4-1/2 Green LED DC 19.999V Digital Volt Meter - JOKE ? - Page 1
Forum section: electronics / general

author: bilko
author_rank: Frequent Contributor
timestamp: May 22, 2013, 10:30:40 pm
content: I saw this on Ebay, quoted accuracy 0.2%, 4 1/2 digit voltmeter and 57 units sold already* It is suitable for the precision measurement .* Good linearity.* Small Temperature Drift.* Extremely high input impedance.* Work stable.* Adopts ultra-bright LED display.* Low Power consumption.* Over-range indication.* Decimal place is adjustable.* Common ground or Not common ground are available.* Over-range indication: "EEEE" or -EEE.* Easy install.Product NameEL8145A-G20V 4-1/2 Red LED Volt MeterPower SupplyDC5VMeasuring rangeDC -19.999V?+19.999VDisplay Type7 segment, 0.56 inchRunning current<80mASampling rate5 readings per secondOperational temperature0?+50?Relative humidity?85%RHAccuracy± 0.2% FSZero settingAutoDimensions8.2mm×26mm×22mmSize of mounting hole69.5mm×24mmMeter Weight30g* For use inIt is suitable for the precision measurement .http://www.ebay.co.uk/itm/4-1-2-Green-LED-DC-19-999V-Digital-Volt-Meter-Excellent-/180712727326?pt=LH_DefaultDomain_0&hash=item2a1351631eSo why 4 1/2 digits when its only 0.2% quoted accuracy

author: Paul Price
author_rank: Super Contributor
timestamp: May 22, 2013, 10:47:33 pm
content: Accuracy, precision, linearity and resolution all have different meanings.Sometimes having the ability to resolve so as to see values or small changes even in the slippery domain of an absolutely inaccurate reading, can be very helpful at times.

author: rolycat
author_rank: Super Contributor
timestamp: May 22, 2013, 10:48:43 pm
content: Quote from: bilko on May 22, 2013, 10:30:40 pmWell, despite the Chinglish the vendor has pretty much nailed it:Quote from: bilkoHigh precision can be useful even if the accuracy isn't great. The vendor also claims good linearity, low drift and stability, which suggest that the reliability of the readings will be good.There's a big difference between precision, accuracy and reliability - i believe Dave covered this in an early video, and there are numerous explanations on the web.

author: Paul Price
author_rank: Super Contributor
timestamp: May 22, 2013, 10:54:19 pm
content: Rolycat,Not really.Precision firstly means accuracy, the correct and non-misleading words for the vendor to use would only be "Suitable for use for high resolution."

author: c4757p
author_rank: Super Contributor
timestamp: May 22, 2013, 11:33:57 pm
content: I always learned that precision = resolution. I distinctly remember this image from high school chemistry:Not that my high school education was worth much, but that's the meaning I still see quite often to this day.

author: pickle9000
author_rank: Super Contributor
timestamp: May 22, 2013, 11:49:00 pm
content: Stability is the real issue at that resolution, I doubt it be stable over time (days weeks months) for that price.

author: Paul Price
author_rank: Super Contributor
timestamp: May 23, 2013, 12:08:22 am
content: Stability can be definitely not the issue in most cases where resolution is more important than accuracy, where some fast or tiny change might need to be noticed in a short period of time of observation.I have on many occasions had the need to measure the drift or level of the tiny millivolt or microvolt offset of an opamp or adj. something to an exact setting relative to a reference. If I was to check with high accuracy a power supply that put out 5.000000 volts and I needed to compare it to a 5-V standard, I would require only an instrument that would reveal the difference, not at all concerned with the accurate magnitude in making this galvanomic measurement.I would be reasonably certain, at a fixed temperature, that the readings would not drift more than a last digit or so over a few minutes or possibly even hours of observational time, even though the absolute reading maybe lacking in accuracy, but not in resolution nor short or long term stability. I only say this with some confidence because at one time in my illustrious career I was a Senior Metrologist and have spent many an hour qualifying and calibrating all types measuring equipment according to their specs. But only hooking the instrument up to a stable, but not necessarily accurate input for some time could really verify my guess. I have not used this instrument.

author: Paul Price
author_rank: Super Contributor
timestamp: May 23, 2013, 12:13:15 am
content: C4757What I see in your diagrams seems a little confused between precise and what could better be called repeatability of readings.The scatter of the slightly off center hits show good repeatability and this is an important measure by itself of an instrument.The problem here is that some dictionaries equate precision with accuracy by definition. These two concepts should not be confused.

