SnowMicroPen ® (SMP5 version)

The SnowMicroPen (SMP, Figure 1) is a variant of a cone penetrometer to measure bonding forces between snow grains at high spatial resolution.

The SMP is developed by the WSL Institute for Snow and Avalanche Research SLF and used for different applications such as snow profiling (avalanche forecasting, snow stratigraphy, ground truth for remote sensing), ski track characterization (ski racing) or snow runway characterization (stability testing).

The instrument is mainly developed for field measurements and specifically designed for operation at low temperatures (-30°C).

As a unique feature, the SMP facilitates a fast and objective characterization of the snow stratigraphy, a profile measurement for a snowpack of 1.25 m takes around two minutes.

To obtain the microstructural information, the SMP drives a rod with a force sensor into the snow and samples the penetration force every 4 micrometers. The force exerted on the tip is a superposition of bond breaking and snow compression.
An example of the measured force signal  is shown in Figure 2.
To illustrate the influence of snow type and layering on the penetration resistance, the signal is aligned with the snow microstructure as seen by micro-computed tomography.
Technically, the SMP mainly consists of a mechanical and a controller unit.

Mechanical unit

The mechanical unit of the SMP comprises the motor and the rod. The metallic rod encloses the piezo-electric force sensor (Kistler Instrumente, sensor type 9207) at its bottom. A thin conical metal tip is attached to the force sensor at the end of the rod for the transmission of the penetration force (see Fig. 3). During a measurement, the rod is penetrating into the snow while the force on the tip is measured.
The rod is driven with a constant speed of approximately 20 mm/s until a user-defined "penetration depth" is reached or an overload force is detected.
To ensure high accuracy in the vertical position, force measurements are triggered by the encoder of the motor. Thereby, natural variations in the penetration speed (e.g. at the transition to a very hard layer) can be compensated.
When the rod is driven back out of the snow, a small brush removes the snow from the gear teeth to avoid jamming of the motor and the rod.
For a stable position of the device during a measurement, ski poles (included) can be mounted on the unit to place it on the snow surface. In addition, the device is shipped with a metal frame for improved operation on hard snow. The metal frame also allows deployment on auxiliary vehicles.