Your comment just now reminds me of an electronics shop class I took in college. Back then we only had analog meters and one day the instructor asked “When you hook up an ammeter into an electrical circuit to measure current, just what do you think you are measuring?”
If you start to think about it, that question should just about blow your mind! After all, amps, the unit for electrical current, is actually electrons per second and an ammeter certainly isn’t measuring THAT.
Here is a very irreverent review by Dave from eevBlog of a Harbor Freight meter that someone sent him to review. I indexed the video to start at the measurement tests. In my opinion his reaction is a mix of surprise and distain.
If I did this correctly, this youtube video should start at 37:30. If it doesn’t, that is where I intended it to. In my opinion a must watch (for about a minute)
Are you sure it’s not just a Very Short Individual sitting in there counting out electrons?? Really?? I could’ve sworn I read that on the Internet somewhere!
I am reading and studying this thread because I would love to build and own a reference source.
But as you alluded to in your post, we are so far removed from the reality of it when we try to measure an electrical quantity, that FAITH always has to enter the picture.
And, an analog ammeter doesn’t measure magnetic flux density either!
Here is maybe an ignorant question. As it would seem, if a highly accurate voltage reference is so easy to come by, why don’t the manufacturers include one in their DMM and use that to self calibrate, say evertime you turn it on. If that were possible, how much extra could that cost. I’m thinking next to nothing once the silicon is done.
I have been out of the field for quite a while but back then calibration was done by checking cardinal points on all ranges, commonly 20, 40, 60, 80, and 100%. Using one reference voltage and calling a meter calibrated is making a big assumption that the observed accuracy carries over to all voltages and range settings.
No one here is calling there meter calibrated after using a reference, but it certainly raises the level of possible precision to know and get a sense of how much your DMM is off from that reference.
I was thinking similarly, I’ve known analog meters to have scaled errors. And even if voltage was calibrated perfectly all across the scale, would that carry over to amperage and resistance readings?
Plus how accurate do we need to be here? I can chop off a log for a cabin square enough with a chainsaw and my eyes, but that obviously wouldn’t work for making furniture. Perfection is nice but do we need accuracy beyond 1/1000th on voltage (or any) readings? I’m NOT wanting to start an argument over accuracy, just thinking aloud.
That is usual not necessary with digital meters, you only need a zero calibration and a near full scale calibration.
The construction of a DAC usual guarantees that it is linear and if not there is nothing you can do.
Yes, with very few exception you need to check all ranges, but knowing the most used voltage range is correct gives some confidence in the meter (Cheap meters will often give wrong readings when the battery is low).
Thanks for all the advice folks. Some of it is deeper than I understand at the moment, but I will revisit all of it when I have time really dig into the concept.
I can get the REF5040's for $3.05 each, plus $7 shipping directly from TI. That is more than I want to spend, so I ordered 3 for $4.53 of Ali. I'm probably being penny smart, but pound foolish. I won't know if these chips will be any good, but I should be able to get a general idea by comparing them to each other and other common references.
Based on the data sheet, seems that powering them with a single li-ion is the way to go. Ideally a 4.3 or 4.35 cell.
EDIT: I received the chips, but couldn't find any 4.3 or 4.35v cells in my collection. So, I connected them directly (in parallel) to a couple different 5V power supplies and got weird results. They all output 2.738 volts. Rereading the data sheet, it appears that it needs various caps to stabilize output. I need to rig up something like below. I don't know anything about High Frequency ("HF") capacitors. At least, the data sheet seems to indicate that the HF cap is optional. If it works, I'll finalize the pcb below.
