Hello, I put a box on a scale to see how heavy it was. The scale only went up to ten kilos but the box was heavier than that. However the needle did spin around 1.5 times so I figured the box was about 15kgs. Then someone came out with the comment, "You're no scientist, are you! It's not a linear scale. Once it goes past that point it is no longer accurate." Is that true.

 

TIA,

 

Jez

Hello' date=' I put a box on a scale to see how heavy it was. The scale only went up to ten kilos but the box was heavier than that. However the needle did spin around 1.5 times so I figured the box was about 15kgs. Then someone came out with the comment, "You're no scientist, are you! It's not a linear scale. Once it goes past that point it is no longer accurate." Is that true.

 

TIA,

 

Jez[/quote']

 

Probably. If the scale depends on a spring, the force depends on the stretch or compression distance squared. You can't trust the system if it isn't calibrated - you can't assume it's linear.

 

However, you could calibrate it yourself. If you take two boxes that are under 10 kg, get their individual masses, and then put them both on the scale you can see if it is actually linear. Please note that there may be mechanical reasons that the scale stops at 10 kg, and damage to the scale might occur if you put heavier stuff on it.

Any instrument technician should be able to tell you (or someone that's had some instrumentation training) measuring instruments of this kind are not exactly linear.

 

If you plotted the output of a measuring device against the known input, you'll find that the graph will be sort of exponential but curving off at the top. A bit like a culmulative frequency curve, not sure if there's a formula for it.

 

There is section of this curve however that is more or less linear which is where the instrument is calibrated to by adjusting its "span" and "zero".

 

In essence the scales will only work within their range on the display.

Here we are

 

http://mechanical-eng.mme.tcd.ie/~clyons/calibrations.pdf