In a 1,000 ml six-part composite sample, if a portion sample was taken when the flow was 5.3 mgd, the average daily flow was 4.8 mgd, what will be the size of this portion?

• A. 132 ml
• B. 222 ml
• C. 156 ml
• D. 184 ml

Answer: (D)

Sizing a flow-proportional portion for a six-part composite sample. In flow-proportional sampling, each portion is adjusted in size based on the ratio of the instantaneous flow to the average daily flow. Start with the base portion: 1,000 ml total divided by six parts equals about 166.7 ml per portion when flow matches the average.

Now adjust for the higher-than-average flow at sampling: 5.3 mgd divided by 4.8 mgd equals about 1.104. Multiply the base portion by this factor: 166.7 ml × 1.104 ≈ 184 ml.

So the size of this portion is about 184 ml. This approach ensures the composite reflects periods of higher flow by contributing more volume to the sample during those times. If the flow were lower than average, the portion would be smaller than the base 166.7 ml; if higher, larger, in line with the ratio.