Multimeters

images wrote:
I should not have assumed all these years the readings I have been getting were pretty accurate because it was from a digital voltmeter.

The digital part only applies to how it displays information, not how accurate the internal electronics are. It is deceiving.

I just installed 2 of the exact same digital VMs in my RV and off the same power source they read 0.2 apart.

I would believe the Fluke meter. John M. Fluke made some good meters.

Depends on the accuracy of the particular instrument and if it is in calibration. For really accurate work they are periodically re-calibrated and certified.:)

From downloaded manuals, the AC voltage accuracy spec of the HF meters is probably either 1.2%+5D or 1.2%+10D. That's either about 1.94 volts or 2.44 volts. Just for curiosity I tested a few HF meters I have lying around.

My most trusted meter varied 123.0 to 123.2 during the testing.
A low-end Sears clamp meter varied 122.6 to 122.8 (about 1/3% difference).
HF #1 - 123.1 to 123.6
HF #2 - 123.1 to 123.8
HF #3 - 121.6 to 122.0
HF #4 - 123.0 to 123.4
HF #5 - 121.4 to 122.5

So if one assumes the 'trusted' meter is correct, the HF meters are all within their spec. It would take a reading below about 121.0 to be out-of-spec. The 4 volt spread the OP saw does look bad.

I've worked in electronics manufacturing for 30 years. Every multi meter we have is sent to our calibration lab once a year for checking and adjustment. I don't have the exact numbers, but recalling from my memory, I believe about 5-10% of them would be out of tolerance and have to be adjusted.

Normally, the variation is fine for 120VAC or 12VDC measurements, but can be a big issue of for very small voltage, resistance, current or capacitance when we were working on circuit cards and components with very stringent tolerances.

By the way, the units that we used to check calibration are calibrated to National Institute of Standards and Technology (NIST) standards. These calibration tools can cost tens of thousands of $$ each.

As mentioned above, the Fluke meters were some of the most reliable we had, and rarely went out of calibration tolerance.

Hey, I'm all for proper calibration, and I'll use a Fluke before I use the Harbor Freight meter (though I have a HF meter that I keep in the RV)...but I'm not all that concerned about 2-3 volt difference....in the day to day stuff.

Yes, low voltage would be an issue for the A/C or some other components in the RV, but I feel that a quick check with the meter is good enough. Now, if the meter is showing a difference in excess of 10VAC at a campsite, I'll be worried.

As someone mentioned, it's really when you are working on tight clearances and low voltages that these numbers matter.
The the average RVer, any meter is better than no meter! And as with most things, try and get the best quality you can afford....it will be worth it in the end:)

I have a Fluke left over from when I had a job. I use it as the benchmark for calibrating my other meters like a lighted volt meter that reads 3 volts low. And my Kill-a-watt that I leave plugged in for voltage display and it reads nearly exactly what the Fluke reads. Unfortunately it isn't lighted.

How accurate the various meters are depends not only on the quality of the mfg, but the capabilities of the meter. Is it average responding rms calibrated, peak responding RMS calibrated? Is it true RMS reading?? etc. And how pure is the sine wave voltage source your measuring. Anymore, the line voltage coming into your house isn't as "clean" a 120V source of years ago, with all the stuff that draws power at the peak of the waveform.

So a true rms meter with good electronics will be able to get the real RMS value of the line voltage. the "average responding or peak responding RMS calibrated" won't, they ASSUME a PURE sine wave with NO harmonics.

Unless the FLUKE is VERY old, more than 20 or so, it is very likely a true rms meter with good electronics and a good RMS converter. Even if it is that old and not calibrated for a long time, it could be giving you the "most correct" AC voltage reading. Greenlee??? don't know. Harbor Freight, well there are cheap rms converter chips out there, but they really aren't that accurate.

Just because it's Digital doesn't mean anything about accuracy. There is digital junk out there and analog junk out there.

But, in reality, for RV applications, I'd say any of the three will give you the info you need.

Next measure your battery voltage. wouldn't be a bit suprised if you also get 3 different answers.

Personally all my test equipment is either Fluke or Tektronix. All NIST calibrated shortly before I retired. I know it's accurate and I know it's rugged and reliable. Equipment from either while it may have a 1year cal cycle, is in reality very very stable and often will be in spec for years or decades if you treat it right.

WyoTraveler wrote:
I would believe the Fluke meter. John M. Fluke made some good meters.

Well you know what they say:

"If it Works! It's a Fluke"

:)

I have a Fluke 77 that is nearly 30 years old. I've used it on and off numerous jobs over that span. Don't know how I'd get by without it. Best part is it was free. My brother, a now retired major tech company exec, gave a talk at Fluke in the mid 80's and they gave it to him as a token of their appreciation. He had no use for it so gave it to me.