Harbor Freight (and other?) Voltmeters issues

[e9999]

I've been inordinately fond of my HF voltmeter in the past. Especially after I realized that I could buy 4 of them for the cost of one replacement fuse for my Fluke. Always amazed me that it is actually possible to make a device like that for something like the $1 or $2 that it must need to cost for them to sell it at $2.99... How can they possibly do that...? Incredible... I like that I can throw this thing around without worrying about breaking it.


anyway, today I was futzing around with an old battery that's going south and measuring voltages when I had an odd feeling about the VM. Not sure why, the numbers looked OK, but it seemed it hiccuped or took a bit longer to stabilize than I would have liked. Anyway, I went to get the Fluke which does not normally lower itself to measuring such mundane things as a truck battery voltage, and sure enough there was a huge difference, like the HF showing 14.4 V and the Fluke 13V. Less difference at lower levels, like 6.8V vs 7.2 or something like that. Still way too much.

So, of course, I don't know for sure which one is bad, but the betting man in me would say it's more likely that the $2.99 one is bad rather than the $300 Fluke. Well, at least I hope so . No trauma on the HF that I recall, except I left it outside for the last 3 or 4 nights and it was a bit humid.

Anyway, yet another painful reminder than one can't completely trust electronics even though one tends to do so when using them all the time... Yikes! Basically, you need to have 3 of everything to be reasonably sure they're reading OK...

 

[porten1134]

check the battery(s) in the fluke, thats about the only thing that could go wrong with it. unless you've dropped down an elevator shaft or dropped it in water or something.

there is a certain charm to the disposable tool (HF), but after 10 years of electrical troubleshooting with one on a professional level......trust the fluke meter!!! and if there is a doubt with the fluke, you've got a lifetime replacement!!! i would kinda have a hard time throwing down 1-5 hundred bucks to have a garage tester(there is the charm in the HF/disposable tool aspect) but if you've got the baddest electrical metering/testing device on the market today, i'd have to put my trust in it!

 

[fj40charles]

I'd find better ways to spend my money than getting POS meters at HF. If you want cheap, get one from Sears for $12 when they're on sale. If you want quality and accurate, then buy a Fluke.

 

[e9999]

I should have said that when I got the HF, I did check it against the Fluke and they were within something like 0.05V or less IIRC of each other at 12V, so the HF was not bad from the start, just likely went south with time or some event. But after a while, so not bad at all for $2.99. And I can likely exchange it at HF for a new one at no cost anyway... Forgive me but I think they're great for truck tool box usage, as in is there a break in the line someplace or is the battery dead kind of thing...

Didn't know about the Fluke lifetime replacement. Good to know. This one is a true RMS with temp measurement, storage and all. Good stuff. (Dang expensive fuses, though.... watch those post amp measurement brain farts... )

 

[Pin_Head]

All meters are a load on the circuit they are measuring and alter the electrical properties of the circuit according to Ohm's law. Good, expenisive meters are more sensitive (measured in ohms per volt and more = better) and put less of a load on the circuit.

Good meters are also more precise and use resistors with a precision of +/- 1% or less, while cheap meters use 5% precision resistors.

The HF meter didn't change. The discharged battery just has a steep I/V curve. Compare their readings on a good battery and they will probably be much closer.

 

[e9999]

All meters are a load on the circuit they are measuring and alter the electrical properties of the circuit according to Ohm's law. Good, expenisive meters are more sensitive (measured in ohms per volt and more = better) and put less of a load on the circuit.

Good meters are also more precise and use resistors with a precision of +/- 1% or less, while cheap meters use 5% precision resistors.

The HF meter didn't change. The discharged battery just has a steep I/V curve. Compare their readings on a good battery and they will probably be much closer.

interesting point, I'll look into that. thanks.

 

[Bret]

Any reason to not measure at the same time. Seems a little like a stretch that a load in the Megs would load a 12v battery but stranger things have happened. If it does the Fluke should show it too.

