We are continuing to try to answer the question: “What is the size distribution of substrate in Muncie Creek?” Unfortunately, I originally posed the question as “how much of the rocks in Muncie Creek are at each size?” which is more confusing. I had deliberately avoided “how many,” because we weren’t counting rocks at this point, but it occurred to me later that we are probably going to do some pebble counts later, so we really will be counting. Anyway, I digress…
I put a sample of dried sediment in the top of a set of stacked sieves and shook them for 10 minutes by hand. We didn’t have a shaker table, and I borrowed the sieves. The school had some sieves already, but unfortunately they were not marked as to the size of the mesh, and rather than try to measure them again, I took the easy way out.
One thing that I stressed was the idea of the simple filter again. I asked them if I wanted to sort a sample like this, should I put the sieves with the bigger holes on the top of the stack, or on the bottom of the stack? Some were still unsure. I took 2 of the sieves and did a little demonstration to convince them that the bigger holes should be on top, so that the smaller stuff falls through to the next sieve down. For those that had it the wrong way around in their head, they seemed to remember the concept better after seeing the physical demonstration and reminding me about how their marble/button filter had worked.
We had the kids break up into groups of 5 (we had 5 things to weigh). Before we turned them loose, we had them all copy down the structure of a data table ahead of time. Mrs. Loeffler felt that it would be more beneficial if the students had that in hand when they went to record their data. Unfortunately, I didn’t label the columns particularly well at first, and that led to confusion later. Rather than naming the columns something more intuitively understandable (i.e., mass of sieve + sediment, mass of empty sieve, mass of sediment), I labeled them ending weight, beginning weight, and sediment, respectively. The students consistently recorded the mass of the full sieve as the mass of the sediment, rather than the ending weight, which was some meaningless term that they didn’t understand.
Since we only had 2 scales, we could only have 1 group weigh the sieves at a time, and since we only had 1 set of sieves, we could only process one sub sample at a time. I had hoped to do 5 sub samples of a composite and repeat the process again the next week, but in hindsight that was unrealistic.
One of the administrative challenges was what to do with the other groups while one was weighing sieves. In the first class, we tried to have 1 person from each group come back, and then relay their information to their group. This method got the data to the class faster, but only allowed 5 people out of about 25 to actually use the instruments and record the appropriate data. We then decided to just bring back whole groups at once, so that everyone could use the balances and have to fill out their own tables by either reading it off the scale or sharing data with each other.
Another challenge was to occupy the interest of the groups not at the scales. We had them do some related reading excercises, and later Mrs. Loeffler had the idea of showing them how to use a google spreadsheet to calculate the mass of sediments and to share the data. Most of the groups were on task for most of the time, but it was pretty chaotic.
It took all of last week and this week to introduce the idea and to allow every group to weigh the sieves and record their data. Next week, we are going to try and calculate percent weight, percent retained on the sieve, percent passing the sieve, and to graph and interpret the data.