Calculation-basis comparison
| Calculation | Required common basis | Output / decision | What it does not establish |
|---|---|---|---|
| BOD bottle reduction | Original sample fraction, same seed suspension and control geometry | Oxygen demand by bottle; explicit numerical exclusions | Full method compliance, batch release, GGA/blank acceptance or BOD-to-CBOD conversion |
| TSS / VSS gravimetry | Same prepared filter, net masses, original filtered volume and reconciled blank loss | Mass and concentration of retained, volatile and fixed fractions | Automatic reporting limit, organic-carbon content or biomass activity |
| Duplicate RPD | Truly paired sample, method, units and reporting basis | Precision; entered relative or low-level absolute comparison | Accuracy, recovery, valid nondetect substitution or whole-batch acceptance |
| Matrix-spike recovery | Native, measured spike and known addition on one explicit basis | Recovery of added analyte; comparison to entered limits | Automatic correction of official concentrations, universal limits or absence of matrix effects |
Direct seed and seeded water are not interchangeable
Direct seed uses the seed-suspension quantity in each bottle. Seeded dilution water uses the fraction occupied by that water, with no additional direct seed in this model. Document the full control-bottle volume and suspension identity. Unseeded blank depletion is a QC check, not a generic term to subtract from every bottle.
Precision and recovery are different percentages
RPD uses the mean of two uncensored paired observations as the denominator. Recovery uses the known analyte addition on a matched basis. A signed percent change against an original value is neither the absolute RPD used here nor a matrix-spike recovery.
Not assessed is an honest output
Zero-mean RPD is undefined. Censored values are inequalities, not measured zeros. No qualifying seed controls means the seeded result lacks a usable correction. Zero dry residue has no defined volatile percentage. Keep these conditions visible for method-specific review.
Mass and volume checklist
- 1 g =1000 mg;1 L =1000 mL.
- For net residue in mg and sample in mL, concentration in mg/L is mass×1000/volume.
- Confirm whether sample volume is original or already diluted.
- Confirm whether added spike volume and native dilution are represented in the reported concentration.
- A user-entered minimum captured mass is not a universally valid reporting limit.
Scope of the calculation
The tools do not contain method-specific holding times, calibration tolerances, universal GGA/blank limits, jurisdictional reporting tables, automatic data qualifiers or treatment recommendations. Use the current approved method and the laboratory's review process.
Sources
- Wisconsin DNR — BOD Seed: direct bottles and seeded dilution water
- Wisconsin DNR — Laboratory Study Guide
- U.S. EPA — Methods for Chemical Analysis of Water and Wastes (160.2 / 160.4)
- USGS — Design, Analysis, and Interpretation of Field Quality-Control Data
- U.S. EPA — Duplicate Relative Percent Difference definition
- Thermo Fisher Scientific — Why and How to Matrix Spike