Engineering Unit Converter: Which Conversions Cause Valve Ordering Mistakes
The engineering unit converter handles the six conversions that come up every week when a datasheet, a datasheet revision and a client specification all use different units: pressure, length, temperature, flow, torque and Cv↔Kv.
Converting is arithmetic. The mistakes happen because two engineers agree on a number without agreeing on the datum or the reference condition. These are the ones worth checking twice.
1. Gauge or absolute pressure
1 bar is 14.5038 psi either way — the trap is barg versus bara. Valve pressure ratings are usually gauge; flow calculations, gas laws and relief valve sizing need absolute. A 100 psig line is 114.7 psia, and using 100 in a gas formula understates the flow coefficient by about 13%.
2. Nm³, Sm³ and scf are not the same
| Unit | Reference condition | Typical user |
|---|---|---|
| Nm³ | 0°C, 1.013 bar | European and Chinese datasheets |
| Sm³ | 15°C, 1.013 bar | Gas industry, some Gulf specifications |
| scf / scfh | 60°F, 14.7 psia | American datasheets and Cv formulas |
The difference between Nm³ and Sm³ is about 5.5%, which is larger than most design margins on gas lines. Always state the reference condition in the enquiry.
3. Cv and Kv are different numbers for the same valve
Kv = 0.865 × Cv. If you compare a European datasheet (Kv) against an American one (Cv) without converting, you will pick a valve about one size too small. The converter does both directions; the Cv/Kv calculator then turns the number into a duty.
4. Torque units decide actuator sizing
ft·lb to N·m is ×1.3558 — but actuator datasheets mix break torque, run torque and safety factors, and some quote in in·lb. A gear operator chosen from a converted number without the break-to-run margin will fail on a cold, high-differential valve exactly when it matters.
5. Temperature: absolute versus interval
20°C is 68°F, but a temperature rise of 20°C is a rise of 36°F, not 68. Insulation, cryogenic extension and heat-tracing specifications are usually written as intervals, and this is where the two conventions get mixed.
6. Pressure classes are not pressures
Class 300 is not 300 psi; it is a rating curve that varies with material and temperature (ASME B16.5). That is why our converter does not offer "Class 150 → psi" — ask us for the rating at your design temperature instead, it takes one message and avoids a wrong selection.
How we use it internally
When a client sends a datasheet in different units from our quotation, we convert in the open and state the converted values in the offer, so both sides are checking the same numbers. It is the cheapest way to avoid a change order later.
Open the engineering unit converter · size the valve with the Cv/Kv calculator · or send the datasheet to websale@sawye.com and we will convert and quote it.