Many good engineers pride themselves on their ability to do quick “back of the envelope” calculations that are accurate enough to be used as the basis for some expedient decisions. The ability to do this stems from experience, where rules of thumb and instinct about the numbers are cultivated, and from good engineering fundamentals which provide the engineer with the tools to develop the right approach and process to do the calculation, even if it is a new problem.

What can be difficult is knowing when and how to question yourself.

I recently came upon a paper (fortunately of limited distribution) where the approach, process and calculations were correct, but all the conversions from metric to English were wrong. Seems that 2.45 was used instead of 2.54 for conversion from centimeters to inches. Though it looked like a case of transposition, in fact the author said that he always thought the conversion was the former. And there lies the potential pitfall to the engineer — assuming he is correct because he always did it that way.

It’s not that I am advocating second-guessing yourself all the time, but rather that periodically we all can use a quick check on some of our fundamental constants, rules of thumb, and calculations methods to both shore up our memories and correct bad habits.

It is sort of like the driving tests that this country doesn’t periodically give to established drivers. I was in a car with a middle-aged driver who tailgated, didn’t use signals, and cut people off. When I tried to bring this to his attention (while making sure my seatbelt was on), he replied that he had been driving for thirty years. Yes, I thought, but you’ve been driving badly for all those years. His experience didn’t change the fact that he was dangerous driver.

If we have bad habits in our engineering calculations or processes, they need to be caught and corrected before they become part of our experience and thus “correct” in our minds. As engineers we need to understand the limitations of our human minds (especially, I am sorry to say, as we age) and bolster our memories with the tools of our trade such as handbooks and structured calculation processes. Though it may cost a bit in terms of pride, it shouldn’t. As far as I can see, no engineer should be faulted — by herself or anybody else — for checking their work. And while you’re at it, you might want to check those handbooks and spreadsheets too. Nothing is sacrosanct where good engineering is required, and the guys that wrote that handbook may have had some bad habits too.

What do you think? Have you ever found yourself using the wrong value, equations, or process out of habit? How often do you go back to examine your fundamentals? Drop us a comment.