I think that any meaningful conversion between the 2 devices is practically impossible, as the silver ones rely upon the elastic properties of a piece of aluminum welded to another piece of aluminum-enough pulls and that property in aluminum is bound to change, I think. I know this is not a satisfying reply, but...

Of the 2 devices, the one with the wheels is much more consistent and better calibrated to the expected bend occurring from a static force and tension in the various caliber wound wires-just a better design overall. I have found that the 'spring and wheel guages' generally give pretty close readings (from one of the same devices to the next). Finally, A loos guage only gets you close to the rigging tension-good enough to be consistent if the device used and wire caliber are consistent, but not good enough to compare between different types of instruments, or be fully accurate based on the material composition. It's a lot cheaper than the piezo electric alternative which cost thousands.