Measuring spring stiffness..

[deejayuu]

Or you could compress a spring of known rate, or the one you want to replace, with 'spring x' (both with guides), in tandem on a longer length of threaded rod, and measure the deflections of the two.

No batteries nor bathroom scales needed

As above. When I was alive we checked all springs by that method on a jig Sykes Pickavant maybe.
Same result using a pull gauge as the known spring.
Now I do this 'cos I'm lazy.

It can easily employ the stiff known spring in tandem, to replace the scale, and is quicker/easier, for me, than threaded rod.
That said I quite fancy replicating the tester from the fifties.