The principle behind the Clegg Impact Soil Tester, also called the Clegg Hammer and Clegg Decelerometer is used to obtain a measurement of the deceleration of a free falling mass (Hammer) from a set height onto a surface under test to determine hardness. The impact of the hammer produces an electrical pulse, which is converted and displayed on the Control Unit in units of gravities "G" or tens of gravities "CIT". Reference ASTM test methods D5874 and F1702.

The standard test protocol developed by Dr. Clegg is to drop the hammer four consecutive times on the same location with the highest value result in the series taken as the Peak Clegg Impact Test result. Since that time, other test protocols have been used based on the materials under test and the application. These other protocols vary the Hammer drop height or use the average result in place of the peak. The Clegg offers the convenience of rapidly scanning compaction variation over large areas. In research studies, 250 tests were performed with the Clegg in a half-day.

Available in following models.

• .5 kg - Used for soft turf, sand areas and golf greens
• 2.25 kg - Used for surfaces consisting of natural or synthetic turf (football, baseball, soccer fields, playgrounds horse tracks, and golf
  greens).
• 4.5 kg - Used for surfaces such as preconstructed soils, trench reinstatement, bell holes, and foundations
• 10 kg - Used for flexible pavement, aggregate road beds, trench
  reinstatement, bell holes, and foundations.
• 20 kg - Used for surfaces such as flexible pavement, aggregate road beds, trench reinstatement, bell holes, and foundations.

Simple and quick test procedure

1. The test procedure is very rapid and can easily be performed by site personnel with minimal training. Each test can be completed in less than 30 sec. The results are immediate.
2. Select a level surface (If needed, lightly scuffing surface with a foot to level the strike area)
3. The guide tube is set into position, secure by placing a foot on the tube's base flange and steadied with the lower leg and knee)
4. The ON/RESET button on the control box is pressed and release to activate the circuitry. The hammer is raised to the drop height indicator mark
5. The hammer is released allowing it to fall freely and cleanly
   Without moving the guide tube or pressing the ON/RESET button, the Hammer is raised o the mark and release three more times
6. The value (CIV) displayed after the fourth blow is recorded and the display automatically powers down 30 sec. after activation
7. Move to the next test site.