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I have two Multimeteres that I'm using to measure resistance of a 10 Ohm resistor. Now, when i measure with the first one it's showing 10.2 Ohms while the other is constantly changing values from 9.9 to 10.8 Ohms. Which one is more reliable?

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    \$\begingroup\$ Probably the more expensive one. \$\endgroup\$ Commented May 5, 2016 at 13:52
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    \$\begingroup\$ Reminds me of Segal's law - "A man with a watch knows what time it is. A man with two watches is never sure." \$\endgroup\$
    – W5VO
    Commented May 5, 2016 at 13:55
  • \$\begingroup\$ The more expensive one is the second one. Shouldn't the resistance be constant? \$\endgroup\$
    – lakistrike
    Commented May 5, 2016 at 13:56
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    \$\begingroup\$ When measuring low resistances like this, if you want to do it really accurately, you need to use a four-wire measurement (typically only available on bench DMMs). Which implies a much more expensive multimeter, to support Marko's comment. \$\endgroup\$
    – uint128_t
    Commented May 5, 2016 at 14:40
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    \$\begingroup\$ If you can't get a stable reading, then you either have a poor connection, or the meter is seriously broken. Fix that issue before proceeding. \$\endgroup\$
    – Dave Tweed
    Commented May 5, 2016 at 15:32

2 Answers 2

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Neither one is probably very accurate if they are inexpensive handheld multimeters.

Perhaps the battery is low or the probe sockets are loose on the one that is varying.

You can consult the manuals for your meters to know what the accuracy was when the meters were manufactured. A typical accuracy spec on an ohms range for a cheap DMM might be 0.8% + 3 counts, but you might have to short the probes and subtract off the ohms read to get that accuracy on the lowest ohms range. For example, if you short the probes and the meter reads 0.3 ohms, then a reading of 10.2 ohms on the display would be more like 9.9 ohms in reality.

Better meters will tend to hold their calibration longer. I have a Fluke handheld that is still within spec on the voltage and ohm ranges after more than 25 years.

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  • \$\begingroup\$ The one which is varying is about $40 (and I changed the battery an hour ago with the new one), while the other fits entirely in my hand. \$\endgroup\$
    – lakistrike
    Commented May 5, 2016 at 14:00
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    \$\begingroup\$ @lakistrike, $40 is a very inexpensive meter. You shouldn't expect too much from either of your meters. \$\endgroup\$
    – The Photon
    Commented May 5, 2016 at 14:40
  • \$\begingroup\$ In addition to @ThePhoton 's comment, I have a $85 meter that I can trust fairly well down to 10.0 ohms, and can use returned values down to about 5.0 ohms with a proverbial "grain of salt." Anything below that, however, would require using oxygen-protected leads (for more predictable lead resistance), and anything below, say 1.5ohms would, IMO simply require a 4-wire micro-ohmmeter to get a reliably accurate reading. \$\endgroup\$ Commented May 5, 2016 at 18:55
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    \$\begingroup\$ @RobhercKV5ROB You can often cob something together to get a 4-wire reading without buying a special instrument. If you can put about 1A through the resistance (and measure it accurately) then a cheap meter with a 199.9mV range will give a resolution of 100\$\mu\Omega\$ (up to 200m\$\Omega\$ of course). At around 20mA you would get a resolution of 5m\$\Omega\$, or about 20x better than the lowest ohms range, and it can be 4-wire so lead resistance doesn't matter. \$\endgroup\$ Commented May 5, 2016 at 18:58
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Its 3w, 10 Ohms, 5%

That means it can range from 9.5 ohms to 10.5 ohms. The meter that has an unstable reading produces an average value of 10.35 ohms and the more stable meter has a reading of 10.2 ohms.

Which one is more reliable?

A little bit of oxidization might produce an unstable reading that is also a little bit high in value so, based on your numbers, there isn't really any conclusion to make.

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  • \$\begingroup\$ Well, what multimeter would you use from now on (out of these two) for future measurment? \$\endgroup\$
    – lakistrike
    Commented May 5, 2016 at 14:36
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    \$\begingroup\$ Nothing to conclude from this data. More data required to make a judgement plus finding out why the reading was wobbling around on one of the meters. \$\endgroup\$
    – Andy aka
    Commented May 5, 2016 at 14:42

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