The Conversion Traps — Read Once, Save a Career
A converter is trivial arithmetic; the engineering is in not converting the wrong thing. These are the traps that recur in plant work:
Gauge versus absolute
Pressure units convert cleanly; pressure references do not. 10 bar(g) is 11.013 bar(a) at sea level, and the difference is a full atmosphere of error in any saturation, density or performance calculation. The unit tells you the scale; only the (g) or (a) suffix tells you the zero. When a datasheet is silent, process pressures are usually gauge and vacuum/condenser pressures are usually absolute — verify, never assume.
The gallon problem
The US gallon (3.785 L) and the imperial gallon (4.546 L) differ by 20%. Flow conversions involving gpm in older British-influenced documentation — common in Pakistani plants of a certain era — have poisoned more than one pump calculation. This converter uses US gallons and says so.
Energy units and the MMBTU
Gas is bought in MMBTU, tariffs argued in kWh and GJ, and boiler datasheets written in kcal — with 1 MMBTU = 1.05506 GJ = 293.07 kWh. The other recurring error is basis, not unit: an MMBTU of HHV is not an MMBTU of LHV (about 10% apart on natural gas). Convert the unit, then interrogate the basis — as in the Heat Rate Converter.
Temperature: values versus intervals
Converting a temperature value needs the offset (32, 273.15). Converting a temperature difference — a ΔT in a heat-exchanger calculation — must not use the offset: a 10 °C rise is an 18 °F rise, not 50 °F. Half the "wrong" heat balances in student and plant calculations alike are this single mistake.
Assumptions behind this tool
- psi uses the exact definition 1 psi = 6,894.76 Pa; mmHg is the conventional 133.322 Pa; kg/cm² is the technical atmosphere, 98,066.5 Pa — within 2% of a standard atmosphere, which is precisely why confusing the two survives undetected for years.
- hp is mechanical horsepower (745.7 W); metric horsepower (PS, 735.5 W) appears on German nameplates and is a different animal.
- Factors are exact definitions or NIST values — conversions here are not estimates, unlike most else on this page.
Worked example (the defaults above)
10 bar = 145.04 psi — and if that 10 bar was a gauge reading, it is 159.7 psia, a distinction worth 10% of a compressor calculation.
References
NIST SP 811 — Guide for the Use of the International System of Units; BIPM SI Brochure.
Related calculators: Heat Rate & Efficiency · Three-Phase Motor