Multimeter accuracy and calibration confusion

As a bit of a follow up to my miliiohmeter project, I’m taking a step back to assess the standard by which I measure things. Having been schooled with a science background (Chemistry, Biology, Physics, Mathematics), the importance of good data, good results, good science is deeply ingrained in me. I believe this important in every walk of life, as an assist for critical thinking and to debunk the media’s annoying tendancy to throw meaningless statistics and skewed numbers at us to convince of whatever they want to convince us of. The tin-foil hat will however remain off tonight. I’ve had three beers after all.

Like most hobbysits, I accept my multimeter as not only the gold standard for everything I do with electronics, but it is also my eyes into what those pesky electrons are actually doing in there. Without it, the study of electronics would be horribly boring. We’d see lumps of circuitry that either did what it was supposed to, or failed in a puff of acrid smelling smoke – the reek of overload.

Recap of the Problem at Hand

In my milliohmeter project, I had reached an impasse. I created the thing to enable my multimeter to measure low resistences (>10Ω) down into the 100s of micro-ohms range since pretty much all DMMs without a dedicated function for this fail badly below 10Ω and especially below 1Ω. To make it work, I need a constant current source. I chose 1A as this made everything line up nicely. Ohm and his law states that 1V = 1A x 1Ω. My DMM, with this box in-between, would clearly read mV as mΩ. I put it together in my usual way of cobbling schematics, lots of fussing and reading.

It worked, after a fashion. It gave me a reading reasonably close to anything I measured with it. Perhaps a bit higher than it’s stated value and tolerance would suggest but close and certainly far better than my multimeter could do. It seems it’s about 2.2% out of where it should be. This could be a number of things or a combination of things. The set resistor that enables the LM317 to act as a constant 1A current source is actually a bunch of parallel resistors to dial into that sweet spot. CircuitLab told me this would be 1.155Ω, the datasheet for the LM317 told me it would be 1.25Ω. The actual measured value I got was approximately 1.245V drop across the parallel arrangement which is close to where it should be, or where I think it should be. I used standard 5% carbon film resistors to make this parallel arrangement with the addition of the critical 100Ω trimpot to calibrate away any oddities in that 5% tolerance of the resistors.

This is dandy, just build it up, trim it up and the things works right? Well sort of.

The datasheet says it should be 1.25Ω, which for 1A means a voltage drop of 1.25V, 50mV off isn’t bad, it’s a 5% error but how can I be sure that will net me 1A out of the thing? The LM317 has it’s limits too based on a variety of factors and that will need to be trimmed out in addition to the 5% resistor tolerance. Then there’s the other things, losses in the protoboard I’m using, loses in the leads, stray capacitance, quantum fluctuations – it never ends.

The only thing I needed to be sure of is that the thing is outputting 1A as close to exactly bang on as possible so that I could get an accurate reading. I needed to calibrate to that.

Unfortunately, as previously chronicled, my multimeter’s current ranges are quite limited to 10A, 200mA, 20mA, 2mA. For 1A I am forced to use the 10A range which gives me an output of 01.00A. Spot on yes, but lacking in that last digit to make sure it’s within the tolerance I need. I need to read 1.000A at least. 1.0000A would be even better! Given that my readings on a 1Ω 1% resistor was 1.022Ω that makes it 2.2% off-tolerance, and I’m pretty sure that’s not the resistor.

Electronics’ Dirty Secret

One of the first things I noticed about electronics when I began playing with it as a child is that the numbers never quite add up in reality. Every time I look at my mutlimeter when I take a measurement, I always shrug and say “close enough” and this can’t be helped. It’s frustrating when one’s math on paper makes nice round exact numbers yet the reality shows us we are just a little bit off. Part of this is due to the fact that we live in the real world and all the things we normally take for granted as not existing – like the resistance of conductors and PCB traces, as well as the noise they induce being antennas, tend to add up and creep into our measurements. Add to that the (in)tolerance of parts and the meter itself you have a mess.

As always, we, the scientists and experimenters, try to minimize this “noise” by buying bigger and better test equipment calibrated by some boffins in lab coats. This is all fine, if you have money. I don’t.

All I have is my Mastech MC8222H Chinese made $70 meter and that is the most accurate instrument I own. To me, this is my de facto gold standard as I simply have nothing better to compare it to.

The Mastech MC8222H is not a bad meter especially for it’s price. It has many annoyances I am not fond of and fluctuates like hell, but it works and has all the features I need for general electronics work on my humble hobbyist bench.

It is s 2000-count 4-digit display which would make this measurement easy if not for two things: it is lacking a 2A range on the current measurement. I can measure 200mA just fine, I can measure 10A just fine, but not in-between and keep that third decimal point. That’s not even the whole story. This is merely talking of it’s display resolution which says nothing of it’s accuracy.

A cursory look at it’s badly translated manual booklet tells me something else I’m not terribly fond of. Though the DCV function has a standard 0.5% accuracy, the DCA on the 10A range has an appalling ±2%+5 accuracy in the best possible case. For those that don’t know, the +5 figure means ±5 digits, meaning the least significant digit could be off by as much as 5 in addition to the ±2% accuracy window.

Acceptance

So here I am, with a bunch of adding intolerances. The resistors to set the constant current of the LM317 can be out by ±5%, the LM317 can be out itself by a bit, the meter I’m trying to calibrate it to can be off by ±2% and then some and that’s before we even take into account all the micro anomalies in terms of materials and construction. What is one to do?

The answer for right this moment is: nothing. I cannot calibrate this thing any better unless I have one good known bit of it I can say is calibrated to within half a bee’s dick of it’s life of where it should be. To me that’s >= 0.1%.

Options

One option that is apparent is to get a 1Ω precision resistor meant for calibration. I did a quick poke about and was unable to find one but I’m sure they exist. With that, I could dial the current source down until it reads 1Ω and then know i’ll be getting the best possible measurements from it. Not counting the error my multimeter will inject of course just being it. At least it would eliminate a couple of error sources straight away.

The other option of course is buy a multimeter worth owning. A brand name, one that is respected. A company that actually calibrates their meters before shipping them out and are known for reliability. The obvious boon here apart from being calibrated are that there will be a much better accuracy on the unit in general. We’re talking at least 10x better on the DCV and at least 2x better on the DCA. If i make it a 10,000 count one, I will get my much coveted missing digit also. The less obvious boons will be a meter I can rely on for twenty years that won’t drift much and has features like auto-ranging that will annoy me far less than the Mastech. I’ll keep both of course, always need two meters at least.

The obvious brand contenders are: Fluke, Keysight (Agilent), BK Precision and a couple of others I will consider after like Extech. I haven’t a budget yet, but when I do I imagine it to be about $300.

With this, i can dial in that current in to my satisfaction. Ironically, I bet some of these more expensive meters come with a low-Ω function which completely negates the purpose of this project but hey – that’s why we do these things, to learn. As you can tell, I’ve learned a lot. Like don’t buy cheap meters.

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  1. [...] referring specifically to my milliohm meter adaptor which I’ve been fiddling with (see here and here oh and here). Last time, I cursed the inexactness of my cheap Chinese multimeter, and not [...]