Water Hardness Calculator (ppm CaCO3) mechanism

Model scope

The arithmetic formula is correct as a rounded standard total-hardness conversion. Verdict is NEEDS_REVIEW because the claimed classification output lacks explicit bands and endpoint rules in the specification; the test vectors use soft <=60, moderately hard >60 to <=120, hard >120 to <=180, and very hard >180 mg/L.

• The hardness formula is standard when Ca2+ and Mg2+ are elemental-ion concentrations in mg/L: the more precise conversion factors are approximately 2.497 for Ca2+ and 4.118 for Mg2+, so 2.5 and 4.1 are accepted rounded coefficients.
• The specification claims a soft/hard classification but does not define the classification bands or how continuous values exactly at 60, 120, and 180 mg/L are handled. Under the stated USGS-style convention, use soft <=60, moderately hard >60 and <=120, hard >120 and <=180, and very hard >180 mg/L.

Units and conventions

• Ca2+ and Mg2+ inputs must be concentrations of elemental calcium and magnesium ions, not concentrations already expressed as CaCO3 equivalent.
• The formula requires both concentrations in mg/L. If a concentration is supplied in g/L, convert using 1 g/L = 1000 mg/L before applying the formula.
• For dilute water, ppm CaCO3 is treated as numerically equivalent to mg/L CaCO3, assuming water density is approximately 1 kg/L.
• Ca2+ and Mg2+ concentrations must be nonnegative.

Inspectable vector

One of 4 source-bound review vectors:

{
  "inputs": {
    "Ca2_mg_L": 20,
    "Mg2_mg_L": 10
  },
  "expected_outputs": {
    "hardness_ppm_CaCO3": 91,
    "classification": "moderately hard"
  },
  "working": "Ca contribution = 2.5 x 20 = 50 mg/L as CaCO3. Mg contribution = 4.1 x 10 = 41 mg/L as CaCO3. Total hardness = 50 + 41 = 91 mg/L, numerically 91 ppm. Since 60 < 91 <= 120, classification = moderately hard."
}

Failure boundary

BREAK_CLAIMED · SELF_FLAGGED

The plain-arithmetic implementation is vulnerable to non-finite results for extreme concentrations and may also accept physically invalid negative concentrations, producing a misleading hardness value and classification.

Other records

Sources and evidence

These pointers bind the page to evidence. They do not grant authority or collapse verification into deployment.

• VERIFIED_LEDGER.json
• evidence-tranche5/tranche5-results.json
• BOUNDARY_HAZARD_CATALOG.json