I use all sorts of units in calculations but, for thinking purposes, I prefer units that I can visualize. I can visualize an inch but not a centimeter - unless I think of a cm as ~0.4 inches. A mile but not a km - unless I think of a km as ~0.6 miles. A pound but not a kg - unless I think of it as ~2.2 pounds. Visualizing a mm is totally out of the question unless I put it in terms of it being 1/25.4 part of an inch - between 1/32" and 1/16" (both of which I can visualize). It's impossible for me to think in terms of meters. The only metric units that I have fully adapted to and like better are the liquid measurements of 3.7849 liters or less. Anything greater, it's a tossup as to whether I use gallons or liters. I'm not real big on quarts and I always think of a fluid ounce as being almost 30ml. And, of course, a ton is 2000 avoirdupois pounds. It contains 29,166 troy ozs. One thousand kg, or ~ 2204 pounds, isn't a ton in my book. It's a metric ton. It contains 32,150 troy oz.
A yard (actually, it's a cubic yard) is 27 ft3. I can visualize a yard - it's a cube that comes up to my navel (I'm 5Ft-9in = 175cm tall). A ft3 will contain 7.5 gallons. A ft3 is one and a half 5 gallon buckets.
And then there's a cup, a tablespoon, a teaspoon, a pinch, a tad, a smidgen, a dash, and a passel.
The original question, asked 8 years ago, makes no sense. Ppm is a measure of concentration. Troy ounce is a measure of weight or mass. Apples and oranges.