Re: IanB
Your discussion about the difference between a "pound mass" and "pound force" is consistent with my discussion of using lb av at the grocery store to weigh food.
However, here is an example of my reasoning not to use lbm:
Imagine a spacecraft whose mass is 100 lbm. We then apply a force of, say, 10 lbf to accelerate it in a desired direction.
Applying Newton's second law straight out of the bottle, F = m A will give the wrong result, unless we throw another factor gN, the defined standard acceleration of gravity.
In careful US usage, one should use pounds of force and slugs of mass, just as in careful metric usage one uses Newtons of force and kg of mass.
I remember in high-school physics class (11th grade), an important initial hurdle for the students was to understand the difference between mass and weight. The instruction was in both customary and metric units, and I learned some important factors for mental calculation, such as 88 ft/sec = 60 mph (exactly).
Indeed. Lecture 1, day 1 of my engineering degree was introducing the subject of units of measure, and the correct use thereof.
Until that point, my high school curriculum had been taught exclusively in SI units. Imagine our surprise when this conversion factor
gc started appearing in all sorts of equations, and how important it was to remember it when venturing off the SI island.
Here is the famous Bernoulli equation as described in a standard industry reference:

We may observe how the mysterious factor "g" appears in some terms. And what the heck is that 144 doing there?

Furthermore, what about that "foot pounds per pound" as unit of measure for head (loss)? Why don't the pounds cancel out to leave just feet? (Actually, they do, if you are a practicing engineer and you know what's going on.) The equivalent SI unit would be J/kg, and maybe that is equivalent to meters? (Yes, it is, as long as you remember the
gc conversion factor when going from one to the other. See? You can't escape
gc even in SI units

)