SPT Hammer Energy Efficiency

GPC equipment used for SPT Energy Measurement per ASTM D4633:

GPC measures the maximum transferred energy (EMX) into an instrumented SPT rod.

EMX / (Rated Energy) = ETR%     (SPT Hammer Efficiency)

Rated Energy = 0.14 kip x 2.5 ft = 0.35 k-ft or 0.62 kN x 0.76 m = 0.475 kJ

ETR% is typically about 80% for automatic hammers. Some auto hammer could have ETR > 100%, this is because the drop height could potentially be more than 2.5 ft.