On English vs. SI/Metric Units

I recently got into a little bit of an argument with a co-worker about the merits of English units.  He cited some grossly inaccurate internet articles to support his point of view, and I wanted to throw an internet article of my own out there to counter.

Up front: I prefer to use English units.  I grew up with pounds, feet, and miles.  If I'm doing an engineering calculation, that experience gives me an instant gut feel for sizes and magnitudes.  However, I have no problem with SI units.  If you grew up with kilograms, meters, and kilometers your experience is probably better suited to using them.  The sanity check is an important tool in engineering, and the choice of units inherently boils down to personal preference (and your organization's practices of course).

On validity:  Both systems are fully consistent and valid.  The SI system has some minor advantages in terms of learning curve due to its use of multiples of ten to subdivide units.  I fully acknowledge this, but still prefer English units having invested the up front effort to understand them.

On the consistency of English units:  The major source of confusion with English units seems to be the units of mass.  The slug is the consistent unit of mass, the pound is the consistent unit of force.  If you use slugs, your calculations will never be wrong due to units.  The slug is defined such that 1 slug will accelerate 1 ft/s^2 when 1 pound of force is applied to it (think F=Ma, (lbf)=(slug)(ft/s^2)).  Now American engineers like to talk in terms of "pounds mass" (lbm) sometimes.  This is a unit of mass that happens to have an equivalent value to its own weight on the surface of earth.  As such, it is 1/32.2 of a slug.  It is not consistent with pounds force and ft/s^2, so a conversion to slugs must often be made when making calculations.  It is a convenient way to intuitively state the mass of something, since us Americans grow up learning our own weight in pounds (vs SI countries, who know their mass in kg).

Metric users often do the same thing in reverse.  You will often hear the terms "kilograms force" (kgf) or "tonnes-force".  These are units of force that happen to be equivalent to the magnitude of their masses at the surface of earth.  They are not consistent with m/s^2 and kilograms, and must be converted to Newtons when making calculations.  It really doesn't matter whether you're describing magnitudes in Metric or English: English uses an inconsistent unit to conveniently describe mass, while Metric uses an inconsistent unit to conveniently describe force.

One final point: a NASA probe once failed due to a unit issue.  It has been incorrectly claimed in some internet circles that this was due to some imaginary inconsistency in English units.  That claim is patently false.  The cause was that multiple organizations were involved in the design of the probe.  One organization used English units, and another SI.  The systems engineer forgot to convert from one to the other, and a trajectory error was made when making calculations. 

To summarize: use the units that you like better if you have a choice.  It's also always good to double check your units when making calculations!