Dirtdad, Just about any scale that measures weight can be positioned at the right spring height under a solid object, like a beam in the garage or basement, can compare each spring you squeeze between the scale and solid object. The scales pad may need a larger flat plate set across the pad so that it does not deflect in the concentrated area that the spring is on so that the space between the scale and solid object does not change when loaded. The method may not be exact yet it can tell you exactly if one spring is stiffer than another spring.
most testers out there are not for low pressure springs and are hard to measure low pressure. the cheaper testers are up to 600 lbs. just looking for something that I'll use once or twice a year. I found the fish scale method too hard to get a consistant reading.
a digial post office scale that covers 60 lbs is fine.. put it under a Drill press and make yourself an adapter for a 1" dial indicator( one the shows reversed readings also) that will also hold the top of the vavle spring. the adapter simply goes in the Drill bit chuck
then find yourslef a 1" indicator setup gauge and 0 the indicator at 1" . Now you can compress a spring to pretty much any height from .900 to 1.050 and that will cover most installed heights you might need for Fhead springs. just read the scale at teh installed ehigth you need
Some folks will use a 2" dial indicator..
a good flat plate on top of your press to put the scale on helps too.
I've used this exact method for years before I bought an Intercomp. I even found a small table top drill press at a yard sale that the electric drill motor was burnt up (paid $1 for it.)
BTW, the digital Intercomp spring tester is VERY accurate at low psi resolution as apposed to a Rimac analog spring tester. I also use it to match clutch springs which are considerably higher in pressure than Briggs valve springs. Great investment as an engine and clutch builder, but for the do it yourselfer, the method that Jimmy described will be sufficient.