Need to calibrate a el cheapo multimeter  - Page 1
Forum section: electronics / beginners

author: UberStrike88
author_rank: Regular Contributor
timestamp: October 12, 2014, 12:06:19 pm
content: Hi guys,i need to fix something fast and my main multimeter broke i'm looking for a second/temp one that i have a thread running about it but anyway i need something fixed fast and i found a el cheapo 830 multimeter in my garage. Something like this one:And as you can guess it is out of calibration :/So does anyone know what i can use as reference without a good multimeter.I was thinking of a usb port on my pc and set it to 5V???Does anyone know how to do it?About resistance it is easy i have some 1% here so i can base it on that.So anyone have a idea on how to do it?ThanksRegards,

author: Rerouter
author_rank: Super Contributor
timestamp: October 12, 2014, 12:21:12 pm
content: when you remove the back there is a single trim pot, this effects the voltmeter function only, there is no way that i know of to calibrate the other functions,USB ports are not an accurate reference by any measure, if you have a linear regulator chip e.g. 7805 floating around it will be much more accurate, but these meters really are the wrong place to go looking for accuracy and repeatability, when the battery voltage droops they read higher, and vary with temperature,

author: rob77
author_rank: Super Contributor
timestamp: October 12, 2014, 12:29:09 pm
content: 7805 is better than a USB port, the ones from ST (at least those i have on stock) are between 5.0 - 5.1 V.better solution would be a voltage reference - for example a LM336 (or some better reference)

author: UberStrike88
author_rank: Regular Contributor
timestamp: October 12, 2014, 12:37:49 pm
content: Can anyone give a a schematic for that (that lm circuit)? I only need to measure tiny voltages like 12V

author: rob77
author_rank: Super Contributor
timestamp: October 12, 2014, 12:50:13 pm
content: sorry, but if you can't figure out how to google the schematic for a 3 pin 7805 or 3 pin voltage reference, then it's worthless to postbtw i'm curious.... how the hell did you realize that the DMM is out of "spec" ? i'm aware of one way how to realize that - measure a KNOWN voltage - and actually once you have a known voltage , then you can "calibrate" against that voltage.

author: BillWojo
author_rank: Regular Contributor
timestamp: October 12, 2014, 12:53:36 pm
content: Do you have a car? Take it to your favorite mechanic and see if he has a decent meter. Shut the car off and see what voltage is present on the battery and adjust yours to match. Or find a repair shop and talk to the owner. Don't know where your from but over here most mechanics pride themselves with there tools. Not unusual to see good Fluke meters in there monster roll around tool box. And if they bought it from the Snap-On tool dealer, they probably paid 3X the price.BillWojo

author: UberStrike88
author_rank: Regular Contributor
timestamp: October 12, 2014, 02:40:06 pm
content: Lol i can make a it i thought just that i had to make somethinb more complicated than that xD. I know that it is out of spec because i mesured a 9v battery and it gave me a 12v reading

author: AG6QR
author_rank: Frequent Contributor
timestamp: October 12, 2014, 11:27:56 pm
content: An unreasonably high reading like that is a common symptom of a weak battery in a cheap meter. The right solution is to replace the battery.Digital meters work by generating a reference and comparing the voltage at the probes to the reference. If the reference is low, all of the measurements will be high.The reference can be generated by something like a zener fed by a resistor connected to the battery, but this only works when the battery voltage is high enough. A good meter will shut itself down when its battery is too low to give accurate readings, but some meters keep providing readings that are inaccurate. There might be some kind of indication of low battery if you know what to look for.If the reference is too low due to a dying battery, calibration will, at best work only until the battery voltage drops further, which it will soon do.

author: UberStrike88
author_rank: Regular Contributor
timestamp: October 13, 2014, 06:54:39 am
content: Quote from: AG6QR on October 12, 2014, 11:27:56 pmOhhh okay i'll buy a new 9V for it then i'll test it again.Or should i wait until it just dies with that battery?

author: tautech
author_rank: Super Contributor
timestamp: October 13, 2014, 07:11:59 am
content: Quote from: UberStrike88 on October 13, 2014, 06:54:39 amDSE (Dick Smith Electronics) in Aussie & NZ used to do a cheap DMM as a kit and I got one for my son at around age 10. The instructions on building and finished calibration were to set to the 2V scale and adjust to 1.55 V while measuring a Silver Oxide button cell. Evidently the silver oxide cell is exactly 1.55 V and adequate for cheap DMM calibration. Checked against other DMM's and a reasonable quality moving coil meter I had at the time the result was quite acceptable for basic use.

author: UberStrike88
author_rank: Regular Contributor
timestamp: October 13, 2014, 07:41:05 am
content: Hmm might try that but i have a electronc sore in the street and it has a old fluke 10 should i use it for calibration? They ask 5euro only...

author: AG6QR
author_rank: Frequent Contributor
timestamp: October 13, 2014, 03:09:46 pm
content: Quote from: UberStrike88 on October 13, 2014, 06:54:39 amI'd test it with a brand new battery right away, and see if that changes anything. If so, then the old battery is too far gone to use reliably. Any sort of "calibration" you do would be useless, because the old battery is steadily changing its voltage, and a meter with a changing voltage reference will give inconsistent readings that vary as its voltage reference changes.On the other hand, if the meter behaves the same when using a new, fresh battery, then maybe it is consistent, but wildly out of calibration.Another reason to try a fresh battery right away is to see if the display is any different. I know that, on one cheap meter I tested, its LCD displayed a box that vaguely resembled an automotive battery when the battery voltage was too low to use the meter properly. But unless you knew that this was the low battery indicator (and there was no mention of it in the instruction manual), you could continue to use the meter for a long time while the displayed voltage would get further and further away from the actual voltage.If you can get an old working Fluke 10 for a good price, I'd probably buy it and use it directly, instead of using it to calibrate an inconsistent meter. Every Fluke I've looked at has the right circuitry so that, as its battery dies, the meter stops displaying readings and shows you a "low battery" indication of some sort. You can't get it to display an inaccurate reading due to a low battery.

author: UberStrike88
author_rank: Regular Contributor
timestamp: October 13, 2014, 03:22:37 pm
content: I can't buy it they say i can borrow it for 1 hour only.But i just bought a new duracell battery tested a 9v again and it gave me 11vAnd it stays on 11.4And with a 1.5V it gives me a reading 3.8V

author: AG6QR
author_rank: Frequent Contributor
timestamp: October 13, 2014, 11:21:24 pm
content: Quote from: UberStrike88 on October 13, 2014, 03:22:37 pmOK, well that tells you that my theory about the reference voltage being low because of a dying battery is the wrong explanation for this particular situation. A low reference voltage would result in all the voltage readings being high by roughly the same percentage, and that problem would probably go away with a fresh battery.It seems like it's roughly 2.3V high on both of those example measurements. Just for curiosity's sake, what does it read when you put it on DC Volts and short the probes together? 2.3V? or 0.0V?I'm not sure what the problem is, but those readings are way out. If a meter has gotten that far out of spec, I would never be very confident in it again. Even if it were somehow brought back into spec, I would not trust it to stay in spec for long. Personally, I'd be shopping for another meter, if for no other reason than to have something to check against.

author: UberStrike88
author_rank: Regular Contributor
timestamp: October 14, 2014, 09:20:18 am
content: It gives me 2.52V :/ ill just wait until my new meter arrives thanks for all the help guys!