NIST WebBook Thermodynamic Properties Skill

Fetches thermodynamic and transport properties of pure fluids from the NIST Chemistry WebBook (webbook.nist.gov) using the fluid properties CGI interface. Covers natural gas components, CO2, water/steam, and common solvents relevant to petroleum and chemical engineering.

No API key required. NIST WebBook is a free public resource.

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API Structure

Base URL: https://webbook.nist.gov/cgi/fluid.cgi

The NIST fluid properties interface returns tab-delimited ASCII text with a header row. All property data is in the requested unit system.

Core Parameters

Parameter	Description	Example Values
Action	Request type	Load
ID	Fluid identifier (CAS-based, e.g. C74828)	See fluid table below
Type	Property table type	SatT, SatP, IsoTherm, IsoBar, IsoChor
Digits	Significant figures	5 (recommended)
THigh	Upper temperature bound	300
TLow	Lower temperature bound	100
TInc	Temperature increment	10
PHigh	Upper pressure bound (for IsoTherm)	10
PLow	Lower pressure bound	1
PInc	Pressure increment	1
RefState	Reference state	DEF (default), NBP, IIR, ASHRAE

Unit Parameters

Parameter	Options
TUnit	K, C, F, R
PUnit	MPa, bar, atm, psia, kPa
DUnit	mol%2FL (mol/L), kg%2Fm3 (kg/m3), g%2FmL (g/mL)
HUnit	kJ%2Fmol, kJ%2Fkg, BTU%2Flbmol
WUnit	m%2Fs (m/s), ft%2Fs (ft/s)
VisUnit	uPa*s (microPa-s), cP (centipoise)
STUnit	N%2Fm (N/m), dyn%2Fcm

Type Parameter Reference

Type	Description	Required Params
SatT	Saturation properties vs temperature	THigh, TLow, TInc
SatP	Saturation properties vs pressure	PHigh, PLow, PInc
IsoTherm	Isothermal P scan at fixed T	THigh=TLow=T, PHigh, PLow, PInc
IsoBar	Isobaric T scan at fixed P	PHigh=PLow=P, THigh, TLow, TInc
IsoChor	Isochoric (constant volume)	DHigh, DLow, DInc

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Fluid ID Reference

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

Fluid	NIST ID	CAS Number	Notes
Methane	C74828	74-82-8	Primary natural gas component
Ethane	C74840	74-84-0	NGL component
Propane	C74986	74-98-6	NGL, LPG
n-Butane	C106978	106-97-8	NGL
i-Butane	C75285	75-28-5	NGL
n-Pentane	C109660	109-66-0	NGL
n-Hexane	C110543	110-54-3	Solvent, NGL
n-Heptane	C142825	142-82-5	Solvent
n-Octane	C111659	111-65-9	Solvent
Carbon dioxide	C124389	124-38-9	EOR, injection design
Water	C7732185	7732-18-5	Steam tables, reservoir
Nitrogen	C7727379	7727-37-9	Injection gas
Hydrogen sulfide	C7783064	7783-06-4	Sour gas
Hydrogen	C1333740	1333-74-0	Hydrogen applications
Oxygen	C7782447	7782-44-7	Combustion
Argon	C7440371	7440-37-1	Inert gas
Helium	C7440597	7440-59-7	Tracer
Ammonia	C7664417	7664-41-7	Refrigerant
R-134a (HFC)	C811972	811-97-2	Refrigerant

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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'

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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:

Column	Description	Typical Units
Temperature	T	K or C or F
Pressure	P	MPa or psia
Density	rho	kg/m3
Volume	v	m3/kg
Internal Energy	U	kJ/kg
Enthalpy	H	kJ/kg
Entropy	S	kJ/(kg-K)
Cv	Isochoric heat capacity	kJ/(kg-K)
Cp	Isobaric heat capacity	kJ/(kg-K)
Sound Spd.	Speed of sound	m/s
Joule-Thomson	J-T coefficient	K/MPa
Viscosity	Dynamic viscosity	uPa*s
Therm. Cond.	Thermal conductivity	W/(m-K)
Phase	Phase label	liquid / vapor / supercritical

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).

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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.

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Error Handling

Condition	Action
Response contains "Error" text	Invalid fluid ID or out-of-range T/P; check ID against reference table and verify T/P within fluid's valid range
Response is empty after header	Temperature or pressure outside the fluid's valid range; adjust bounds
Phase boundary crossed in IsoTherm	Two-phase region encountered; switch to SatT query for that range, then IsoTherm above and below saturation
Fluid ID not in table	Search NIST WebBook manually at webbook.nist.gov/cgi/cbook.cgi?Name=FLUID&Units=SI and copy the fluid ID from the "Fluid Properties" link URL
Mixture requested (e.g., natural gas)	NIST WebBook covers pure components only; compute mixture properties by component fraction weighting, or recommend NIST REFPROP for rigorous mixture EOS
Very large T/P range returns too many rows	Increase TInc or PInc to reduce row count

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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.

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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