How does 2-wire-compensation work on Fluke 5700 calibrator? - Page 1
Forum section: electronics / general

author: e-doc
author_rank: Contributor
timestamp: March 08, 2016, 11:35:44 pm
content: 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 SNSon calibrator:ON2-wire-compon calibrator:ONNULL/Relativeon instument = set on calibrator "0" andONIs 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 onDatron 4808:5700: (where is the 2-wire-comp-device, how does it work?)Myspeculation: 2-wire-comp-device, how 2-wire-compcouldwork: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?

author: uncle_bob
author_rank: Unknown
timestamp: March 09, 2016, 12:21:07 am
content: HiIf 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

author: e-doc
author_rank: Contributor
timestamp: March 09, 2016, 01:27:49 am
content: Hi Bob!Quote from: uncle_bob on March 09, 2016, 12:21:07 amThis 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 amMy 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?

author: uncle_bob
author_rank: Unknown
timestamp: March 09, 2016, 01:45:11 am
content: Quote from: e-doc on March 09, 2016, 01:27:49 amHiThe 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

author: e-doc
author_rank: Contributor
timestamp: March 09, 2016, 01:46:03 am
content: Exactly!

author: uncle_bob
author_rank: Unknown
timestamp: March 09, 2016, 01:47:37 am
content: Quote from: e-doc on March 09, 2016, 01:46:03 amHiPage 40 in:http://exodus.poly.edu/~kurt/manuals/manuals/Fluke/FLUKE%205700A,%205720A%20Getting%20Started.pdfChapter 4 in the manual.Bob

author: e-doc
author_rank: Contributor
timestamp: March 09, 2016, 02:23:43 am
content: Thank you Bob!Manual p. 4-16, chapter 4-22:QuoteWe 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?

author: uncle_bob
author_rank: Unknown
timestamp: March 09, 2016, 02:40:56 am
content: Quote from: e-doc on March 09, 2016, 02:23:43 amHiYou 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

author: e-doc
author_rank: Contributor
timestamp: March 09, 2016, 02:51:34 am
content: Quote from: uncle_bob on March 09, 2016, 02:40:56 amYes of course...Quote from: uncle_bob on March 09, 2016, 02:40:56 amThat 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...)

author: uncle_bob
author_rank: Unknown
timestamp: March 09, 2016, 11:18:09 pm
content: Quote from: e-doc on March 09, 2016, 02:51:34 amHiIt 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

author: e-doc
author_rank: Contributor
timestamp: March 10, 2016, 10:44:00 am
content: Thanks for your reply, I will have a look at the service manual later, maybe at the weekend...