Rotational Penetrometer

mobile firmness and stability testing
using an Instrumented Surface Indenter (ISI)

Rotational Penetrometer

mobile firmness and stability testing using an Instrumented Surface Indentor (ISI)

rotational penetrometer (RP)

The Rotational Penetrometer (RP) is a portable Instrumented Surface Indenter for objectively testing the firmness and stability of ground and floor surfaces in the field.

tested & proven

The results of ASTM F1951 surface testing correlate with the results of the Rotational Penetrometer surface testing on playground surfaces as shown in the peer-reviewed article, “Use of Two Test methods to Ensure Accurate Surface Firmness and Stability Measurements for Accessibility.” The National Center on Accessibility has used the RP to conduct longitudinal studies of various trail and playground surfacing materials to determine how well those surfaces maintain firmness and stability that meets federal requirements. This means that an agency can now implement a maintenance schedule using the BD Series 100 RP to verify that required firmness and stability is maintained. For more information, please visit the following pages:

Click: U.S. Access Board – Accessible Exterior Surfaces

Click: Disability and Rehabilitation: Assistive Technology Journal

convenient mobility

Can’t take your trail, playground, or flooring to the surface testing lab? The only device of its kind, the RP is a reliable, objective, portable instrument for testing all types of indoor and outdoor surfaces on-location.

The Rotational Penetrometer 100 Series is easily transportable, whether by plane, automobile, or even a trail assessment HETAP cart! The fully assembled device weighs only 18 pounds and stands at 28.25 inches tall. When collapsed, the RP may be securely stored in a special carrying case, which measures 26 x 18 x 8 inches. Contrast this with the cost of shipping a crate holding an entire ASTM F1951 instrumented wheelchair, and you will see why many are choosing the RP to perform surface testing.

long-term durability

Constructed of stainless steel and aluminum, along with surface reference plates made of finished marine grade plywood, the RP is designed for durability and long-term outdoor use. The indenter assembly uses a precision spring that presses a caster wheel, as the indenter, into the surface being tested. A precision caliper measures the amount of indentation into the surface. The fully-assembled device can be transported without a carrying case, as is typical for HETAP trail assessments.

0.001 accuracy

reliable accuracy

Research has shown that the Rotational Penetrometer produces repeatable and reproducible results that correlate with the amount of work required to propel a wheelchair across a surface (as measured by ASTM F1951) and the amount of physiological energy required by persons with and without disabilities attempting to negotiate a surface. The RP provides objective and highly accurate firmness and stability readings without the performance of off-site ASTM F1951 testing.

objective surface measurements

The RP provides objective surface measurements which allow individuals to know the exact quality of a surface. Without objectivity, surface descriptions such as firm or soft are empty and meaningless.

Objective surface measurements also provide builders, architects, and site planners with specifications to design and verify they are creating firm and stable, and therefore accessible indoor and outdoor surfaces.

objective firmness

Objective surface firmness measurements are obtained by pressing an indenter into a surface with a precision spring while measuring the exact amount of vertical penetration of the indenter into the surface.

objective stability

Objective surface stability measurements are also obtained by rotating the indenter back and forth on the surface, measuring the total amount of vertical displacement of the indenter after rotation.


While there may be many ways to subjectively test the firmness of a surface, an objective measurement is essential. An objective firmness value of a given surface may be determined by pushing an instrumented surface indenter into the surface with a calibrated spring. Then the amount of indentation into the surface, shown by the vertical displacement of the indenter into the surface can be measured.


As defined, stability is the degree to which the surface resists change from a contaminate or force and returns to its original condition. A stable surface will resist change and will not be permanently affected by normal trail use and weather. An unstable surface will shift and change shape when used normally, such as when a foot or tire changes direction or pivots while in contact with the surface. This is what happens when a wheelchair user or an ambulatory person turns or changes direction.

Using an Instrumented Surface Indenter, an objective stability measurement can be obtained by first applying a specified force vertically into the surface, then pivoting the indenter on the surface. The vertical displacement created by the pivoting into the surface can then be measured.