Source: https://guthlabs.ai/docs/instruments/seawater-density-simplified-eos/mechanism/

instrument mechanism / verified-clean

# Seawater Density (simplified EOS) mechanism

The inspectable method record for Seawater Density (simplified EOS), kept separate from implementation and qualification.

Status: **VERIFIED_CLEAN**Lifecycle: **verified-clean**

## Model scope

The atmospheric-pressure UNESCO EOS-80 polynomial tested here is standard and gives the expected values. The instrument cannot be approved as specified because it advertises optional pressure without defining the pressure-dependent equation, units, range, or whether pressure is gauge or absolute. The honest-label statement is appropriate only if the exact EOS-80 approximation, validity range, and pressure limitation are displayed.

- The specification does not state the exact UNESCO EOS-80 density equation or its coefficients, so 'simplified linear/UNESCO-80-style polynomial' is not uniquely implementable.

- Pressure is listed as optional, but no pressure correction, pressure range, or pressure unit is specified. The atmospheric-pressure polynomial alone is not valid for nonzero pressure.

- The implementation should explicitly distinguish practical salinity S in PSU from mass-fraction salinity; PSU is dimensionless and should not be treated as g/kg without qualification.

## Units and conventions

- Temperature is in degrees Celsius on the IPTS-68/ITS-90-compatible EOS-80 temperature scale.

- Salinity is practical salinity S in PSU, numerically dimensionless, with S^(3/2) interpreted as S multiplied by sqrt(S).

- The tested polynomial computes in-situ density at zero gauge pressure, conventionally P=0 dbar; pressure-dependent density requires the UNESCO pressure correction.

- Output density is in kg/m^3.

- The atmospheric EOS-80 polynomial is intended approximately for -2 <= T <= 40 degrees C and 0 <= S <= 42 PSU; these constraints should be enforced or documented.

## Inspectable vector

One of 4 source-bound review vectors:

````
{
  "inputs": {
    "temperature_C": 0,
    "salinity_PSU": 0,
    "pressure_dbar": 0
  },
  "expected_outputs": {
    "density_kg_m3": 999.842594
  },
  "reference_data": {
    "rho_w_coefficients": {
      "a0": 999.842594,
      "a1": 0.06793952,
      "a2": -0.00909529,
      "a3": 0.0001001685,
      "a4": -0.000001120083,
      "a5": 6.536332e-9
    },
    "source_convention": "Widely tabulated standard value, UNESCO EOS-80/UNESCO Technical Papers in Marine Science 44 atmospheric pure-water density polynomial"
  },
  "working": "Use rho_w(T)=a0+a1*T+a2*T^2+a3*T^3+a4*T^4+a5*T^5. At T=0: rho_w=999.842594+0.06793952*0-0.00909529*0^2+0.0001001685*0^3-0.000001120083*0^4+0.000000006536332*0^5=999.842594 kg/m^3. At S=0, all salinity terms are zero, so rho=999.842594 kg/m^3."
}
````

## Failure boundary

**BREAK_CLAIMED** · SELF_FLAGGED

The negative-salinity vector produces an undefined real result and must be rejected. The convention probe also exposes a unit-label hazard: interpreting an unlabeled temperature value of 10 as Celsius versus Fahrenheit changes the expected density substantially.

## Other records

[01**Overview**](<https://guthlabs.ai/docs/instruments/seawater-density-simplified-eos/>)[03**Implementation**](<https://guthlabs.ai/docs/instruments/seawater-density-simplified-eos/implementation/>)[04**Qualification**](<https://guthlabs.ai/docs/instruments/seawater-density-simplified-eos/qualification/>)

## Sources and evidence

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

- `VERIFIED_LEDGER.json`

- `evidence-tranche7/tranche7-results.json`

- `BOUNDARY_HAZARD_CATALOG.json`