author: rolycat
author_rank: Super Contributor
timestamp: May 23, 2013, 08:57:38 am
content: Paul,First you state that precision and accuracy are the same:Quote from: Paul Price on May 22, 2013, 10:54:19 pmThen you say that they are not:Quote from: Paul Price on May 23, 2013, 12:13:15 amI think you may be the one who is confusedPart of the problem may be that there seem to be several slightly different uses of the term 'precision'.In computer science it means the detail with which a quantity is expressed - the resolution.In engineering or physics it means the reproducibility or repeatability of a measurement - as neatly illustrated by c4757p's diagrams.And in common usage it is often equated with accuracy...Just to confuse matters further, ISO5725-1 defines accuracy as being composed of both 'precision' and 'trueness', but personally I think they are full of it

author: Unknown
author_rank: Unknown
timestamp: May 23, 2013, 09:58:26 am
content: Quote from: bilko on May 22, 2013, 10:30:40 pmPerhaps because it has an adjustable decimal place you could use it with a shunt; then move the decimal place to measure currents larger than 19.999.Or maybe you could add some resistors and attenuate the input voltage to measure higher than 19.999.Because it says: "It is suitable for the precision measurement ." but doesn't give any real facts about the accuracy or chipset etc. I'd be inclined to perhaps print-out some of c4757p targets, then just throw these modules at the targets to see what precision or accuracy I could achieve ;-)

author: T4P
author_rank: Super Contributor
timestamp: May 23, 2013, 10:07:21 am
content: F87V has a accuracy rating of 0.05% +/-10counts on 4-1/2" modeDoes that make it a joke? OF COURSE NOT.If it's precision you need that's what it will do, accuracy over time probably not.

author: Paul Price
author_rank: Super Contributor
timestamp: May 23, 2013, 10:12:38 am
content: RolyCat,I am not confused, I saw this definition in a dictionary, to point out how these words become used and create confusion when using them to describe the accuracy of measurement.

author: Fsck
author_rank: Super Contributor
timestamp: May 23, 2013, 10:28:24 am
content: This is the idea I think of when I compare accuracy and precision, but I'm more scientist than engineer.

author: amyk
author_rank: Super Contributor
timestamp: May 23, 2013, 11:26:50 am
content: I wonder what type of voltage reference it uses... looks like an all-in-one ADC chip but I can't see the part number.

author: bilko
author_rank: Frequent Contributor
timestamp: May 23, 2013, 11:57:55 am
content: Quote from: T4P on May 23, 2013, 10:07:21 amQuoting 4 1/2 digits at 0.2% is a JOKE because19.999V X.02% = .0399VThis means that the 2 LSD are nonsense, that's the joke !!The absolute voltage accuracy of the module can be calibrated out at one particular voltage level.No figures are given for linearity, stability or resolution.The module could use an 8 or 10 bit AD Converter, this is again nonsense with a 4 1/2 digit meter

author: bilko
author_rank: Frequent Contributor
timestamp: May 23, 2013, 12:03:20 pm
content: Quote from: amyk on May 23, 2013, 11:26:50 amSome of the cheap meters use the A/D converter of a PIC chip or similar, its nonsense to derive 4 1/2 digit accuracy from these devices.I think to establish any credibility for these meters, the manufacturer or seller should quote the chip used.

author: rolycat
author_rank: Super Contributor
timestamp: May 23, 2013, 12:56:32 pm
content: Quote from: bilko on May 23, 2013, 12:03:20 pmAs many people have pointed out, 4 1/2 digit resolution has nothing to do with accuracy, and there may be perfectly valid reasons for using such a meter.Whether you would trustanyof the specifications claimed by this seller is another question.QuoteCredibility? From a Chinese eBay widget-shifter? Not their strongest suit, I'm afraid.

author: Unknown
author_rank: Unknown
timestamp: May 23, 2013, 01:16:24 pm
content: From the Ebay site; if you hit the image host site with the url without the image size params, you can get the natural size of the image (slow image server!)hk3.image.pushauction.com/ViewPicture.aspx?Key=c2f5261f-1ad6-4802-a127-60f55de3097b_f53dfb35-da9f-4e8f-8f8f-c86da27c7de1I think it's a near all in one solution, like this one; if you do a parametric search on: 28 pin, 4 1/2 digit, 20000 count etc.http://www.gme.cz/dokumentace/320/320-038/dsh.320-038.1.pdf"widget-shifter" LOL- I like that! ;-)

author: madires
author_rank: Super Contributor
timestamp: May 23, 2013, 01:33:34 pm
content: Quote from: jucole on May 23, 2013, 01:16:24 pmAt the top right between the orange cap and the precision trimmer there's a nice TL431 :-)