I agree that there are times when you don't need absolute accuracy. I have plastic calipers myself for crappy jobs. But I don't say that they're accurate to 0.1mm because they're not.
> Also, no, 50C is not a very conservative range for that purpose, that would be ±5C around the steady 20C temperature inside the average house.
That is true until you realize many people keep their tools in the garage or in a cellar or an attic, where the temperature can go anywhere between -20C and +45C. Then you measure a small nut or screw as often used in, exactly, home appliances, and between the thermal coefficient, slop, bad geometry, and an inaccurate digital potentiometer you arrive at 4.8mm instead of 4.1mm and you order a 5mm replacement for your 4mm screw and have to wait another week or two after you figure that out. I've had this exact story happen to me in the past, and my tools were stored in, as you say, "±5C around the steady 20C temperature inside the average house" and not out of plastic - just still not accurate enough.
For measurement of small nuts you apply an error calculated for 10cm. If the nut is small, the error will be also small, not 0.5mm, but something like 0.02-0.03. So if you measure 4.0-4.03 you won't make a mistake which nut it is. You can live with that as a DIY-er unless you need to measure 10cm+ nuts with high precision, I doubt that.
No, that's incorrect, because for small measurements the slop as well as the influence of bad geometry is going to be larger. Lack of accuracy is a problem that multiple issues contribute to.
I don't know where you got your numbers from. I doubt it's anything other than you thought the numbers just looked fine and illustrated your point.
> Also, no, 50C is not a very conservative range for that purpose, that would be ±5C around the steady 20C temperature inside the average house.
That is true until you realize many people keep their tools in the garage or in a cellar or an attic, where the temperature can go anywhere between -20C and +45C. Then you measure a small nut or screw as often used in, exactly, home appliances, and between the thermal coefficient, slop, bad geometry, and an inaccurate digital potentiometer you arrive at 4.8mm instead of 4.1mm and you order a 5mm replacement for your 4mm screw and have to wait another week or two after you figure that out. I've had this exact story happen to me in the past, and my tools were stored in, as you say, "±5C around the steady 20C temperature inside the average house" and not out of plastic - just still not accurate enough.