T i m wrote

I would like to read the output of a strain gauge (measuring
weight (0-5kg) in this instance) from a PC as part of a general
experiment but not sure what bit does (and maybe limits), what.

So, strain gauge (or gauges) connects to a SG
amp and that turns the output into volts?

The SG varys the resistance with weight.

Then you could read said volts with say an Arduino and then have
a USB interface to PC, effectively streaming the output of the strain
gauge in ASCII (numbers) to the PC (and the rest is software etc) .

The other approach is to use the resistance to vary an
oscillator and use a counter to measure the frequency.

However, I understand the Arduino's only have a 10(12 with a tweak)
bit ADC and so with 10 bits I'm only going to get 1024 output values?

The counter approach fixes that and is very easy to interface.

eg, If that was a 1kg 'scale' then I could read around 1g increments
but if it was a 5kg scale then ~5g would be the smallest increment
(that may be fine but I am just testing my understanding here).

That's basically correct.

Or is the output of the little eBay strain gauge amps already serial
so if it was a 24 bit device (16777216) it would be resolving down to
~1/3000 of a g (mathematically at least) and so would just use the
Arduino to turn it into ASCII over a USB interface (no ADC needed)?

I know you can buy commercial strain gauge to ASCII
over USB devices but they aren't exactly cheap (~£250+).

Quite a few digital scales that report the weight using bluetooth
but they are mostly bathroom scales, not kitchen scales. And even
with a 5KG kitchen scale they don't usually report continuously.