Something you might like to look into:

Hand Probe: This standard piece of equipment, sometimes referred to as a trier, is constructed of brass or aluminum. Probes come in various sizes with standard lengths of 5, 6, 8, 10, and 12 feet. The type of carrier dictates which probe length shall be used. There are two types of hand probes: compartmented probes in which slots in the outer tube match compartments in the inner tube and open throat probes in which the inner tube is open. Open-throat probes tend to draw more of their sample from the top portion of the grain while compartmented probes draw a representative sample from each layer. All official grain probes are compartmented probes with a 1-3/8 inches in diameter (outer tube). Make sure the probe reaches the bottom of the carrier. A 5 or 6 feet probe will be sufficient for most farm trucks while hopper-bottom carriers may require a longer (6, 8, or 10 feet) probe.
Mechanical probe: There are two types of mechanical probes which are recommended for sampling stationary lots of grain in trucks, railcars, or other open-top carriers. The GRAVITY-FILL PROBE function is similar to compartmented hand probes except that after the compartment is filled it rotates to an inner tube where it is forced up by air. The core probe functions by forcing the sample up into the core as the probe is pushed down and then using air to transport the sample to the output point. A third type, the in-load suction probe which uses negative air pressure to suck the sample into the bottom of the probe, is not recommended since it tends to overestimate foreign material.
http://pearl.agcomm.okstate.edu/agecon/farm/wf-224.html