DIY DMM (Multimeter) Voltage Calibration (Centech P37773/Mastech M9508) PART ONE
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- Опубліковано 4 чер 2013
- WARNING: Experts will tell you this is a waste of time, as low quality DMM units (such as the one presented in this video) are not accurate enough to even bother calibrating and not accurate enough to use to calibrate other meters. They are probably right. There is definitely an over-the-top approach that is the textbook correct way to do anything, and then there is the irresponsible and maybe even negligent approach to do anything. This falls somewhere in the middle.
In this video, I show you how I "calibrated" the voltage input reading on my Harbor Freight CenTech P37772 (aka Mastech M9508). I put calibrated in quotes because 37 out of 39 experts will agree that this is an exercise in futility as a $15 digital multimeter can't really be calibrated. Full disclosure, here is the documented accuracy specs for this multimeter:
Range ---- Resolution ---- Accuracy
200mV ---- 0.1mV ---- ± 0.5% of rdg ± 1 digit
2V ---- 1mV ---- ± 0.5% of rdg ± 1 digit ← What I was using.
20V ---- 10mV ---- ± 0.5% of rdg ± 1 digit
200V ---- 100mV ---- ± 0.5% of rdg ± 1 digit
1000V ---- 1V ---- ± 0.5% of rdg ± 2 digits
Now that that's out of the way, let's get to it!
So, as demonstrated in the video, I used a brand new pack of Duracell AA batteries as a reference voltage source. There are much more professional ways to get this done, but lots of folks seem to agree that an un-loaded brand new Duracell AA battery should read around 1.64 volts (this is still the "unknown" and the most likely challenge with this process). EDIT: I later found the reference voltage to be more like 1.619. See PART TWO. With that information, I check the voltage using the multimeter, and then show you how I took it apart and adjusted the calibration using the variable resistance potentiometer on the circuit board. In the end, I end up with what I personally consider a "tuned" multimeter. It's good enough for what I use it for, and I would argue that it is as calibrated as you can get without spending money or getting access to professional equipment.
Stay tuned for the next video (PART TWO), where I will share the results of how close I got. I'll take one of those reference batteries and check it against a couple of Fluke DMMs to see how the readings compare, and will let you know what I come up with.
Oh...and the most important thing to learn from this...
Man with one watch always knows the time, man with two watches never quite sure. ; )
Comments and feedback is always welcome. If you have better ideas, please share!
I have three ajustable R in my cheapo multimeter.
To know (instead of guess) which one is for voltage, parallel a 1k R on the ajustable R and if the reading changes, that's it.
You can buy a precision reference for some $, look up LM4132A for example (A is for 0.05% accuracy).
You can buy a high tolerance resistors accurate to within one tenth of a percent or even greater for calibrating the ohmmeter part of it also
Thanks for the very informative video. Have you tried checking the voltage on a new Duracell AA battery over time? The 1.64 volt reference that you mentioned might apply to brand new Duracell batteries. The voltage appears to drift down over time. I bought a new package of Duracell AA batteries in March 2022. The date marking on the batteries was March 2031. The voltage of a selected cell was measured with five different digital multimeters. The readings ranged from 1.610v to 1.621v.
The same battery was checked a little over one month later. Four of the meters showed a drop of 2 millivolts and the fifth meter showed a drop of 3 millivolts. A package of unused Duracell batteries purchased three years ago was also checked. Those batteries measured 1.57 volts. The difference between 1.64v and 1.57v is 0.07 volts or 70 millivolts. I plan to continue the measurements over the next few months. To minimize variations I take the readings at approximately the same temperature each time.
If the nominal 1.64 V is, in fact, 1.640 (you can't assume it is), then 1.635 V is off by only 0.3%, which is well within the listed specs.
Hi, you happen to know how to adjust the value 0 in the resistance measurement, it happens that I change the cables and now I have a difference of several ohms.
My multimeter only has VR1 and VR2 as tuning options.
Thank you.
Hola, por casualidad sabes como ajustar el valor 0 en la medicion de resistencia, sucede que cambie los cables y ahora tengo una diferencia de varios ohmios.
Mi multimetro solo tiene VR1 y VR2 como opciones de ajuste.
Gracias.
I appreciate the vid! My thinking: To figure out what pot effects what without actually changing any you could use a high value resistor and jump from one side of a given pot to the wiper terminal. It seems a better process would be for someone to create a matrix of pot number vs effected range setting. With this information one could determine exactly what pot to turn for a given range. For instance VI might change the cal of the readout and thus effect all ranges - not what is wanted. In a given cal one needs to pay attention to both range and scale and be sure what they are adjusting.
