Exactly. I've considered making one of these. It wouldn't be terribly difficult to make a setup to test a specific spring, you just need a setup that allows you to compress the spring to varying degrees and a second setup that allows you to load the spring with a specified weight and measure the compression that results. A test would be to load the spring with Y pounds and measure the compression that results as a percent of the new, uncompressed spring length. Then compress the spring to X percent of its new, uncompressed length for a period of Z days. Then decompress the spring, load the spring with Y pounds again and measure the compression again. Repeat with different levels of compression and varying amounts of time.