purchased a volt for my voltmeter

I've been in the process of building up my home lab. Of course, a bit of that process involves purchasing equipment that either I've gotten rid of in the past, or that I've never had in the first place. One of those pieces of equipment is an old but nice 6.5 digit HP 3455A bench voltmeter. I got it off eBay for roughly $100 after shipping, which is slightly below the average going price. Sometimes newer models such as 3456A and the 3457A can be found for not much more if you're patient, but those typically get listed as "for parts only", which doesn't necessarily mean it doesn't work fine, just that the seller doesn't know or doesn't want to guarantee that it does. In my case, the seller listed it as being pulled from a working environment, but since it had a "calibration not required" sticker it could have been quite a bit out of calibration. I took the risk.

There were a few reasons I opted for this particular model rather than, say, the 3457A. For one, you're likely to get a 3455A in better condition for around $100 than you would a newer model. Secondly, I prefer the LED display over the 3457's LCD. Not only is it easier to see, it would be easier/cheaper to repair if the display were to go faulty. Lastly, calibration on the 3455 is stored mechanically via trim pots and resistors, whereas it's done electronically on the 3457, with the settings maintained in volatile memory backed up by a battery. I figured I had better chances of finding a 3455 closer to spec than I would have a 3457. But how would I know one way or the other? More on that later.

So I immediately popped the hood on my 3455 after receiving it to glance in awe at it's analog circuitry (sorry, no pics but there's plenty online), and to make sure the electrolytic caps didn't look like they were about to spew their guts all over the digital section's PCB. I was very pleased by just how clean and dust free it was inside. The two of us got off on the right foot.

That's all well and good, but if I'm probing some circuit how do I know I'm getting an accurate measurement at the level of resolution the meter is capable of? If I can't trust the readings, the best I can do is believe them. In the real world of electronics (and pretty much everything else for that matter), very few things are exact, but rather have some allowable tolerance for deviation from the actual, or the specified nominal value. This truth extends to measurement devices as well. For example, the 3455A has a resolution of +/-14.99999 volts in the high resolution 10 volt range selection with an accuracy of +/-0.002% + 1 digit, which means if I measured a voltage source that was exactly 10 volts, the 3455 could read anywhere from 9.99979‬ to 10.00021 volts and be within spec. And that's only within 24 hours of being calibrated, and used in a 23 +/- 1°C environment. Sheesh. High accuracy and resolution is hard.

That's all well and good, but how can I stop believing and instead start trusting? Well, I can do what the pros do, and send my meter off to a calibration lab where they'll adjust it to be in spec, but that would cost quite a bit more than I paid for the meter - a hard pill to swallow for those of us who buy used lab/engineering grade equipment for pennies on the dollar. Which brings me to the whole point of this post... I bought a volt for my voltmeter... 10.000000 (give or take 5 microvolts) of them to be exact...

This is my trimmed 10V voltage reference source from voltagestandard.com 

The way this works is that there's a voltage reference chip on the board (the rightmost black square) which outputs something damn close to exactly 10.0000 volts right out the box. Conveniently, there's a way it can be trimmed to be even more exacting to more digits to the right of the decimal point. This trimming is done with the help of a fancy and currently calibrated 8.5 digit voltmeter. This is a middle man of sorts, and gives me something to check my 3455A against. The closer my meter reads 10.00000V when hooked up to this, the higher my confidence is that the DC voltage reading function of my meter is in spec. So what did my meter read?

Well I'll be, uhh, not damned.