Why is there no standardization of indentation hardness measurements? The last hundred years have seen a great degree of standardization of units, and the expression of units of measurement in standardized, instrument-agnostic, objective terms. 
So I wonder why this has not happened in the case of the indentation hardness of solid matter. 
It seems that the measurement of indentation hardness is fragmented into one scale per instrument, with one or more instruments being customarily used for each material, and it seems like the scales are mostly semi-qualitative and probably non-linear -- and they make it difficult to compare dissimilar materials. 
Off the top of my head, I can name: 


*

*Rockwell B: Used for soft steel, mostly. 

*Rockwell C: Used for hardened steel and similar materials

*Brinell: One of the more quantitative-seeming scales, but uses an instrument that has some downsides. Used mostly for nonferrous metals and soft steel. 

*Shore 00 and A durometers: Used for elastomers and soft gelatinous materials

*Shore D durometer: Used for "hard" plastic

*Knoop: Uses a weird-shaped indentor, usable for metals but mostly used for ceramics and other brittle stuff. 

*Vickers: uses a pyramid-shaped indentor but not the same one as Knoop, apparently generates quasi-SI hardness measurements but isn't that widely used. 

*Mohs Scale: Incredibly nonlinear empirical scale based on what materials can indent each other


Is it just because of the "There are now 15 competing standards" issue? Or is there a definitional problem in the concept of "indentation hardness" that makes it hard to reduce to a single value?
 A: I think there's a fundamental problem with how the materials react under a point/local load. You can't do hardness testing on rubber with a pointy sharp probe that's going to split the material open, and you'd have to produce a huge force to push a dull/blunt probe into a hardened steel sample far enough to be able to measure it with the precision available to you. You have to have a large enough range of dimple depths across your material category to be able to tell the difference between two materials of very similar hardness. To build a single instrument capable of measuring the hardness of both tool steel, which remains relatively rigid while probing, and silicone, which would deform and compress, you'd have to have a probe with a huge stroke an a very very accurate and repeatable sensor capable of sensing the entire range of forces produced by a measurement.
Building multiple instruments brings up the question of cross-device and cross-material category calibration/matching, which, practically speaking, is a nightmare. Multiple instruments for an inspection shop with limited counter space? An extra calibration/certification to get? No thanks. Hope that helps, would love to hear others thoughts on this.
