Using a Control Chart to Detect Problem in Analysis

Suppose  you are working at your local sewage treatment facility.  One of your responsibilities is to monitor the effluent’s biological oxygen demand (BOD), which is a measure of the amount of biologically available organic material in the effluent.  Basically, a high BOD means that bacteria present in the water will require a large amount of O2 to decompose the available organic matter; thus, an effluent with a high BOD will ultimately have a lower concentration of dissolved O2.  A high BOD is undesirable as it may indicate that a water system is unable to support aquatic life.  As part of your job you use control charts to monitor the sewage treatment plant’s analysis of BOD levels.  Several scenarios are described below.  For each, discuss how you can use a control chart to detect the problem; feel free to sketch the resulting control chart if it helps to illustrate your answer.

Scenario A. The BOD levels are measured using an amperometric electrode that is positioned behind an O2-permeable membrane.  With time, the membrane begins to fail and the amount of O2 crossing the membrane decreases.

Scenario B. BOD measurements are temperature sensitive and appropriate temperature corrections must be made if results are to be accurate.  During a brief 3-day heat wave, the analyst forgot to make the appropriate temperature corrections.

Scenario C. A new standard is introduced to calibrate the BOD electrode; unfortunately, that standard has a negative determinate error.

Scenario D. The analysis is assigned to a less experience chemist whose work shows an increase in indeterminate errors.

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