Yep, I missed the humor. ?? Hey, it happens..... Yes, there is a relationship between volume (of water) and mass but that is just a happy (but planned) coincidence. It does not define the unit of mass, it merely 'links' it with volume in a clever, and sometimes useful way. And hey, it is certainly a better link thank, say, "furlongs per fortnight" is for measuring speed.
I <think> you are missing the point of what NIST has done here regarding mass. Mass is a fundamental unit of measurement that cannot depend on being compared with anything else that is not fundamental..... and that lump of stuff in France is NOT a fundamental unit in physics :-) The point of the balance is that it allows a way to define a basic unit, in this case mass, with another fundamental unit, in this case force. This is the key: no matter where (or perhaps even when) you are in the universe, you can always grab a handful of quantifiable force. So the objective becomes to define ALL fundamental units based on at least one of the other, defined and quantifiable units.
So how about a practical example: suppose you find yourself visiting the city of Hemorrhoid on the planet Uranus and you want to weight a rock (or at least what you truly and deeply is a 'rock' that you just found in Uranus). But you do not have that pesky piece of French 'stuff' that you know weighs a kilogram (well, it <masses> a kilogram, weighs a Newton but let's not make this problem we have in Hemorrhoid any worse than it already is). What to do, what to do? You can measure the lump of 'stuff' you have but then you would have to know its density and that is most certainly NOT a fundamental unit of any kind. In fact that is multiple <not> fundamental units. What to do. I know- what if there was a standard fundamental force that you could compare, using some kind of machine or formula or other duplicate method, which ALWAYS works because it is a CONSTANT, and then derive your unit(s) of mass? Hey, that would work. But comparing differing units is often impossible, for example, how many gallons in a foot? See? What we need is some key that we can use to 'jump categories' and directly compare fundamental units of.... well, stuff. Why just a few weeks or months ago this would have been impossible but in today's mail, we find a flyer for how to build your very own Kibble balance, which will allow YOU (read: anyone, anywhere, anytime) to compare Force to Mass directly. Now as force is a fundamental unit, you can grab yourself a "pound" of force, or better yet a 'metric pound, or Newton (the SI unit of force..... in the English system it is really 'Slugs' (really) but we cheat and use the world Pound and just put a little 'f' to indicate we mean pound, force, not pound, weight), and compare it with your 'thing' from Uranus and you will know EXACTLY how much mass it has! You may not be sure what it is but at least you know how much of it you have (kinda' like being in the chit or in deep chit- big difference!). And even better yet, you can show all of the local Hemorrhoids just how to measure their own 'stuff' regarding mass with nothing other than force, which they too will have in abundance (though I understand some Hemorrhoids are more forceful then others but again, I digress).
So like when the Ancient Greeks (who may or may not have known about Hemorrhoidians) discovered Pi, the point was not how accurately they could calculate it nor how fast, and it did not matter what units they expressed it in (degrees, radians, quadrants). The point is that they discovered a fundamental relationship between a circle's circumference and its radius: there are always, exactly, two Pi radians in the circumference of each circle, and Pi is the 'tool' we have to convert one to the other. Same thing with measuring length with light- light is the constant of the universe and so can be directly linked to length, meaning length then becomes a fundamental unit in and of itself.
As far as friction of the pendulum used to measure time, I would not worry too much about that.... time is NOT a constant anyway so it becomes a trivial measurement. Unless, of course, you happen to be waiting for a public stall to open up on Uranus.... then it could become critical. But still not a constant.
Brian (for homework, see if you can determine how many ergs of energy it took me to type all of that foolishness......)
P.S. Thermal expansion is measured in units of length (of expansion) per units of length per units of temperature change (inches per inch per degree change in temp. or Delta T). This one time, at banned camp, I gave an answer to a thermal expansion question in these units: inches, inches, degrees Centigrade. It was marked wrong but it was not wrong; it was merely me amusing myself and hopefully, those who read it. Not so much to the Professor who marked it wrong and then corrected my grade..... although begrudgingly. We agreed that 1) it was a correct answer. 2) it was in 'bad taste' to state it that way (mixing English and SI units in one expression, technically correct but NOT NORMALLY DONE or, in this case, well received). 3) One of us found it amusing......
Yeah, sometimes my humor goes un-noticed, or should I say it becomes repurposed into seriousness. Kind of like when your best stuff wasn't having any effect on your Doctor. More personally, a "My name isn't Hanz." type of reaction. The shoe did sort of fit me so I tried to wear it for humor. I tried this once before and it failed (humor) then too, but I'm too lazy to search for it...in a many years ago posting. So to be clear now, I was just kidding and took no offense. Kind of playing with how some can take things one way and they (on the internet mostly) take it one of the ways most likely to cause themselves offense.
Anyway let's move on. So I think my question about practicality is answered. A Kimble balance sounds expensive and if it's man made (same as a pendulum) then it's ability to microscopically cancel opposites out perfectly is suspect. For instance even a pendulum can have more bearing resistance when swinging on one side of straight down that isn't equal to the resistance of the other side. So, no real practical improvement, at least that's the conclusion I have come to until I read or find out otherwise
So getting back to theory. There is something that they didn't mention or perhaps I missed. The gram. The gram was defined as the weight of one cubic centimeter of water held at 4 degrees C, right? And a Kilogram was defined by a mass of something that was 1000X the weight of a gram, or as close as they could get. Now that a kilogram is defined by a constant, (or in practicality, by a master man made machine designed to measure that constant) is a gram now defined as 1/1000th of that result or is it still defined by the weight of a certain volume of water at a specific temperature?