Have you heard the story of the Gimli Glider? An Air Canada flight ran out of fuel on July 23, 1983, at an altitude of 41,000 feet, midway through the flight. The cause? It was a mix up of pounds and kilograms when calculating the weight of fuel to load for the flight. (Give it a read; it’s an interesting tale of mistakes.)
A compressed air auditor has a similar story. Early in his career, he was helping an important customer right-size their compressor. The existing unit was at the end of its useful life, but was also greatly oversized for the job. The recommendation was to reduce the capacity of the compressor and purchase a new VSD unit.
Flows were measured and calculations done, and the unit was shortly purchased and installed. As soon as the compressor went into service, the technician setting up the compressor noticed the loading was much higher than expected — and so an inquiry was made.
Calculations were examined and it was found that there had been a mix up in volume conversions. The new compressor was specified in cubic feet per minute, but the actual flow requirement assumed liters per second … so a compressor half the specified size was purchased!
It turns out that with all the safety factors applied to the sizing, the compressor exactly matched the load, but had no margin for growth.
Lesson learned — always check and double check your unit conversions.
Some units are the wrong size for the quantity being measured. Pascals as units of pressure is possibly the worst, although millimeters for large distances is also a poor selection.
Of course wit the metric system having had units sized to make the math convenient instead of being sized to be units of useful size, what else could be expected??
Metric bolt sizing is probably the best example of something being created by mathematicians rather than engineers.