Alright, what I am looking for is a way to coil and un-coil about 10m of thin (0,5mm) wire and accurately measure the length.
Jeweller's line looks promising - it is cheap, small diameter and made of steels strands
https://www.gal-art.pl/l3a02-linka-jubilerska-0-5mm-55m-szpula.html
Unfortunately , coiling a wire makes it go in circles, so 1 turn of a coil is nothing close to integer. Therefore I settle with making manufacturing easier and make a roll of 80mm diameter by...
so 35mm wide ( 20mm if wire is laid tightly, so let there be some space).
Next - auto-return. Something like this device but with longer extension.
Two ways to investigate:
1) direct drive which makes no sense as 40-coil spring would be massive and expensive.
2) mechanical transmission. Which makes perfect sense as 10-turn springs are readily available and 1:4 gear ratio is sensible and relatively easy.
At the output I expect to have force that pulls back the wire with about 200gf (gram-force), so around 2N. That gives a torque of
at the output reel. Since step-up gear ratio is 4:1 , spring shaft has to make 10 turns at 160Nmm.
Lesjofors caalogue states that 20-wind spring has a toque of 150-709Nmm whil having dam of 116mm and tape width of 10mm. Caalog no 8990. Makes sense to use 10 windings only, since the torque will not rise that dramatically.