The classic "R"-value is the mechanical travel of the mallet on rebound.
It is affected by its friction on the guide rod, the friction of the drag pointer on the scale, the influence of gravity during its travel, the relative velocity between unit and mechanical parts.
This is true for every rebound hammer currently on the market.
The "Q"-value is virtually free of all the above error sources. It is thus the indicator of choice to be used as a basis to convert to compressive strength.
The rebound hammer SilverSchmidt acquires the "Q"-value by measuring the velocity of impact and of rebound immediately before and after the impact.
The "Q"-value measured by the rebound hammer does not to be corrected for impact direction.