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What I'm worried about is that my meter may read exactly 5 volts at 5 volts but could still read 12.5 volts when it should read 12.1 volts and I'm unknowingly dealing with an almost dead battery.
Interesting. I took my boat and car multimeters (both $20 ones from Amazon) to work, and checked them against a calibrated meter. They were really close - about 50mV at 12V.
You try to collect a bunch multimeters (maybe from other sailors at the marina), and compare what the readings are. If they are all the same, then it is probably correct... works for the supreme court anyway.
If you have a known good meter that reads the voltage reference as something other than what the maker says it is supposed to be, is it your meter or the Vref thingie? How would you know?
Now, if their pretty little voltage reference device is exposed to an environment which causes it to be inaccurate, such as salt spray or just dampness on the board, what good is it?
I pay more to have my meters calibrated and certified that most people pay for their meters. I also usually have a meter that I will lend to anyone for a $10 deposit. I have checked it against a calibrated, certified meter. If you smoke the thing, I keep your $10. I get them from Harbor Freight when they are either free with a coupon or on sale for a couple of bucks. I don't get many repeat offenders.
When I get 'em, I open the critter up and adjust it until it reads correctly on the VDC scale. I usually check the current and AC volts also but I have found that if the meter is right on DC it is often close enough on the other voltage and current scales.
Buy a GOOD meter and have it certified then buy another GOOD meter. Then when you see a difference, you know there is a problem. You don't have to pay $1000.00 to get a good working meter. You can get them on ebay for $100 to $300 and pay a cal shop to make sure they are right.
I pay more to have my meters calibrated and certified that most people pay for their meters. I also usually have a meter that I will lend to anyone for a $10 deposit. I have checked it against a calibrated, certified meter. If you smoke the thing, I keep your $10. I get them from Harbor Freight when they are either free with a coupon or on sale for a couple of bucks. I don't get many repeat offenders.
So, I guess that begs the question: What has your experience been with the cheepo Harbor Freight meters? How close are they normally? I've gotten two from them. They read about .6 volts different from each other (the difference between 95% charged and 25% charged).
david, I made up something similar from scratch but using 10.00 volts as the standard. There are only so many voltage reference standards out there to work with, and I figured ten point zero zero (and yes it is guaranteed to four decimals) is a pretty good place to test the same scale that 12.6 will need.
Of course that fellow has the advantage over me, I didn't put mine into mass production.(G)
A diode and resistor will work--but not to four digits. Both have variations and while you can find resistors spec'd to 1%, 5 or 10% is more common. Plus, you're still only scaling 1.5V and that's a whole 10x off from the range you want to work with.
Since a lot of meter inaccuracy can come from the "plus or minus three LSD" side of things, you really want to see four digits, all dead on, to know how that last digit is behaving. In '12" volt use the difference between 12.30 and 12.60 is significant--but either reading would still be "the same" with normal meter float.
"What has your experience been with the cheepo Harbor Freight meters? "
heheh. I've got a couple of those HF meters and they were worth every cent I didn't pay for them. Once calibrated they seemed to work OK, but out of the box? They're about as accurate as a test lamp. "YES/NO" would be more accurate.
And then again, I also bought a pair of "Shenzen special" 4-digit 20-volt scale self-powered DC volt displays two years ago. One would say 14.3 while the other said 14.4 connected to the same power source. Once the display covers warped from sunlight I decided not to bother looking for the calibration screws.
Twelve volt systems? You know how they do auto inspections at the garage? "Horn works, try the lights. OK, it works."
There's something to be said for the $20 LCD meters that are just designed to plug into a car cigarette lighter. Simple, cheap, and often "good enough" unless you want to roll your sleeves up and really dive in.
I've had those cheap VOM meters give weird results that caused me hours of searching for problems that were in the meter itself. Bu good meters. Even the Radio Shack meters are crap. For checking a 12V battery, why not use a hydrometer?
"why not use a hydrometer? "
Ergh, because the jeans that the straydrop of acid eat a hole in might cost $50, and the jacket $100, or the shirt $25, or, hey, how did that get onto the carpet in the trunk? might cost a couple hundred more.
I own a hydrometer, but I find it way cheaper, faster, and more enjoyable to spend the money on a voltmeter and new batteries instead of new everything else. And that's not just a hydrometer, it is one of the ones with a built-in explosive penetrator so it can be used on sealed batteries as well. (VBG)
Don't need that on the voltmeter, either.
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