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#21
I was not very clear. You are right but that is pretty much like any measuring device (except maybe a ruler). To better illustrate my point about consistency, look at scales for example - we'll use a bathroom scale. They may be calibrated to be most accurate at 150 pounds. This means a 50 pound kid may show up as 47 pounds. And a 250 pound man may show up as 260. But a 150 pound person would be (hopefully) 150 pounds - EVERY TIME they step on the scale.
The key thing is, if off, it should be consistently off by the same amount EVERY TIME. That is, a 50 pound kid should always be 47, and the 250 person, always 260. If today, the scales say the 50 pound kid weighs 47 and tomorrow it says 52, and the next day it's 45, that's a much bigger problem than consistently reading 47.
Now ideally, the measuring device would be spot on across it's entire operating range. But we don't live in an ideal world.