On Wed, 20 Feb 2019 20:59:15 -0000, "NY" wrote:

I suppose one could argue that *any* device (not just weighing scales) that
uses a spring to balance out the position of a needle on a gauge against an
analogue of the quantity that you are measuring (eg voltage proportional to
speed, weight, oil pressure, engine revs, temperature) is equally "****"
because you may not get a linear reading across the scale or may get
different readings for several measurements of the same quantity, because of
non-linearities in the rate of the spring (constant of deflection versus
force) or because the needle sticks slightly on its bearings.

Aaaah, thereyougo, you got it. Just leave away the "uses a spring" bit, and
it'll be more correct!

There is whole host of terms to describe the different ways a measurement is
****: repeatability, linearity, temperature dependence, systematic errors,
random errors, drift, ...

A balance scale, a simple two-bowls-onna-beam thing, will fail systematically in
many ways: a nicely sneaky one its the buoyancy in air of the objects to be
weighed in air. Different *volume* of the weights and the object enters into it,
at the rate of about a kilo per cubic meter, and dependent on the barometric
pressure. It's pretty good in that it compares masses, and the local gravity
pretty much cancels out. (Though there are special scales made to detect
differences in local gravity -- and these scales are tabletop models, not huge
things, see Cavendish and Eötvös). The there's friction, heat and temperature
gradients, the beam bending under loads, dirt on the weights, down to gasses
sticking to the weights and diffusing into them.

That said, I have a Soehnle kitchen scale to 15 kilograms, reading to one gram,
probably strain gauge, ~25‚¬. A test with a set of weights (once certified, but
out-of-date) showed yeah, off by one or two grams above a kilo -- more than OK
for kitchen work.

Beware some bathroom scales have cheat software to show the same weight if
nearly-the-same-weight is applied within a short time!


Thomas Prufer