Determining accuracy of voltmeters - Page 1
Forum section: electronics / testgear

author: electricthot
author_rank: Newbie
timestamp: February 03, 2018, 03:30:13 pm
content: I am an off/grid solar user, and depend on my solar equipment for all my power needs. I have several voltmeters, some built into my system. I also have two handheld voltmeters (cheapies) that I need to set certain values, such as bulk and float voltage set points on my charging equipment. But, every single voltmeter I use gives a different reading. Additionally, the difference itself can vary, as the voltage in my system varies, which typically can range from 12.0vdc to 15.0vdc through a 24 hour period. I would like to find one voltmeter whose accuracy I could trust. Any suggestions? Is money spent the primary answer? Or would it be brand name? Please help.

author: rstofer
author_rank: Super Contributor
timestamp: February 03, 2018, 04:01:08 pm
content: You can't expect much from a Harbor Freight $7 meter that is often a give-away item. Yes, price is an indicator of quality. So is brand name with Fluke being at the top of the list.What exactly do you want to measure and how many decimal digits do you require. For example, you probably care about DC voltage over some range. Similarly, you might care about AC voltage over a range. You probably don't carry about microamps either AC or DC.Displayed digits is somehow related to what is called 'count'. A 9000 count meter can display 8.999 whereas a 50000 count meter can display 4.9999. You're gaining resolution. 120.0 versus 120.00 - that extra digit comes in handy.When you have figured out what range you want to measure, you will also have some idea of the % accuracy required in that range. Then it's just a matter of picking a meter and reading the datasheet. Look at the long term accuracy, you won't be sending it out for calibration every 30 days.Here is a typical datasheet (Fluke 179). On page 12 they list the accuracy for various measurements. Note that AC Volts is a 1% kind of measurement while DC Volts is 0.09%.http://assets.fluke.com/manuals/175_____umeng0000.pdfThe Fluke 179 is a reasonably priced meter (compared to the 289) but 35 times the cost of that hypothetical Harbor Freight meter:https://www.amazon.com/Fluke-EDA2-Industrial-Electronics-Multimeter/dp/B01K4GSQMS/ref=sr_1_fkmr1_1Dave (our host) sells a rebadged Brymen BM235 and it's a pretty decent meter:http://www.eevblog.com/files/BM235-Manual-EEVblog.pdfSee around page 24...You can compare the specs for yourself. The Brymen is good but it's no Fluke!Notice also that in the Fluke document, when they discuss a scale, they show you exactly how many decimal digits you will get.Then you can chase CAT ratings down a rathole. The 179 is rated for CAT IV 600V which is probably adequate. The more important part is that the ratings are believable. The cheap meters will have similar specs but, upon review of the internals, no reason to believe them.There is a lot of information in the stickies above.Price matters, brand matters, intended application matters. A meter targeted at electricians may have different ranges and accuracies than one aimed at EEs.

author: Kean
author_rank: Unknown
timestamp: February 03, 2018, 04:47:09 pm
content: You could buy yourself a voltage reference. From your usage description I don't think you need high precision, you just want to get a feeling for "correctness".Something like the DMM Check Plus or PentaRef:http://www.voltagestandard.com/Or for a cheap one from China search ebay for "AD584" - the ones in the clear acrylic cases are nice. These are often using second hand parts, but again good enough for your application. Just don't believe the "calibration" values to 4 decimal places on the side/back.

author: phil from seattle
author_rank: Super Contributor
timestamp: February 03, 2018, 07:29:58 pm
content: I think the key question is how much accuracy do you need? It's been my experience that even the cheapo DMMs aren't THAT far off. Maybe 1% though I'm sure there are some that are worse. For the kind of stuff you do - maintaining your solar setup - do you need 0.01% accuracy? I would certainly test the DMMs you have - a fresh battery is a good starting point. If bothof your DMMs read close then it's a good chance they aren't terribly out of whack. Then I'd use one of them to verify your built in voltmeters. Keep good notes. You can get super accurate references but only you can judge whether the cost is worth it - they aren't cheap.By the way, there are a lot of other reasons to buy a good quality DMM - features, safety, ergonomics. That's been written about extensively in this forum and elsewhere so I'll not duplicate it.

author: GreyWoolfe
author_rank: Unknown
timestamp: February 04, 2018, 01:26:57 am
content: I had 2 of the Harbor Freight meters that pretty much matched each other, but both read ~240R on a resistance measurement that should of read between 15-20RCheap meters can truly give a headache but I have 2 Anengs, the 8008 and 8009 and the resistance measurement matches my 5.5 digit meter for my use. They could be a possibility for low voltage measurements with a fair bit of accuracy. Being cheap, if one fails, it is cheap to replace.