Process Units Conversion

Process Units Conversion

Overview

Deterministic unit conversions for the common process-engineering quantities (flow, pressure, viscosity, thermal conductivity, energy, temperature, length, area, volume, density, mass). All calculations are SI under the hood; converting to or from a non-SI unit goes through the SI value.

Prerequisites

1. uv available.

When to Use

• An input is given in psia, bar, lbm/hr, gpm, cP, ft, in, etc., and you

need SI before calling another skill.

• The user requests output in non-SI units (e.g. shop-floor reporting).
• You need to sanity-check a vendor data sheet expressed in mixed units.

Don't use for

• Computing properties that depend on state (density, viscosity, heat

capacity) — those need a property package (thermo-process-properties).

• Currency, financial, or non-physical conversions.
• Unit groups that are NOT pure conversions (e.g. API gravity to density

depends on water reference; check the underlying definition).

Utility Scripts

• uv run scripts/convert_units.py convert --value 100 --from-unit psi --to-unit Pa --output /tmp/p.json
• uv run scripts/convert_units.py convert --value 2.5 --from-unit gpm --to-unit m3_s --output /tmp/q.json
• uv run scripts/convert_units.py list-units --output /tmp/units.json

Procedure

1. Identify the source unit and the target unit.
2. Confirm the conversion is purely dimensional (no T/P/composition

dependence).

1. Run the script; inspect the JSON for the SI value as well.
2. If the result is suspicious (off by a factor of 10), check whether you

confused similar units (e.g. psia vs psig, bar vs barg, mol/h vs kmol/h).

Pitfalls

• Treating gauge pressure (psig, barg) as absolute (psia, bara). The skill

is unit-of-quantity, not gauge-vs-absolute; you must subtract / add atmospheric pressure yourself.

• Mixing mass and molar flow ("kg/s vs kmol/s"); they are different

dimensions and the skill will refuse them.

• Treating "Bbl" or "STB" without specifying the reference (oil vs water).
• Treating Celsius and Kelvin as interchangeable for delta-T calculations

(they are; for absolute, they are not). Convert intentionally.

• Treating cP and cSt as the same; cP is dynamic viscosity, cSt is

kinematic viscosity (= dynamic / density).

• Converting GPM to m³/s and forgetting US-vs-Imperial gallons.
• Inputting flow in "mol%" when the schema expects mole fraction.
• Reporting a converted value with more digits than the source warrants.

Fallback Strategies

• If the requested unit is not in the registry, surface to the user; do

NOT guess the conversion factor.

• For non-trivial conversions (e.g. API gravity, gas SCF at standard

conditions), use a dedicated property routine instead.

Verification

• Run the listed script with representative inputs and an --output file when a deterministic calculation is available.
• Confirm the JSON result contains ok: true, expected units, and no unhandled warnings.
• Check result magnitudes against the stated assumptions, references, and a hand calculation or known operating range before reporting them.

References

• references/unit_conventions.md — supported quantities and units.

Anti-Patterns

• Embedding a "convenient" conversion factor in another script instead of

routing through this skill.

• Reporting bar and psi values together without naming gauge vs absolute.

Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: jskherman
• Source: jskherman/engg-skills
• License: Apache-2.0

Install and usage instructions live in the source repository linked above.