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Old 06-20-2013, 06:43 AM
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DaveInDenver DaveInDenver is offline
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Join Date: Jun 2006
Location: Larimer County
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Originally Posted by Rzeppa View Post
I have yet to encounter a modern DMM that isn't accurate enough for general automotive applications. Even the little $4 jobs that HFT sells. They work just fine for our automotive applications. We're not measuring cold fusion here.
Yes, exactly. My point, apparently poorly stated, was that most portable DMMs are not sufficient to measure the true resistance of a large size wire. It's just beyond their design criteria, even though a $4 ICL7106-based meter can be quite good, you are just at the limit when you are operating your ADC at the LSB range on the V x T curve with budget-minded op-amps. I also said that it does not matter for this because as you have followed proper engineering principles when you sized the wire, so you are just validating your workmanship with the meter.

The Fluke 87 I carry considers conductivity for purposes of making the little beeper beep anything up to about 100 Ω, so in the same vein no reason to be hypercritically worried about 5 decimal places and resolution. The only reason I even mentioned it was to point out limitations with DMMs at the edges of ranges. With 3-1/2 digits on a 200 or 400 Ω scale there is a lot of error for measurements lower than a few ohms and certainly so for less than 100 mΩ or so, both due to resolution and accuracy.

'91 Pickup - Imelda
'08 Tacoma TRD - Donna

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