 

[e9999]

this is odd.

I just measured it again on another battery and this time the readings are within 0.02V of each other at around 13.6V whether measured at the same time or not.

This is a good battery (motorcycle). Don't know if that is the reason per PH post. Or if the HF multimeter recovered somehow (drier, warmer, colder? who knows).

I always assumed that a VM would not affect the rest of the circuit significantly at all...

I'll run both on the bad battery at the same time to see if there is a difference or not, I think I may have done it separately earlier.

 

[porten1134]

depending on how the meters are built, you could be somehow measuring through the other meter as well. this might alter the readings somewhat depending on how much voltage the other meter uses to make its readings, or on what lead is where.i'd stick to the one meter at a time. also the battery may read 12v or whatever when there is no load on it, but may not produce the proper amperage under a load......like a slightly drained battery may run the stereo or lights but may not have enough amps to crank the starter.

my experience with 12v automotive systems is fairly limited. most everything i did was an ac system.but in theory it's all based on the same principals. i'd stick by the flukes readings (with a fresh battery(s) in the fluke) and make sure your reading the same way on the same settings every time. use the same lead from the meter on the same terminal on the battery, make sure the meter is set on the same setting every time....that kinda thing. everything being everything which lead is on which terminal shouldn't matter too much, but if there is a difference and your trying to find out whats going on try to eliminate as many as the variables as possible by doing everything the same way.

rechargable batteries will behave differently than disposable types. lead acid (car batteries), ni-cad (cordless tools) and other re-chargables will give you different readings depending on the conditions. a lot of times if you let one sit for a while even after use things will....well, settle down inside the battery just enough to let some of the little electrons go back to the plates.its not re-charged at all, but will give you a smidgeon more power (wattage) for a few seconds. you can drain a cordless tool out, remove it and let it sit for a half hour or so and stick it back in the tool and it'll give you a little more power for a second ot two.
not that i would assume you are, but if the battery is still hooked up to the car or motorcycle you could get different readings. temperature will effect the readings as well....heat = resistance, resistance = less power.

the meters themselves should be fairly consistant, but like pin-head stated the components have tolerances that may effect the readings. the cheaper the tool the bigger the difference may be. the meters may have different settings for different systems....usually a dial or something and on it there may be a settting for 1-12v, <120V, 120v+,or whatever.if its on the wrong setting the measurements will be different.

i'm just throwing this out there......not trying to assume you dont know what your doing. i think if you eliminate most of the outside factors and measure the same way consistantly things should be fairly close.still, my money is the fluke with a fresh battery in it. good luck boss, wish i could be more helpful.

 

[Pin_Head]

The meter discrepancy has more to do with differences in the meter sensitivity than tolerances. Tolerances will effect accuracy. Sensitivity means that the meter can be used to accurately measure voltages in low power circuits (like TTL) because its impedance is so high it does not significantly alter the circuit. Good meters have a sensitivity of 100K ohms per volt, while the cheap meter may be more like 10K. The meters will read the same on high current circuits, like a good battery, but give very different readings on a low current circuit, like a discharged battery.

Always trust the more sensitive meter. The reason that sensitive meters are much more money is they use high sensitivity op amps and high resistance precision resistors.

 

[e9999]

well, I went back and did some more checking.
Strangely, the 2 meters show now very close readings, within 0.05V whether at 6V with the bad battery or a 12V good one, whether used singly or together.
So I don't think the problem was the bad battery, which would have amazed me.
No idea what caused this though, maybe a bad connection in the HF someplace? Maybe intermittent? Maybe cuz a tad wet?
So back to using it for easy measurements, but now I'll double check things with the Fluke every so often. Lesson learned...

I have to say though, that using a $3 meter for an unknown level amp reading, possibly above the max amp level, before putting the $300 Fluke on is good peace of mind. I like that.