Back to main page

Some particle analysers can work with only small samples, and thus it is sometimes desired to obtain a subsample of sand that has the same particle-size distribution as a larger source sample. For example, if a laser diffraction particle analyser requires sample size of five grams of sand, how can those five grams be obtained from a larger sample, such that the five gram subsample has the same particle-size distribution?

Despite usually appearing to be homogeneous, dry sand is quite prone to being or becoming inhomogeneous when jostled. Grains in a bag of sand are almost certainly not evenly distributed by size; smaller grains will tend toward the bottom, due to percolation (they fall downward between the gaps of larger grains). Stirring or shaking 'to mix it well' can actually make things worse, depending on the sizes present, as movements provide more opportunity for the smaller grains to fall downward.

This is a well-known, well-studied problem. Subsampling techniques are reported to vary substantially in their expected sampling error:

Method | Estimated maximum sample error | |
---|---|---|

Spinning riffling | 0.4% | (best method) |

Chute splitting | 3.4% | |

Table sampling | 7.0% | |

Scoop sampling | 17.1% | (a common method) |

Cone and quartering | 22.7% | |

Table 1. Reliability of selected subsampling methods [Chemical Engineers' Handbook, 7th edition]

With spinning riffling, the best of the subsampling methods above, the entire sample is slowly deposited into a series of containers, each of which collects a fraction of the sample (figure 1 and 2). Rotation, either of the collectors or of the depositor, spreads the deposit evenly over the containers. Further divisions can be obtained by repeating the process with one (or more, combined) of the resulting fractions, until the desired subsample quantity is obtained.

The objective is to deposit a stream of grains such that any concentration of grains of a particular character is spread over several (ideally, all) collection bins. Thus it is best to have a high number of revolutions while the grains are flowing. A rule of thumb is that at least 20, better 100, rotations ought to occur during the time it takes to deposit the sample.

Commercial spinning rifflers are available, eg., US$2000 on eBay. First, however, we thought we'd give it try as a 'do it yourself' project. This web page describes what we did.

Fig 1. Paper cone riffler. Funnel is moved. After Gerlach (2002).

Gerlach (2002) describes a simple paper cone riffle splitter (figure 1a) constructed from a piece of paper with a few 'Orgami' folds. A square of paper is cut into an octagon (by getting a second identical square, centering it over the original, then rotating it 45 degrees; cut off the exposed parts of the underlying original). The octagon has four pairs of opposing points; fold the octagon in half such that the fold goes through one pair of opposing points; unfold and do it again for the other three pairs, always folding in the same direction (this creates the ridges illustrated by dashed lines in figure 1b). Then create the 'valley' folds in the opposite direction by folding the octagon in half, bringing opposite edges together, for each pair of edges. The result should be a cone-shaped 'mountain' with eight 'valleys' each separated by a ridge, as in figure 1b.

Containers are placed at the mouth of each trough of the paper cone, as in figure 1a. A funnel (which could also be constructed from paper) with a small opening is loaded with sand and then moved in a uniform circular motion around the apex of the splitter, such that sand falling onto the slopes and into the eight troughs falls into the collection containers. The funnel should complete at least 20 circuits while draining.

Alternatively, the collectors and cone could be mounted upon a turn-table, so that the funnel could be held steady, along the lines of the spinning riffler discussed next.

Fig 1b. Spinning riffler. The collectors rotate, funnel is fixed.

Spinning riffling involves dropping the sample onto pie-shaped containers arranged in a circle (figure 1b). The containers need not be the same width.

Below is a video of a do-it-yourself spinning riffler operating, with collection bins made of aluminum foil over cardboard, rotated using an old phonograph turntable at 33 rpm. The collection bins are one-eight, one-quarter, one-eight, and one-quarter sections.

Fig 2. Video of an operating spinning riffler, with funnel.

Here are some construction considerations:

- For the collection bins, use a material that doesn't attract sand (eg., electrostatically or mechanically), to avoid losing small grains. Aluminum foil or glass is good.
- The bins can be any fraction of the circumference. Variety gives flexibility. Fractions can be combined. The arrangement in fig 2 has two sections of 1/8 of the circumference, and two sections of 3/8. However, sample error likely varies inversely with bin width.
- A higher rotation rates means better spreading and allows a greater funnel throat diameter, but a higher rotation rate will increase the chance of material bouncing and flying out of bins.
- A narrower funnel throat means better spread (more rotations), but there is a limit to how small the throat can be and still have flow.

The pie-wedge-shaped bins in figure 2 were constructed of uncorrogated cardboard surfaced with aluminum foil held by a thin layer of epoxy, as described below. One corner of each bin was left open so that material could be poured out.

Fig 3. Cardboard backing for bins

To prevent grains from falling between bins, the foil on one edge of each bin was folded downward so that it would overlap with the edge of the neighbouring bin, as shown in figure 4 below.

Fig 4. Aluminum foil overlap to cover the gap between bins

With an old phonograph turntable providing 33 revolutions per minute, to reach the desired number of revolutions (20 to 100) requires the sand to be depositing onto the spinning riffler for at least a minute, and better, two minutes. To a limit, this can be accomplished by making the opening of the funnel smaller. However, eventually the opening becomes so small that grains jam and some other method is required to deliver the sand to the spinning riffler. For aeolian dune sand with a median diameter of about 300 microns, this became a problem when the total sample size was less than about 10 grams.

Sampling dune sand (obtaining a good sample to start with).

Gerlach, Robert W. et al, 2002. Gy's sampling theory in environmental studies. 1. Assessing soil splitting protocols. Journal of Chemometrics 16: 321-328.