire HI/LO and EX+/EX- across the solenoid terminals?

Thanks!

[jdude]

By the way the expected resistance is only around 12 ohms.  Will I be able to get a fairly accurate measurement using the 9219 with such a low resistance?

Thanks again.

[johnsold]

Four wire measurements are preferred for measuring low resistances.  One pair of wires carries the excitation current and the other pair connects to the voltage measuring instrument, which, ideally draws no current.  Thus any voltage drop on the wires carrying the excitation current is not measured and does not contribute an error.

I am not familiar with the 9219, so I cannot tell how accurate your measurement will be.  If the resistance of the lead wires from the instrument to the coil under test is equal to or greater than the maximum acceptable measurement error, then the  4-wire technique will be required to meet your accuracy specifications.

Lynn 

[JB]

Here is a general info about the 4-wire resistance measurement. There are many others on the web !

Message Edité par JB le 10-24-2008 04:47 PM

[Ray.R]

Resistance thermometer wiring configurations:

Two-wire configuration
 
The simplest resistance thermometer configuration uses two wires. It is only used when high accuracy is not required as the resistance of the connecting wires is always included with that of the sensor leading to errors in the signal. This applies equally to balanced bridge and fixed bridge system.

Three-wire configuration
 
In order to minimize the effects of the lead resistances a three wire configuration can be used. Using this method the two leads to the sensor are on adjoining arms, there is a lead resistance in each arm of the bridge and therefore the lead resistance is cancelled out. High quality connection cables should be used for this type of configuration because an assumption is made that the two lead resistances are the same.

Four-wire configuration
 
The four wire resistance thermometer configuration even further increases the accuracy and reliability of the resistance being measured. In the diagram above a standard two terminal RTD is used with another pair of wires to form an additional loop that cancels out the lead resistance. The above Wheatstone bridge method uses a little more copper wire and is not a perfect solution. Below is a better alternative configuration four-wire Kelvin connection that should be used in all RTDs. It provides full cancellation of spurious effects and cable resistance of up to 15 Ω can be handled. Actually in four wire measurement the resistance error due to lead wire resistance is zero.

Message Edited by JoeLabView on 10-24-2008 01:00 PM