Switching a pressure between bar, metres of water head and kPa using simple conversion factors.
Plumbing pressures get quoted in different units, so being able to convert between bar, metres of head and kilopascals is a handy skill. This worked example uses chosen figures.
The conversions. About 10 metres of water head is roughly 1 bar; 1 bar is 100 kilopascals (kPa); and 1 kPa is 1000 pascals (Pa), so 1 bar is 100,000 Pa. The 10 metres figure is a rounded rule of thumb.
The situation. A system is running at 3 bar and we want that expressed in metres head and in kPa.
Step 1 - bar to metres head. At about 10 metres per bar: 3 x 10 = about 30 metres of head.
Step 2 - bar to kPa. At 100 kPa per bar: 3 x 100 = 300 kPa, which is 300,000 Pa.
Working the other way. If a cistern gives 2.5 bar at an outlet, that is about 2.5 x 10 = 25 metres of head and 2.5 x 100 = 250 kPa, so you can move between any of the units the same way.
Sense check. The 10 metres per bar rule is why people say you need roughly 1 bar for every 10 metres you have to push water up. For precise work use the exact relationship from pressure equals density times gravity times height, and keep your units consistent.
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