Discuss Ohmmeters such as 17XX and Impedance Meters such as 25X
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Joined: Tue Jul 22, 2014 10:38 am
Real Name: Will Rogers


Post by wilber100 »

I have a question about the accuracy of this model.
The published 2-wire, 3-wire, and 4-wire specifications on ranges 4 and 5 are the same.
Should the 4-wire specs be tighter? Should the 3-wire and 2-wire specs be wider?


Posts: 56
Joined: Fri Feb 29, 2008 2:13 pm
Real Name: Andy Brush

Re: R1L-D1

Post by abrush1 »

To start with, I'm very sorry that no one noticed this question for so long. Your question is very valid. Normally, 2-wire and 4-wire specifications differ substantially, but in this case, I think that footnote 1 of the table in the manual tells all: It says,
1 Does not include test lead or contact resistance
This footnote indicates that, for 2-wire mode, the R1L-D1 will measure the resistance under test plus the test leads to the indicated accuracy. The 2-wire measurement will have the accuracy error from the table, plus all the resistance of the test leads as error. The test leads will differ from installation to installation, so it's up to the user to compute this contribution to the error.

The three-wire situation is more complicated. The third wire lets the R1L-D1 measure the voltage drop in ONE of the test leads, and this drop is (effectively) then doubled and subtracted from the voltage across the "source" leads at the instrument. Obviously, this is an approximation that assumes both source leads have exactly the same voltage drop. The lead-related error for three-wire is the probable difference in resistance between the two source leads. It's clearly smaller than the two-wire case, but also can only be quantified by the person who selects and cuts the wire.

Note also that the lead resistance error does not have a distribution. While the instrument error expresses a confidence interval, the lead resistance error is constant.

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