The Schmidt hammer is a device designed to estimate the strength of concrete (but which can be used on rock) by measuring the rebound of a spring-impelled hammer striking on the surface of the mass. Several varieties of the hammer differ in the energy imparted by the spring – the L type is generally the most satisfactory for rocks.
The rebound number is converted to unconfined compressive strength by means of graphs, which are based on laboratory testing evidence. Correlation between rebound number and UCS is not absolute, and rather like the use of Point Load Strength index, it might be better used for comparison between rocks if the link to UCS were avoided.
The value of rebound is affected by the attitude of the hammer, whether vertically pointing down or up, horizontal or inclined.