Tips to avoid the Filtering Faux Pas

Filtering is an essential part of processing LMVP water samples. To ensure the data are of the highest quality possible we issue an annual “report card” to our volunteers summarizing the precision of their filtering technique. The grades, which are based on the difference between both chlorophyll filters’ values, provide valuable feedback to volunteers by reinforcing good techniques and revealing bad techniques.

Usually the difference between the filters is small and the volunteer gets a good grade. In 2007, for example, nearly 90% of the paired filters rated either “excellent” or “good.” Occasionally, however, the paired values are very different from one another and a poor grade is given. If you received a poor grade on your card last year, it is probably due to a common error that is easily correctable.
Here’s a review of some of the common errors that we see with volunteer processed filters. The same issues apply to both chlorophyll and suspended solids filters. While these errors don’t occur frequently, they are common enough to warrant addressing.

Labeling: Don’t write on the filter “house” when the filter is still inside. The pressure from the pencil can damage the filter. If the damage occurs before filtering, then water and the material we’re measuring can pass through the tiny holes created by the pencil. If the damage occurs after filtering, the wet filter can be mangled and pieces of it lost. Measuring the volume: Be certain that the correct volume of water passes through the filter and that the volume is recorded on both the filter house and the data sheet. When we see poor replication, we often notice that one filter will have twice the concentration of chlorophyll found on its duplicate. This is probably the result of accidentally doubling the volume of water going through a single filter. This can happen if a volunteer (or even a lab employee) forgets that they’ve already filtered the correct volume of water through the filter and repeats the process unnecessarily.

Folding the filter: When you remove the filter from the filtering apparatus, be careful that you don’t tear the filter. Occasionally a piece of the filter might break off. If this happens, place the piece inside the filter before folding. When folding the filter, be sure that the stained area is on the inside of the folded filter. Also, double check that all of the colored circle on the filter is enclosed within the fold. This prevents the material we’re measuring from rubbing off and sticking to the inside of the paper “house” we use to transport the filter.

To protect the integrity of the LMVP dataset, we will discard values that don’t replicate well. Poor replication, while rare, creates holes in the dataset which represent both a lost opportunity and wasted effort. By going slow and being careful, you can prevent that from happening.

5 Common Filtering Issues

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