Technically Speaking, It All Adds Up...
is a series of articles for people who want to obtain the best possible results from their fluorometer. Last month Technically Speaking described how to optimize your fluorometer installation when monitoring chlorophyll a in a continuous flow setup, using a fluorometer such as the Turner Designs AlgaeWatch. This month's article will describe how algae buildup on the sample cell in the AlgaeWatch impacts the displayed chlorophyll reading; and some cleaning tips to minimize the impact of the buildup.
Determining When to Clean a Fluorometer Sample Cell
Even if you are using the Basket Strainer on the inlet to the AlgaeWatch Sample Cell, described in last month's article, you will still need to clean the sample cell periodically to optimize your measurement accuracy and repeatability. When to clean? If your fluorometer is remotely located, then minimizing the frequency of cleaning becomes an important issue because of the impact on operating expenses, (time).
The following procedure will give you an appreciation for how to optimize the schedule for cleaning the sample cell.
Step 1 - Decide on acceptable change in reading due to fouling buildup.
This is an arbitrary value, and will generally be a value greater than the AlgaeWatch accuracy specification, (±3%), up to a maximum acceptable change in reading due to buildup on the sample cell. For this explanation, it will be assumed that the change in reading due to sample cell fouling should be less than -10%.
Step 2 - Measure without cleaning!
To determine if the fluorometer sample cell needs cleaning, start by injecting some calibration solution and noting the reading - be sure that you don't clean the cell before making this first reading, since the value will be used as a benchmark.
Step 3 - Clean the Sample Cell
Clean the sample cell by inserting the accessory cleaning brush for the AlgaeWatch, and vigorously clean the cell. (The cell is located behind the desiccant plug on the sample chamber, and can be used to confirm how far to insert the brush when cleaning). Continue brushing until the cleaning brush has removed all the debris and/or fouling, (equal to the bristles being clean when the brush is removed ).
Step 4 - Measure the calibration solution
Using the same calibration solution as in Step 2, inject a sample of the remaining solution until the reading stabilizes again to within ±0.2. Note the new reading of the same solution. If this new reading differs from the reading obtained in Step 2 by more than the arbitrarily established value of 10%, then the time from the last cleaning was too long, and appropriate adjustments to the cleaning schedule should be made, (reduce the cleaning interval).
Less than 10% change means that all the data collected since the last cleaning of the sample cell is within the arbitrarily established limits of Step 1.
Note that it is unlikely that fouling buildup has a known relationship with time, (and it will vary with the seasons, etc.). This means that it is not possible to determine a correction factor for the fluorometer readings for the time back to the last cleaning.
To reduce the chance of having to discard data, it is better to clean the sample cell as frequently as possible, noting the change in reading after cleaning, and making appropriate changes to the cleaning schedule.
Next month's article will discuss how the environment affects the measurement of chlorophyll. For more information on cleaning the sample cell, view the video.