What Is a Penetrometer?

A penetrometer is a device used for measuring the compaction of soil. The device is occasionally also used for measuring the density of other materials, such as pharmaceuticals and food. A range of different styles and sizes of penetrometers exist for use in diverse settings, including civil engineering, agriculture, geology, and scientific exploration.

When testing soil, individuals usually use cone penetrometers, which consist of a steel cone attached to a long shaft with a pressure gauge or level on top. The cone may be pushed into the soil at a constant rate or hammered in, depending on whether the penetrometer is static or dynamic. Dynamic penetrometers are designed to be sunk with hammer blows and are often used in civil engineering, while static penetrometers, sunk at a constant rate, are used in agriculture. Graded marks on the penetrometer shaft indicate depth. The penetrometer measures soil compaction in pounds per square inch (psi) or kilopascals (kPa).

Soil compaction is an important issue in agriculture because it directly affects how well crops can grow. The cone penetrometer is designed to determine the level of subsurface compaction. High levels of compaction adversely impact the ability of plant roots to extend into the soil. A resistance level of over 300 psi (about 2065 kPa) will prevent roots from penetrating, so if the penetrometer finds this level of pressure, a deep tillage process known as subsoiling may be required to increase crop growth.

Civil engineering relies on soil science to create functional infrastructure. Compacted soil is required to support buildings and roads. Highly compacted soil can also be a concern, since it tends not to absorb rain and may lead to excess erosion and runoff. In other words, groundwater flow is sometimes reduced by soil compaction. Civil engineers use penetrometers to determine soil compaction in these scenarios so that an appropriate course of action can be followed.

Pocket penetrometers are much smaller than cone penetrometers and can be held in one hand. These spring-loaded devices are used to test the compaction of soil, concrete, or other materials. They are most frequently employed in industrial settings, laboratories, and backyard gardens. Other small laboratory penetrometers are mounted on stands and used for precision testing of the firmness of fresh fruit, pharmaceutical products, and any products in which a specific density is necessary.

Penetrometers are widely used in science as well as industry. The Huygens space probe, which landed on Saturn’s moon Titan in 2005, was equipped with a penetrometer to measure the consistency and composition of Titan’s soil. Deep sea expeditions have also made use of penetrometers to collect data from sediments on the ocean floor.