Length of convexity

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Dear Programmers,

I am looking for a CMM measuring solution for ground outer diameter measurement. Requirements are the follows: 
- Konvex 0,003-0,006mm 
-Length of convexity is 20±1mm. Simmetric on 30mm long cilinder length

Question is the length measurement opportunity.

If do anybody has solution for this, please share with me.  

Thanks in advance