Darcy-Weisbach Pressure Drop Calculator mechanism

Model scope

The two displayed Darcy-Weisbach formulas are standard and mutually consistent because ΔP=ρg·hf. The instrument cannot receive a PASS until the exact automatic-friction-factor inputs, flow-regime rules, Colebrook implementation, unit conventions, and validation behavior are specified. The empty honest_label is a concern: the product should state that automatic f requires roughness and fluid-viscosity data and that the result is frictional loss only, excluding fittings, valves, elevation, acceleration, and other minor losses.

• The Darcy-Weisbach formulas are correct when f is the dimensionless Darcy friction factor: hf=f·(L/D)·(V²/(2g)) and ΔP=f·(L/D)·(ρV²/2). The specification must explicitly distinguish Darcy friction factor from the Fanning friction factor, which differs by a factor of four.
• The stated automatic Colebrook option is underspecified: it requires pipe absolute roughness ε, fluid viscosity μ or kinematic viscosity ν, and a defined Reynolds-number calculation. The Colebrook equation applies to turbulent flow; laminar and transition-flow handling is not specified.
• The Colebrook equation and solution method or convergence behavior are not provided, despite the engine being described as plain arithmetic.

Units and conventions

• f is dimensionless Darcy friction factor, not Fanning friction factor.
• L and D must use the same length unit; V must use length per time; g must use length per time squared; ρ must use mass per volume; ΔP then has pressure units consistent with those quantities.
• For SI calculations, use L and D in metres, V in m/s, g in m/s², and ρ in kg/m³; outputs are metres of head and pascals.
• The standard gravitational acceleration assumption is g=9.81 m/s² unless the user supplies another value.
• The pressure formula assumes a consistent mass-based unit system. US customary lbm-based inputs require the gravitational conversion constant gc; using lbf-based density/force conventions must be explicitly defined.
• For automatic f, absolute roughness ε and fluid viscosity are required in addition to the listed inputs.

Inspectable vector

One of 4 source-bound review vectors:

{
  "inputs": {
    "f": 0.02,
    "L": 100,
    "D": 0.5,
    "V": 2,
    "rho": 1000,
    "g": 9.81,
    "units": "SI"
  },
  "expected_outputs": {
    "head_loss_m": 0.8154943935,
    "pressure_drop_Pa": 8000
  },
  "working": "L/D = 100/0.5 = 200. V²/(2g) = 2²/(2×9.81) = 4/19.62 = 0.2038735984 m. hf = 0.02×200×0.2038735984 = 0.8154943935 m. ρV²/2 = 1000×4/2 = 2000 Pa. ΔP = 0.02×200×2000 = 8000 Pa."
}

Failure boundary

BREAK_CLAIMED · SELF_FLAGGED

A zero pipe diameter produces undefined division in both formulas and must be rejected. The specification also leaves the explicit-f versus auto-Colebrook branch-selection convention and the sign convention for pressure drop insufficiently explicit.

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-tranche4/tranche4-results.json
• BOUNDARY_HAZARD_CATALOG.json