Composite Sampler Pacing Calculator

Program a flow-paced composite sampler: enter the bottle size, the number of aliquots, and the flow to be sampled, and get the aliquot volume and pacing interval.

Composite bottle volume (L)

Number of aliquotsAliquots collected over the compositing period.

Total flow to sample (gal)Expected flow over the compositing period.

Flow meter pulse (gal/pulse)Optional — for pulse-paced samplers. Leave blank to skip.

Flow-Paced Composite Setup

Item	Value

How the calculation works

Flow-paced composite setup
Aliquot volume (mL) = bottle volume (L) × 1,000 ÷ number of aliquots
Flow per aliquot (gal) = total flow (gal) ÷ number of aliquots
Pulses per aliquot = flow per aliquot (gal) ÷ pulse volume (gal/pulse)

Notes & limitations

Flow-paced (flow-proportional) composites are the standard for NPDES wastewater compliance sampling (see 40 CFR 122.21 and your permit’s sampling section): each aliquot represents an equal volume of flow, so the composite reflects the true flow-weighted concentration.

Keep individual aliquots at 100 mL or more when practical so the lab has enough volume for every analysis plus QC.
Refrigerate the composite at ≤ 6°C during collection; observe the holding time for each parameter in 40 CFR 136.
Time-paced composites are acceptable only where the permit allows them — they bias toward high-flow periods in variable flows.
Verify the sampler intake velocity and purge settings per the manufacturer’s manual and your sampling plan.

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