Suspected low voltage to an outlet this past weekend so I tested it and it was only 112vac. Tested other outlets and got same results. Tested breaker box - only 112vac! Called the power company to report the issue. Tech came out and tested. 121vac using 2 diff meters. We then compared his results with mine. My meter is 10 volts off!! Embarrassing! Decided to see if it was possible to calibrate it before tossing it and found your video. Thank you for taking the time to create/post your vid.
8/12/2016 checked 3 new durcell aa alkaline and they read consistently 1.61 volts not
1.64, believe depends on batch and manufacturer.
Something else. When you're on ohms, and touch the probes together, you SHOULD read a small value of resistance. That's the R of the leads. Some meters, like my Fluke 87III, have a "relative" feature that let's you null out that resistance, but most do not.
Anyone find out which pot adjusts the ohm setting?
The DVM does not calibrate itself when the leads are shorted out in ohm position. Shorting the leads is a self-test to confirm your leads and meter are working. It is just reading the resistance thru the leads, which should be close to 0, or a few tenths of ohms.
Thanks for the info brother, I used an AD584 calibration standard that has known calibrations of 2.499,4.999,7.500, and 10.000 dc volts and I used the VR1 resistor pot to adjust mine and it worked great across the complete line. This was an 25 year old Craftsman 82175 that was made by Mastech. It was off about 20-35 mV on all settings but now spot on. That standard also has a 100 ohm, 1K ohm, 10K ohm, and a 100K ohm known at is was also accurate with these. I have a fluke 117 also, but thought this might help someone with similar older device. Usually the VR1 is the correct one, but I'm not an EE so do any of this at your own risk, I am not responsible if you screw up your meter or worse. And of coursed don't do it if under warranty.
a good reference is a atx 3.3v rail ( it is always between 3.33---3.30) or a raspberry pi 3.30v stabilized rail.i used the one in the raspberry pi to calibrate my cheap multimeter
What CAN happen on the cheapies is variance between the 200mV 2V 20V and 200V ranges. Generally only one pot is provided for DCV and if you try and correct a meter that is reading 12.4V when it should be 12.6V, you succeed only to find that the 2V range you use on torch cells that was perfect... is now high. Which sucks.
The solution for that is to zero out the multi-meter on each scale. Sadly cheap didgitals don´t have this feature. Hapily, cheap analog ones do.
thanks for this. does it matter? It can be in the case of 12V lead acid batteries where there is a big difference between 12.4 and 12.6V. 12.4V is underpar and may struggle to start eg a motor bike, 12.6V is a sound battery. I have had my cheapo MM for a very long time it claimed at new .08% accuracy, but over the years seem to have drifted way off - a new Duracell battery is 1.547! as were all 4 were in the pack. The batterys were dated to expire in 2029 ( today is 2020)
I had no idea! I have 4 dmm one of them being an old 15 yr old fluke, I'm curious to see how off it is
Yeah, but won't the AA's get a little difference from the time they have been on the shelter and temperature they have been stored and all that kind of stuff.
Also changing VR1 isn't there a chance other values like reading resistance or current might be changed???
I have a crappy multimeter, just like your yellow one, but in red, and wanted to calibrate it as much as I can, because I don't need much accuracy.
0.05V failure on low voltage for me is fine.
Which one pot is for adjusting the temperature sensor?
Thanks man, it is just what i was looking for.
how about a 5.0v cellphone charger as reference?
По-вашему, батарейка это образцовый источник напряжения?
sweet! thanks for the feedback!
very comprehensive research
no problem. glad it helped!
Just to mention that after all, the problem was the low battery of my multy meter... the 9V battery was dieing... it became very hypersensitive with its readings cause of this.
Este tutorial en español, no salen los subtítulos traducidos
Calibration costs a minimum of about $200 for a basic DMM. Cost can be substantially lower under annual contract of many instruments. Probably better to buy a new DMM of the same or better accuracy every once in a while to confirm the approximate calibration state of your DMMs.
Calibration should be done using a standard/meter that is 4x more accurate or better than the product being calibrated.
Thanks for the feedback.
thank you!
I have one of these and the continuity buzzer does not buzz...any fix for this?
I would just exchange it for another.... No telling what is wrong, but I personally wouldn't waste time trying to figure it out.
