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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.

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In beta. This lesson was written for Hone and has not yet been checked by a licensed engineer. Practice material, not professional advice. What that means.

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 SI
The border to cross.
Q = 40 gal/min × (3.785 L / 1 gal) × (1 min / 60 s) = 2.52 L/s
Gallons cancel, minutes cancel. Litres per second remain.
p = 60 psi × (6.895 kPa / 1 psi) = 413.7 kPa
One factor for pressure. About four bar.
Each factor is a fraction equal to one, so the value has not changed, only its name
Why 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

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.

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