NIST WebBook Thermodynamic Properties Skill

Unit Parameters

Type Parameter Reference

Fluid ID Reference

See references/api_reference.md for the complete fluid list. Key fluids for petroleum and chemical engineering:

Example API Calls

Saturation Properties — Methane from 100 K to 200 K

curl -s "https://webbook.nist.gov/cgi/fluid.cgi?Action=Load&ID=C74828&Type=SatT&Digits=5&THigh=200&TLow=100&TInc=10&RefState=DEF&TUnit=K&PUnit=MPa&DUnit=mol%2FL&HUnit=kJ%2Fmol&WUnit=m%2Fs&VisUnit=uPa*s&STUnit=N%2Fm"

Saturation Properties — Water from 100 C to 374 C (steam tables)

curl -s "https://webbook.nist.gov/cgi/fluid.cgi?Action=Load&ID=C7732185&Type=SatT&Digits=5&THigh=374&TLow=100&TInc=10&RefState=DEF&TUnit=C&PUnit=MPa&DUnit=kg%2Fm3&HUnit=kJ%2Fkg&WUnit=m%2Fs&VisUnit=uPa*s&STUnit=N%2Fm"

Isothermal Properties — CO2 at 50 C from 1 to 20 MPa

curl -s "https://webbook.nist.gov/cgi/fluid.cgi?Action=Load&ID=C124389&Type=IsoTherm&Digits=5&THigh=50&TLow=50&TInc=0&PHigh=20&PLow=1&PInc=1&RefState=DEF&TUnit=C&PUnit=MPa&DUnit=kg%2Fm3&HUnit=kJ%2Fkg&WUnit=m%2Fs&VisUnit=uPa*s&STUnit=N%2Fm"

Isobaric Properties — Methane at 3000 psia from 200 F to 400 F

curl -s "https://webbook.nist.gov/cgi/fluid.cgi?Action=Load&ID=C74828&Type=IsoBar&Digits=5&THigh=400&TLow=200&TInc=10&PHigh=3000&PLow=3000&PInc=0&RefState=DEF&TUnit=F&PUnit=psia&DUnit=kg%2Fm3&HUnit=kJ%2Fkg&WUnit=m%2Fs&VisUnit=uPa*s&STUnit=N%2Fm"

Parse Tab-Delimited Response (bash + awk)

curl -s "https://webbook.nist.gov/cgi/fluid.cgi?..." \
  | awk -F'\t' 'NR==1{print; next} /^\s*$/{next} {print}' \
  | column -t -s $'\t'

Workflow

Step 1 — Identify Fluid

Map the user's fluid description to a NIST fluid ID. Use the table above or the extended reference in references/api_reference.md. If the user specifies a mixture (e.g., "natural gas"), note that NIST WebBook covers pure components only — compute mixture properties by combining component data using mixing rules, or advise using NIST REFPROP for rigorous mixture calculations.

Step 2 — Identify Property Type and Conditions

Determine the appropriate query type:

• Saturation curve (vapor pressure, boiling point, latent heat): use SatT or SatP
• Single-phase P-v-T at fixed temperature: use IsoTherm
• Single-phase H-S at fixed pressure (process design): use IsoBar
• Specific P-T point: use IsoTherm with PHigh=PLow and small pressure range

Convert user units to NIST parameter values. Common conversions:

• 200 F = 93.3 C = 366.5 K
• 3000 psia = 20.68 MPa
• 1 cp = 1000 uPa*s

Step 3 — Build URL and Fetch

Construct the URL using the base URL and parameters above. Execute with curl. Check that the response starts with column headers (Temperature, Pressure, etc.) — if it starts with "Error" or "Not Found", the fluid ID or range is invalid.

Step 4 — Parse Response

The response is tab-delimited text. Key output columns:

For saturation queries, NIST returns two rows per temperature: one for the liquid phase and one for the vapor phase.

Step 5 — Identify Phase and Produce Output

Note the phase from the Phase column. Flag if conditions are:

• Supercritical: T > Tc and P > Pc (CO2: Tc=31.1 C, Pc=7.38 MPa; CH4: Tc=-82.6 C, Pc=4.60 MPa; H2O: Tc=374.0 C, Pc=22.06 MPa)
• Two-phase: user requested single-phase type but conditions cross the saturation curve — split into separate saturation query plus single-phase queries above and below

Include a brief note on engineering relevance (viscosity for wellbore flow, density for hydrostatic head, Cp for heat duty calculations).

Output Format

Present results as a markdown table capped at ~25 rows for readability. Always state temperature, pressure, and fluid name in the table title. Then provide a narrative summary.

## Methane Properties — Isothermal at 300 K, 1-20 MPa

| P (MPa) | Density (kg/m3) | Enthalpy (kJ/kg) | Viscosity (uPa-s) | Phase |
|---------|-----------------|------------------|-------------------|-------|
| 1.0     | 6.33            | 864.2            | 11.2              | vapor |
| 5.0     | 34.0            | 836.8            | 12.4              | vapor |
| 10.0    | 76.3            | 793.1            | 14.9              | vapor |
| 15.0    | 131.2           | 737.6            | 19.1              | vapor |
| 20.0    | 194.4           | 672.3            | 25.8              | supercritical |

**Summary:** Methane at 300 K transitions from dilute gas behavior at low
pressure to a dense, increasingly liquid-like fluid above ~15 MPa.
Density increases 30x from 1 to 20 MPa. Viscosity increases only 2.3x
over the same range — important for reservoir flow models, which are
sensitive to viscosity but less so at these values (methane remains low
viscosity even at reservoir conditions). At 300 K, methane is above its
critical temperature (190.6 K) at all pressures shown — no phase change
occurs. For WVU Appalachian wells at ~2000-4000 psia (14-28 MPa) and
200-300 F (93-149 C), methane is always supercritical.

Error Handling

Caveats and Data Limitations

• Pure components only. NIST WebBook does not calculate mixture properties. For mixtures (natural gas, reservoir fluids), use component-level data with appropriate mixing rules or advise the user to use REFPROP or a process simulator.
• Reference state matters for enthalpy and entropy. Values are relative to the chosen reference state (DEF = NIST default, usually 0 K or NBP). Do not mix enthalpies from different reference states.
• Data accuracy varies by fluid. Equation of state accuracy is highest for well-studied fluids (methane, water, CO2, propane). Less common fluids may have higher uncertainty, especially near the critical point.
• Near-critical region is difficult. Within ~10% of Tc and Pc, properties diverge rapidly (infinite Cp at Tc, Pc). Use smaller increments near the critical point.
• Engineering correlations vs. NIST. The tNav/tnav-reservoir-sim skill uses Standing, Beggs-Robinson, and similar empirical correlations calibrated to specific fluid types. NIST WebBook is more accurate for pure components. Use NIST for precise single-component property lookups, and correlations for reservoir fluid mixtures where compositional data is unavailable.

Implementation Notes

• Use bash_tool with curl to make requests; pipe through awk or python to parse the tab-delimited output
• For unit conversion: multiply uPa*s by 0.001 to get cp (centipoise); multiply kJ/kg by 0.4299 to get BTU/lb
• See references/api_reference.md for the complete fluid ID table (50+ fluids), all unit codes, and worked examples in Go
• Critical constants for key fluids are in references/api_reference.md