Multimeter issue: inaccurate readings of higher resistances (or dud resisitors)

DarkDeepRed

Member
2016-01-04 10:50 pm
Hello all,

I recently received a bundle of new resistors, of values ranging from 1 Ohm to 10 Meg Ohm.

I went through the bundle testing them with my multimeter to make sure everything was as it should be. I started with the lower values and worked up.

Up to around 500K, the readings seemed to be more or less as expected, although I'm dubious about the stated 1% tolerance.

From 1 Meg to 10 Meg however, the readings are way off the stated values; 1M are reading around 600k, 1M5 reading around 900K, 2M reading around 1M etc. until you get to 10M reading around 2 or 3 Meg.

The meter is a Fluke 87. The 9v battery measures 8.2V (I don't have a new one here to swap it with).

Is it the meter or bad resistors, and why are only the high value readings off?

Thanks in advance :)
 

DarkDeepRed

Member
2016-01-04 10:50 pm
Measure several known accurate, but smaller resistors in series, and compare that result with a single resistor
of about the same value.

Good idea. 4 x 470K = ~1 Meg. Hmm. The meter hasn't been calibrated for as long as I can remember, but I assumed calibration wasn't the issue as the lower values read OK. Maybe I was wrong about that.
 

rayma

Member
2011-04-29 8:37 pm
Good idea. 4 x 470K = ~1 Meg. Hmm. The meter hasn't been calibrated for as long as I can remember, but I assumed calibration
wasn't the issue as the lower values read OK. Maybe I was wrong about that.

Maybe it was damaged by measuring high voltage on the Ohms setting. Fluke meters do seem to hold up well, though.
Fluke claims to give a lifetime warranty on these meters, but I haven't ever had to use the waranty.
 
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Exactly how did you perform the measurements? You may have been measuring the resistance between your left hand and your right hand - somewhere around 3M?

Tee hee hee… I've done that.

Tho' a few hours after I got my first 3rd hand VTVM (yep tubes!), I found my dry-hand body resistance to be about 1 meg or so … but clearly depended a lot on how hard I was squeezing the probes…

These days, it might not be so simple to figure out - digital ohmmeters tend to "measure and hold". So the values don't wiggle around. So there's no feedback that your finger-to-finger resistance is in parallel with the measured devices.

That's one idea.

ANOTHER is that there is some much more stable resistance in parallel with the probes of an otherwise OK multimeter. Leakage? That'd be bad. Manufacturing error - some solder smut or something giving a high impedance leakage path internally. Could be! The values that the OP reports point to a 1.5 or so MΩ leakage-path resistance.

Or it could be worse in a systematic sense: if the multimeter is one of the mass-produced, arbitrarily cheap models, it could just be that its chip is bonkers, and is internally not biased right. So, OPEN LEADS measure 5 MΩ or so.

OP: does the multimeter register infinity when the leads aren't touching anything?

OP: have you been using your fingers as clamps, you sneaky guy?

Anyway, there's the thoughts. Newbie or internal FUBAR.

GoatGuy
 

DarkDeepRed

Member
2016-01-04 10:50 pm
Exactly how did you perform the measurements? You may have been measuring the resistance between your left hand and your right hand - somewhere around 3M?
Tee hee hee… I've done that.

OP: have you been using your fingers as clamps, you sneaky guy?

Maybe.... apparently I'm a 2Meg kinda guy. The good news is that my trusty old Fluke works fine. My brain however... not so fine. :rolleyes:

Thanks for the replies everyone. I'll get my coat. :)
 
Because most of us on this forum are human, most of us made the same mistakes so we recognise the symptoms when someone else does them.

There are exceptions of course: this forum includes people who are not subject to the placebo effect, have accurate long-term acoustic memory, and whose hearing is equal to the famous 'wife in the kitchen'. Some are even subject to different laws of physics and mathematics from the mere humans like you and me!