Other hand-held equipment that has been studied or is in use today includes two mechanical penetrometers: the Compact-O-Gauge and the dynamic cone penetrometer. A prototype of the Compact-O-Gauge was invented in 1991 by Greg Ruark, when he was a Forest Service researcher in North Carolina. The device simulates an acceptable vehicle pressure or load (as defined by contracts or other specification). The device works well, but only when assessing the top 200 mm of soil. In a recent conversation, Greg Ruark thought the hand-held electronic cone penetrometer could perform the same function as the Compact-O-Gauge but at greater soil depths.

The dynamic cone penetrometer uses a slide hammer dropped from a specific height to force a cone into the soil (figure 12). The recorded depth of penetration, weight of the slide hammer, and drop height are used to calculate the soil penetration resistance. Even though the two instruments function differently, MTDC conducted a quick side-by-side test of the dynamic cone penetrometer and the GP40 electronic penetrometer. Figure 13 shows the results. It takes about 5 min to collect data for one 500-mm probe with the dynamic cone penetrometer, compared to about 17 s with the electronic penetrometer. Dynamic cone penetrometers range cost from $1,200 to $2,500, depending on options and accessories.

Figure 12—The slide hammer on a dynamic cone
penetrometer is dropped from a given height for a
given number of times and the probe's depth is recorded.
This device is less expensive ($1,200 to $2,500) than
the handheld electronic cone penetrometers,
but it takes much longer to use in the field and
the cone index must be calculated after use.

Some other untested instruments include the Clegg Impact Soil Tester, Humboldt GeoGauge, and the nuclear density meter. Generally, these instruments just look at the top 100 to 200 mm of soil.

Figure 13—Relative differences between a hand-held electronic
cone penetrometer and a dynamic cone penetrometer.