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

[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!

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.

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.

I would strongly suggest reading the massive library of Fluke app notes and manuals to come up to speed on the 5700.

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]

Hi Bob!

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.

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.

I would strongly suggest reading the massive library of Fluke app notes and manuals to come up to speed on the 5700.

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?

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!

[uncle_bob]

Exactly!

Hi

Page 40 in:

http://exodus.poly.edu/~kurt/manuals/manuals/Fluke/FLUKE%205700A,%205720A%20Getting%20Started.pdf

Chapter 4 in the manual.

Bob

[e-doc]

Thank you Bob!

Manual p. 4-16, chapter 4-22:

For calibrating a meter with only a two-wire resistance mode such as a typical handheld
DMM, refer to Figures 4-4B through 4-4D. For resistances of 19 k? or lower in two-wire
mode, compensation circuitry inside the calibrator is available to remove errors
introduced by resistance in the path between the front panel terminals and the precision
resistor. Depending on how you connect the meter, you can use two-wire compensation
referenced at the UUT terminals (Figure 4-4C) or at the ends of the UUT's test leads
(Figure 4-4D). See ìResistance Outputî for information on turning on and off two-wire
compensation circuitry.
Figure 4-4B shows a meter connected in a two-wire connection with the two-wire
compensation circuitry turned off. For low resistances where uncompensated lead
resistances are significant, use the two-wire compensation circuit and the connection in
Figure 4-4C or 4-4D. Use the connection in Figure 4-4C if you want to calibrate the
meter referenced at its terminals. Use the connection in Figure 4-4D if you want to
calibrate the meter referenced at the end of its test leads.

We used connections as described in Fig. 4-4C: (see attached picture)

When EX_SNS and 2-wire-comp is ON, the results seem to be correct!

More and more I think about it, I come to the conclusion, this must be the correct method, because without external sense wires and with EX_SNS=OFF the 5700 cannot measure voltage drop over the "current-leads".
Can you agree?

What do you think about my speculation how the 2-wire-compensation physically works?

[uncle_bob]

Thank you Bob!

Manual p. 4-16, chapter 4-22:

For calibrating a meter with only a two-wire resistance mode such as a typical handheld
DMM, refer to Figures 4-4B through 4-4D. For resistances of 19 k? or lower in two-wire
mode, compensation circuitry inside the calibrator is available to remove errors
introduced by resistance in the path between the front panel terminals and the precision
resistor. Depending on how you connect the meter, you can use two-wire compensation
referenced at the UUT terminals (Figure 4-4C) or at the ends of the UUT's test leads
(Figure 4-4D). See ìResistance Outputî for information on turning on and off two-wire
compensation circuitry.
Figure 4-4B shows a meter connected in a two-wire connection with the two-wire
compensation circuitry turned off. For low resistances where uncompensated lead
resistances are significant, use the two-wire compensation circuit and the connection in
Figure 4-4C or 4-4D. Use the connection in Figure 4-4C if you want to calibrate the
meter referenced at its terminals. Use the connection in Figure 4-4D if you want to
calibrate the meter referenced at the end of its test leads.

We used connections as described in Fig. 4-4C: (see attached picture)

When EX_SNS and 2-wire-comp is ON, the results seem to be correct!

More and more I think about it, I come to the conclusion, this must be the correct method, because without external sense wires and with EX_SNS=OFF the 5700 cannot measure voltage drop over the "current-leads".
Can you agree?

What do you think about my speculation how the 2-wire-compensation physically works?

I instrument +   o------>-------o + calibrator OUT-----------------                         |                                                                    |-                         |                                                               ---------    ^      variable voltage                         |                                                                  ----        | (resulting voltage drop = 0)                         |                  + calibrator SENSE                     |+                         L_________o-------------V+--------------> o    |                                                           (voltage drop)         \   |                                                                                           \ |                                                                                            Rx           EX_SNS = ON                                                                                           / |                                             - calibrator SENSE                /   |                         __________o<-------------V- ------------- o     |                         |                               (voltage drop)              |                         |                                                                    |-                         |                                                               ---------    ^      variable voltage                         |                                                                  ----       | (resulting voltage drop = 0)                         |                                                                    |+    instrument -   o------<-------o - calibrator OUT------------------

Hi

You would have to get out the service manual and take a look at the schematic to see how they actually do it. The normal approach is to stick a voltage in series with the "standard" and drive that off of the sense leads. To keep things working right, you need to have zero current in the sense leads.

Bob

[e-doc]

You would have to get out the service manual and take a look at the schematic to see how they actually do it.

Yes of course... 

The normal approach is to stick a voltage in series with the "standard" and drive that off of the sense leads. To keep things working right, you need to have zero current in the sense leads.

That is what I've drawn, isn't it?
Sense wire only measures voltage drop over the current lead (no current flow, high impedance voltage measurement).
Same voltage in reverse in series to the current lead results in "zero" voltage drop in addition.

Maybe this is implemented by using differential amps (gain = 1) with differential outputs?

(I really need some sleep now, it's 4:00 am local time...)

[uncle_bob]

You would have to get out the service manual and take a look at the schematic to see how they actually do it.

Yes of course... 

The normal approach is to stick a voltage in series with the "standard" and drive that off of the sense leads. To keep things working right, you need to have zero current in the sense leads.

That is what I've drawn, isn't it?
Sense wire only measures voltage drop over the current lead (no current flow, high impedance voltage measurement).
Same voltage in reverse in series to the current lead results in "zero" voltage drop in addition.

Maybe this is implemented by using differential amps (gain = 1) with differential outputs?

(I really need some sleep now, it's 4:00 am local time...)

Hi

It may be implemented inside a software loop that drives a pair of DAC's that do the entire output process and simulate a resistance at the load.

Bob

[e-doc]

Thanks for your reply, I will have a look at the service manual later, maybe at the weekend...