Lessons · Engineering · a unit-conversion chain from a spec
A datasheet in one system, a design in another
A conversion chain is one multiplication per unit, each factor written as a fraction equal to one, so the old units cancel on paper before any number is touched.
Hone is a place to practise a career, one idea a day. This is one of its lessons, written out in full and free to read without an account.
What it is for
The pump is American, the pipe is metric and the client's spec is in bar. Every number on the datasheet has to cross at least one border, and the chain is how it crosses without being dropped.
How to think about it
Write the datasheet number with its full unit. For each unit that has to change, multiply by one fraction. Cancel on paper. When only the wanted units are left, do the arithmetic once.
Worked example
A pump datasheet: 40 gal/min at 60 psi. The design is in SIThe border to cross.
Q = 40 gal/min × (3.785 L / 1 gal) × (1 min / 60 s) = 2.52 L/sGallons cancel, minutes cancel. Litres per second remain.
p = 60 psi × (6.895 kPa / 1 psi) = 413.7 kPaOne factor for pressure. About four bar.
Each factor is a fraction equal to one, so the value has not changed, only its nameWhy the method cannot go wrong if the units cancel.
Your turn
30 gal/min to L/s. Finish the chain.
Q = 30 × 3.785 / 60 = L/s
Solve one, graded on the server
The trap
US gallon versus imperial gallon. 3.785 L and 4.546 L; a British datasheet and an American one both say gallon, and the pump is 20 % wrong either way if the wrong one is picked.