How to read a catalogue and pressure gauge
The prefixes kilo and mega mean one thousand and one million pascals. The unit psi means pound-force per square inch. The notation psig denotes gauge pressure and psia absolute pressure. If a value lacks this distinction, verify its meaning before using it in the specification.
Watch out for pressure differences
Filter pressure loss may be stated in mbar, kPa or bar. First put the values into the same units, then add the losses along the entire flow path. For example, 200 mbar is not 2 bar. Conversion between bar(g) and bar(a) is different from unit conversion; it requires knowledge of atmospheric pressure.
What to check in practice
- Use one unit and explicitly state the pressure type in the technical specification.
- Round catalogue values only after conversion.
- Distinguish the sensor range from the operating value and the equipment's allowable pressure.
Example scenario
A machine requirement of 0.6 MPa(g) is 600 kPa(g), or 6 bar(g), approximately 87 psi(g). At an atmospheric pressure of 1 bar(a), this corresponds to 7 bar(a), not 6 bar(a).
| Unit | Equivalent |
|---|---|
| 1 bar | 100,000 Pa = 100 kPa = 0.1 MPa |
| 1 kPa | 1,000 Pa = 0.01 bar |
| 1 MPa | 10 bar |
| 1 psi | ≈ 6,894.76 Pa ≈ 0.0689476 bar |
| 1 bar | ≈ 14.5038 psi |
What to watch out for
- Confusing MPa and bar, causing a tenfold error.
- Losing the gauge-pressure designation when transcribing a foreign-language catalogue.
- Treating the same numerical value in different units as a comparable pressure.
How precisely should psi be converted?
For ordinary specifications, 1 psi ≈ 6.89476 kPa is sufficient. For guarantees and measurements, retain the required precision and the units stated in the report; additional decimal places do not improve the accuracy of the sensor itself.