Electronics > Metrology

How does 2-wire-compensation work on Fluke 5700 calibrator?

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e-doc:
Sorry for asking you this, but I could find no answer in the documentation of this calibrator.
Maybe someone can explain it to me...

We got a Fluke 5700 as a replacement for the old Datron 4808 for a while, and now we have massive (out of tolerance) problems calibrating some multimeters in the lower ohms ranges, that were calibrated one year before using the Datron 4808. Normally nearly all used to be "in tolerance" after one year.

The only way to get the correct reading on several Yokogawa 7551 in 100-Ohms-range:

 

EX SNS on calibrator: ON
2-wire-comp on calibrator: ON
NULL/Relative on instument = set on calibrator "0" and ON
Is this the correct way to calibrate an instrument in 2-wire-Ohms mode and how does the 2-wire-compensation of the 5700 works?

The 4808 calibrator I used before only had "2-w" and "4-w" modes (REM SENSE on and off).
The nominal values for 2-w were higher than the nominals for 4-w (about 0.5 Ohms, what is the resistance of the Pomoma cable used in calibration of the calibrator plus calibrator internal wiring to "Rx").
This is plausible to me.


This is how I understood 2-w/4-w on Datron 4808:



     5700: (where is the 2-wire-comp-device, how does it work?)




My speculation: 2-wire-comp-device, how 2-wire-comp could work:



The 5700 uses the sense-wiring from instrument to calibrator sense to measure voltage drop by instuments measurement current I and connects a voltage source with same voltage in series but in reversed polarity into the current path... (sorry for my confused writing...)
In other words: the resulting voltage drop along the current path equals zero, so voltage at the instruments input is equal voltage over Rx.

Anybody agree or disagree?


uncle_bob:
Hi

If you do not compensate for your lead resistances you will get crazy high errors calibrating low resistance measurements. You *must* use some sort of 4 wire technique. I would strongly suggest reading the massive library of Fluke app notes and manuals to come up to speed on the 5700.

Bob

e-doc:
Hi Bob!


--- Quote from: uncle_bob on March 09, 2016, 12:21:07 am ---If you do not compensate for your lead resistances you will get crazy high errors calibrating low resistance measurements. You *must* use some sort of 4 wire technique.
--- End quote ---
This is obvious to me, so I try to find the proper method.
 
We used this "half"-4-wire-wiring:



The old 4808 had the lead resistance "calibrated" in 2-w mode, because these leads (Pomona) were used during the adjustment of the calibrators nominal values, so no additional "compensation" was used.


--- Quote from: uncle_bob on March 09, 2016, 12:21:07 am --- I would strongly suggest reading the massive library of Fluke app notes and manuals to come up to speed on the 5700.
--- End quote ---
My collegue and me searched the manual (PDF) for "2 wire comp" with poor results.
Only few hits and nothing really helped us further.

Do you know concrete spots, where we can find more about the topic?

uncle_bob:

--- Quote from: e-doc on March 09, 2016, 01:27:49 am ---Hi Bob!


--- Quote from: uncle_bob on March 09, 2016, 12:21:07 am ---If you do not compensate for your lead resistances you will get crazy high errors calibrating low resistance measurements. You *must* use some sort of 4 wire technique.
--- End quote ---
This is obvious to me, so I try to find the proper method.
 
We used this "half"-4-wire-wiring:
instrument + o--------------o + calibrator OUT
                       \________o + calibrator SENSE

instrument - o--------------o - calibrator OUT
                      \________o - calibrator SENSE 

The old 4808 had the lead resistance "calibrated" in 2-w mode, because these leads (Pomona) were used during the adjustment of the calibrators nominal values, so no additional "compensation" was used.


--- Quote from: uncle_bob on March 09, 2016, 12:21:07 am --- I would strongly suggest reading the massive library of Fluke app notes and manuals to come up to speed on the 5700.
--- End quote ---
My collegue and me searched the manual (PDF) for "2 wire comp" with poor results.
Only few hits and nothing really helped us further.

Do you know concrete spots, where we can find more about the topic?

--- End quote ---

Hi

The way I understand your picture, you took the output and sense lines from the calibrator and tied them together at the front of the instrument you are calibrating. Is this what you did?

Bob

e-doc:
Exactly!

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