My Dad has one of the LSM spring testers. Like everything else they have it wasn't cheap but it seems to work very well. It is only meant to do seat pressure though as it doesn't have enough travel to test open pressures.

One interesting thing in the instructions is that you need to tap on the spring lightly with something once you get the height set to relieve any tension between the inner and outer spring, makes a noticeable difference in the reading.