English/metric measurement error in the Mars Climate Orbiter

[mklotz]

It is a big mistake to attempt to see anything reasonable in the imperial system or its offshoots. That comes from someone brought up on the system. My advice is to burn all rulers not graduated in mm.

Good advice, Tony.

One of my favorite examples of the idiocy of the inferial* system is having a volume unit depend on what is contained in said volume...

There are seven different barrel sizes used in the USA, with the size being dependent on the contents. Their names and metric equivalents are as follows: US cranberry (95.5 liters), US dry (115.628 liters), US liquid (119.24 liters), US federal (117.348 liters), US federal proof spirits (151.416 liters), US drum (208.4 liters), US petroleum (135 kg.), US petroleum statistical (158.99 liters)

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* my term for the further botched version of the Imperial system used in the USA under the name "US Customary Units". "Inferial" is a portmanteau of "inferior/infuriating" and "Imperial".

[tonyfoale]

Good advice, Tony.

One of my favorite examples of the idiocy of the inferial* system is having a volume unit depend on what is contained in said volume....

It is not only the idiocy of the units system that causes confusion and problems. Possibly the source of more mistakes is the misuse of the units that are there. The infernal system does have well defined and consistent units (maybe not in everything) but very few people use them or use them correctly. One VERY common misuse is with mass and weight.
The ISO system has kg for mass and Newtons for force. We all know that F=ma, so 1 kg accelerated at 1 m/s2 requires a force of 1 N. Simple and mistake proof. Nobody in a technical environment gets this wrong. Supermarkets are unlikely to weigh things in Newtons, they use kgf which they call kg. but this is unlikely to make a bridge fall down.
On the other hand it is common in imperial land to use lb for mass and lbf for force. The problem here is that F != ma it should become F = ma/g. Looked at in another way you have to express the acceleration in Gs not ft/sec2. I have had countless people ask me "I know that I have to insert "g" but does it go on the top line or the bottom". When using lbs for mass and ft/sec2 for acceleration the correct unit of force is NOT lbf but poundals pdl (or maybe PDL, I try to forget). If you insist on using lbf for force then there is another unit of mass called the slug which is lbf x g or numerically a slug is 32.2 times a lb. The slug is common in the imperial aircraft industry.
I have been told that the Mars error was due more to this F = ma or is it F = ma/g then again perhaps it might be F = mag, than just a simple case of using metric or infernal.
One of the things that really get me steaming is when I see imperial engineering profs. and the like talking about weight density. What type of evil is that? It is unbelievably crazy, confusing and mistake prone. The worse thing is that these utterances are from technical people brought up in that crazy units system. They more than anybody should use it correctly.