1 resistance on 2 DMM's simultaneously - Page 1
Forum section: electronics / testgear

author: electrolust
author_rank: Unknown
timestamp: November 30, 2016, 07:53:26 pm
content: A lot of videos show a volt reading on 2 DMMs in parallel, for the sake of comparison of the DMMs.Is it possible to take a resistance reading on 2 DMMS simultaneously? I think not. Even if you wire the DMMs in serial you have the problem that both DMMS are trying to power the DUT.But maybe I'm wrong and there is a way? Maybe using 4-wire mode?

author: Vgkid
author_rank: Super Contributor
timestamp: November 30, 2016, 09:13:22 pm
content: No. Though you can use another multimeter to measure the voltage developed across the resistor.

author: MosherIV
author_rank: Super Contributor
timestamp: December 01, 2016, 09:43:14 am
content: I have not tried this but in theory it might work for hand held (battery powered) meters if the 0V of both is NOT common, then each meter can apply the measuring voltage without affecting the other ie in practical terms put the + and - opposite way round with each meter.I cannot guarantee this will work, probably will not but worth a try.

author: Alex Nikitin
author_rank: Super Contributor
timestamp: December 01, 2016, 09:55:06 am
content: Quote from: MosherIV on December 01, 2016, 09:43:14 amEven in theory it won't work. One way I can think of to measure one resistance on two meters really simultaneously (and not by switching it from one meter to another by a relay, for example) is to measure it on DC from one meter and on AC from another meter (probably an LCR meter in this position, as all multimeters I know of do use a DC method). There are some problems with this arrangement - for instance, the output impedance of the DC Ohmmeter on AC should be sufficiently high, and so on.CheersAlex

author: TiN
author_rank: Super Contributor
timestamp: December 01, 2016, 11:20:02 am
content: Common way to measure same resistance by multiple instruments would be using automated switch box, like Keithey 7001 with scanner card and use kelvin connections to remove wiring/contacts resistance. This is technically speaking, not simultaneous measurement, but with second or two delay between measurements it's still good enough for generic test conditions, as it's unlikely that your resistance will change much over 2 seconds.If requirement are not very right, this would work rather OK with proper wiring and care. If high resistances need to be measured, low leakage switch card would be needed.

author: omgfire
author_rank: Regular Contributor
timestamp: December 01, 2016, 02:49:19 pm
content: I would guess, that Keysight's Smart ohm might survive other meter:QuoteBut I don't see how that help original problem.

author: David Hess
author_rank: Super Contributor
timestamp: December 01, 2016, 05:58:42 pm
content: It helps to understand what the "ohms converter" in the meter does. It provides a stable current inversely proportional to the ohms range and the meter then reads the voltage across the resistor using its non-divided input range which will usually be 2.000 or 200.0mV for a 3.5 digit meter. The input shunt resistance usually present from the normal voltage divider is removed and the divider is usually used as part of the ohms converter circuit to provide a calibrated output current in decade steps which is why only one ohms calibration is required in this arrangement. Modern cheap meters and digitally calibrated meters may have different arrangements.The meter excitation currents will add together roughly doubling the returned voltage making the resistance look roughly twice as high. I say "roughly" because different meters may use different excitation currents and modern meters may use calibration after the reading is taken so their excitation current may be stable but not calibrated.So if I use a pair of my old meters which have calibrated excitation currents, they do indeed read twice the resistance when used in parallel. You could set one meter to read voltage and the other ohms however the shunt resistance of the voltage reading meter will be in parallel with the resistance to be measured. This is solved by using the voltmeter in high input resistance mode in which case both meters will read the same value although the decimal points will not line up in every range.

author: Fungus
author_rank: Super Contributor
timestamp: December 01, 2016, 07:34:08 pm
content: Is there really any point to doing this?Internally the meters are reading volts. If you compare them side by side reading volts then the Ohms is the same.

author: tooki
author_rank: Super Contributor
timestamp: December 02, 2016, 07:53:03 pm
content: Quote from: electrolust on November 30, 2016, 07:53:26 pmWhich is dumb anyway -- now you've got two readings and don't know which is correct!Just use one ohmmeter and call it a day. Calibrate the damned thing if you care.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: December 03, 2016, 01:36:34 am
content: Maybe on a pot. One DMM on pins 1 and 2, the other on pins 2 and 3. Then you should see the total resistance of the pot by adding both readings...Works great here!edit: as long as at least one DMM is floating, I guess!

author: joeqsmith
author_rank: Super Contributor
timestamp: December 03, 2016, 06:05:58 pm
content: Quote from: Fungus on December 01, 2016, 07:34:08 pmExcept it does not account for the current source.Quote from: tooki on December 02, 2016, 07:53:03 pmYou can say it's dumb but if I make a review people like to see how they compare with other meters.

author: 3db
author_rank: Frequent Contributor
timestamp: December 05, 2016, 03:54:36 am
content: Quote from: tooki on December 02, 2016, 07:53:03 pmNOT so dumb if they are both fed from a known voltage reference.3DB