Meanings for symbols on circuit and process flow diagrams

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Engineers can communicate via visual languages with symbols that show things like electronic circuits and chemical processes.

 

Back in high school I learned the circuit symbols for electronic components by reading articles in Popular Electronics magazine. I have known them for so long it never would occur to me that someone unfamiliar with them might have completely different interpretations, as is humorously shown above in Randall Munroe’s xkcd cartoon for August 21, 2023. (I have added the correct interpretations in red. There is a detailed discussion in the article at the EXPLAINxkcd web site).

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Chemical engineers instead use process flow diagrams. Some symbols from the Wikipedia page are shown above, with one for a valve highlighted in yellow. Famous physicist Richard P. Feynman told a great story about them in his memoir, Surely You’re Joking, Mr. Feynman. It is in an essay titled Los Alamos From Below, on pages 106 and 107 of the 1986 paperback edition. During World War II he was sent to Oak Ridge to help them fix the process for uranium to prevent an accident. The Army had not realized that neutrons were much more effective in setting chain reactions when slowed down (moderated) by water. He says:

 

“How do you look at a plant that isn’t built yet? I don’t know. Lieutenant Zumwalt, who was always coming around with me because I had to have an escort everywhere, takes me into this room where there are these two engineers and a loooooong table covered with stacks of blueprints representing the various floors of the proposed plant.

 

I took mechanical drawing when I was in school, but I am not good at reading blueprints. So they unroll the stack of blueprints and start to explain it to me, thinking I am a genius. Now, one of the things they had to avoid in the plant was accumulation. They had problems like when there’s an evaporator working, which is trying to accumulate the stuff, if the valve gets stuck or something like that and too much stuff accumulates, it’ll explode. So they explained to me that this plant is designed so that if any one valve gets stuck nothing will happen. It needs at least two valves everywhere.

 

Then they explain how it works. The carbon tetrachloride comes in here, the uranium nitrate from here comes in here, it goes up and down, it goes up through the floor, comes up through the pipes, coming up from the second floor, bluuuuuurp – going through the stack of blueprints, down-up-down-up, talking very fast, explaining the very, very complicated chemical plant.

 

I’m completely dazed. Worse. I don’t know what the symbols on the blueprint mean. There is some kind of a thing that at first I think is a window. It’s a square with a little cross in the middle, all over the damn place. I think it’s a window, but no, it can’t be a window, because it isn’t always at the edge. I want to ask them what it is.

 

You might have been in a situation like this when you don’t ask them right away. Right away it would have been OK. But now they’ve been talking a bit too long. You hesitated too long. If you ask them now, they’ll say, ‘What are you wasting my time all this time for.’

 

What am I going to do? I get an idea. Maybe it’s a valve. I take my finger and I put it down on one of the mysterious little crosses in the middle of one of the blueprints on page three, and I say, ‘What happens if this valve gets stuck?’ – figuring they’re going to say, ‘That’s not a valve, sir, that’s a window.’

 

So one looks at the other and says. ‘Well if that valve gets stuck –‘ and he goes up and down on the blueprint, up and down, the other guy goes up and down, back and forth, back and forth and they both look at each other. They turn around to me and they open their mouths like astonished fish and say, ‘You’re absolutely right, sir.’

 

So they rolled up the blueprints and away they went and we walked out. And Mr. Zumwalt, who had been following me all the way through, said, ‘You’re a genius. I got the idea you were a genius when you went through the plant once and you could tell them about evaporator C-21 in building 90-207 the next morning,’ he says, ‘but what you have just done is so fantastic I want to know how, how do you do that?’

 

I told him you try to find out whether it’s a valve or not.”