Linearity

Most transducers are “fairly linear”. Double the input and the output more or less doubles, within the working range. But not exactly. It’s quite common for there to be a slight curve, or for the graph to not quite go through zero. More worryingly, if you force the graph to go through the origin (because you want the instrument to read zero at zero) then there can be “not-insignificant” non-linearity over the bottom part of the range. Which is another reason not to buy an instrument with a much bigger range than you need. 

This graph also shows some systematic noise, giving scatter which is constant over the whole measurement range, (rather than being a constant proportion of the signal).