https://www.goodson.com/VST-60G-Anal...Spring-Tester/ The engine builders use something like this, one of the lesser expensive tools in their shop. They also make a small hydraulic portable unit that is used to check springs in the engine but it is much less accurate. Valve spring pressure is a critical element in engine design but unfortunately the optimum pressure is a trial and error endeavor. Goodson is the go to place for engine building equipment, check out their catalog and you can begin to appreciate the cost in building a racing engine.
Honest Dad himself
https://www.goodson.com/search.php?m...keep_https=yes A guy Could make himself a mechanical stop or plug to limit the travel of his machinist vice for repeat ability and use one of these little scales in a low production (back yard) environment, still lacking a little in precision. When choosing a gauge keep in mind your most accurate readings will be at mid scale of the gauge.