How often should the skimmer "throat" and collection cup be cleaned? Is skimmer "performance" degraded by dirty equipment? Here is how I answered these questions.
Skimmer performance is defined here as the amount of material removed over time. This quantity is calculated by multiplying the skimmate volume by the concentration of the material in the skimmate. Unfortunately, determining the concentration of this material is typically out of reach of most of us. Selecting a surrogate measure that is proportional to the concentration of the material would be an approach. Skimmate color intensity is the surrogate I selected for this study.
Assume that color intensity (light absorbance) of the skimmate is proportional to the concentration of material in the skimmate (I have "circumstantial evidence" for this). To calculate the relative amount of material in the skimmate, I define a quantity "color total" determined by multiplying the skimmate volume by the intensity of the color or absorbance. The color intensity is defined as absorbance of blue light passing through the skimmate sample. In most cases, this number is dilution adjusted. The absorbance was measured with a digital camera. I monitored the daily "color total" of my counter current DIY 4" diameter skimmer between cleanings.
The first plot below shows the "color total" collected versus the number of days since skimmer cleaning. Three post-cleaning periods were observed. The plots indicate that skimmer performance declines over time. Since the amount of solid material deposited on the skimmer surface was not observed, the conclusion comes with the caveat "based solely on skimmate content". The spike in "color total" on day 6 after the second cleaning was due to an airflow adjustment. The large amount of dilute skimmate resulting from this adjustment also resulted in a large "color total". This increase in material removed is consistent with the notion that collecting wet skimmate removes more material per unit of time. The second plot is the average of daily "color total" and makes the decline trend easier to see.
My take away from this quick study is that I will try to clean my skimmer more often than 9 days and consider increasing air flow to achieve a wetter skimmate. I guess this confirms the current best practices.
Skimmer performance is defined here as the amount of material removed over time. This quantity is calculated by multiplying the skimmate volume by the concentration of the material in the skimmate. Unfortunately, determining the concentration of this material is typically out of reach of most of us. Selecting a surrogate measure that is proportional to the concentration of the material would be an approach. Skimmate color intensity is the surrogate I selected for this study.
Assume that color intensity (light absorbance) of the skimmate is proportional to the concentration of material in the skimmate (I have "circumstantial evidence" for this). To calculate the relative amount of material in the skimmate, I define a quantity "color total" determined by multiplying the skimmate volume by the intensity of the color or absorbance. The color intensity is defined as absorbance of blue light passing through the skimmate sample. In most cases, this number is dilution adjusted. The absorbance was measured with a digital camera. I monitored the daily "color total" of my counter current DIY 4" diameter skimmer between cleanings.
The first plot below shows the "color total" collected versus the number of days since skimmer cleaning. Three post-cleaning periods were observed. The plots indicate that skimmer performance declines over time. Since the amount of solid material deposited on the skimmer surface was not observed, the conclusion comes with the caveat "based solely on skimmate content". The spike in "color total" on day 6 after the second cleaning was due to an airflow adjustment. The large amount of dilute skimmate resulting from this adjustment also resulted in a large "color total". This increase in material removed is consistent with the notion that collecting wet skimmate removes more material per unit of time. The second plot is the average of daily "color total" and makes the decline trend easier to see.
My take away from this quick study is that I will try to clean my skimmer more often than 9 days and consider increasing air flow to achieve a wetter skimmate. I guess this confirms the current best practices.
and have a picture and scratch n' sniff of skimmate.
