Convert common pressure units without losing a factor of ten or one thousand.
Core idea
The core relationships are 1 bar = 1000 millibar and 1 bar = 100 kilopascal. Therefore 1 kilopascal = 10 millibar. Write the relationship first, then decide whether the numerical value should become larger or smaller in the new unit. The pascal itself is one newton of force acting on each square metre (1 Pa = 1 N/m²). That is a very small pressure, which is why plumbing figures are given in kilopascals, bar or millibar. Rearranged, force = pressure × area, with the pressure in pascals and the area in square metres. Mains water at 300 kPa (300,000 Pa) acting on a valve washer over a 20 mm² orifice (0.00002 m²) pushes on it with 300,000 × 0.00002 = 6 N; multiplying 300 by 20 without converting the units gives a meaningless 6000.
What to recognise
- Convert bar to millibar by multiplying by 1000.
- Convert millibar to bar by dividing by 1000.
- Convert bar to kilopascal by multiplying by 100.
- Convert kilopascal to millibar by multiplying by 10.
- Keep gauge and absolute pressure descriptions separate when the context uses them.
Apply it
For 1.5 bar: 1.5 × 1000 = 1500 mbar and 1.5 × 100 = 150 kPa. For 21 mbar: 21 ÷ 1000 = 0.021 bar and 21 ÷ 10 = 2.1 kPa. A quick magnitude check catches most errors.
Limits
Converting units correctly does not tell you a safe test pressure or operating limit: use the procedure, the instrument's range and resolution, and the manufacturer's data for the actual system. Never use bar in place of millibar in a gas procedure just because both measure pressure.
Gauge and absolute pressure
A pressure gauge reads zero when it is open to the air, so what it shows is gauge pressure: how far the pressure is above atmospheric. Absolute pressure adds the atmosphere back: absolute = gauge + atmospheric. Taking the atmosphere as about 1 bar, a gauge showing 2.0 bar means about 3.0 bar absolute. System pressures, test pressures and relief-valve settings are normally gauge figures; gas-law calculations need absolute pressure.
Trapped gas and temperature
For a fixed amount of gas in a sealed, rigid vessel, absolute pressure rises in proportion to absolute temperature: P2 = P1 × T2 ÷ T1, with each temperature in kelvin (°C + 273). Air at 2.0 bar absolute warming from 7 °C (280 K) to 35 °C (308 K) reaches 2.0 × 308 ÷ 280 = 2.2 bar absolute. The Celsius figure has risen fivefold, but the pressure only by a tenth.