When I was a young lass, we were barely out of the Stones Age (moving on to the New Wave Age). The Oil & Gas industry uses both Imperial and Metric units (no kati and tahil in sight). Computers were slow, so it was faster to remember unit conversion constants. I still remember 0.14504 to convert from kPa to psi (or is it the other way around), and the classic 25.4 to convert from inches to mm. How about 7(ish) to convert from cubic meters to barrels (US, mind you).
The following books were your friends (along with deodorant).
No, I didn’t use logarithm tables at the office.
Wata - Malaysian Oil and Gas Engineer 