DIY DMM (Multimeter) Voltage Calibration (Centech P37773/Mastech M9508) PART ONE

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!