Check if the wire going to the buzzer got disconnected and needs to be resoldered
This is a method of "How to UNcalibrate a multimeter". A 1/1000 part of a volt does not metter in real world. You calibrate at a 1.xxx volt "reference". If you measure 500 volts you will expect to read the last milivolt?
Thanks for a great article.Keep on tinkering. I will exchange the unit and take a new battery and get a 1.619 reading.
I have this same DVM, but recently my display reads only half of the numbers. Meaning the top portion of the numbers are showing and the bottom is not. Any ideas way this all of a sudden happened ?
Not sure...sounds like your display went bad. I don't know of any easy solution.
Maybe check if the ribbon is seated well??? Sounds like that's what it would be cause of the rotating display.
Yeah I checked that already. II think the heat from being in my car must' have delaminated it Ill figure it out. Ill look again to see if its well seated. Thanks
I got rid of it and bought a Fluke,
if touching them together set to ohms does not come out to 0, disconnect 1 probe and stick the tip of the other probe and touch it in where the one you removed connected the do the same thing on the other side with the other probe. If one comes out to.
That yellow meter is the contacts on the probes. They are coated with something that makes it not make good contact.
DC voltage is 0.5% accuracy rated.
~0.082V
The battery in the meter will tell you when it's low. It being new doesn't always mean good. Heavy Duty batteries (forgot what the proper name is) don't have a shelf life like the alkalines.
When you take a meter apart be careful with some because the function knob has tiny metal balls to make it have the clicking feel
Go to your electronics store and measure something with the display models. If they don't turn on tell them you want to look at the meter and ask them to put batteries in it then act like you just happen to have what you want to meter for reference in you pocket , measure it and tell them you are going to think about it and you will be back to make purchase
Where the other units and ranges affected?
I couldn't say 100% for sure, but I haven't found any adverse effects in other areas. There are other adjustment points, so I can only assume that perhaps other units are adjusted using those...
Sure calibration of any DMM may prove futile.
But it is the wise man that knows the relative accuracy of the DMM in use.
Of course for maybe 95% of applications, accuracy of better than 5% will rarely be needed.
Just did my Cen-Tech harbor freight style, i guessed and got lucky also it was VR1. Thanks Alot
My meter is identical except i have the P61593
I am sure if you try to measure the same battery with the same multimeter at another room temperature you will obtain another result. If your meter fall from your banch, goodbye calibration.
"Reference Batteries"? You need to use a Reference Voltage.
Atx power supplies are not a safe option. They vary on loads.
very interesting... being a tinkerer, myself...
My right ear enjoyed your intro
A man after my own heart.Dont this But I’m going to anyway.Tinker thinker.
I do have a reliable reference. A US made Tektronix 4 4/5 digit, 50,000 count .05% basic DC accuracy and a NIST traceability certificate to back it up. After you get past the garbage leads, lack of input protection and cheap rotary switch, I have found mine to be pretty darn close. The Intersil clone chip that is the heart of the meter is used in meters that cost a lot more. As far as only one calibration pot goes, that is sufficient if the resistors in the voltage divider are reasonably accurate. Accuracy is not the biggest problem. it is good enough for most home users. It is the things I have mentioned, above. If you subtract the lead resistance on the low ohms range it will get you close. On the higher ranges it does not matter. It is also only 1 megohm input impedance. Better meters are 10 megohms. When used on a high impedance circuit it can cause loading and give a false reading. This meter is fine for checking batteries, usb voltage or the presence of AC it has a not often seen, transistor checker, that does work. People keep them in the glove box or other places where a quick check is needed. If you paid more than zero for yours, you got ripped off. Harbor Freight often gives them away with any purchase.
get a proper voltage reference, man ! atleast something like lm385 diode, that will give u 1% tolerance.
I don't understand why you bothered for 5 milivolts? Couldn't you at least borrowed someone's calibrated meter for a reference instead of using batteries for a reference? The meter could have been closer before you started and you wouldn't have even known... Batteries are an unknown value no matter what someone says a new battery should be. This was not thought out at all. Poorly done and metrology fail!
Sony guts.
🤦♂️
even worse than my $5 dollars meter
No disrespect here at all but you sound like kermit the frog
don't try this at home pmsl.
wrong, wrong and... wrong again. a new brand battery have over 1.5v
Everybody must be wrong, and you are right. Congratulations!
@@EVERYTHINGcpo you cant use battery as ref voltage, try to use a 7805
mate u r full of it!!!! get to the point dude, no wonder everyone hates a sparky??? its not rocket surgery