[This fragment is available in an audio version.]

The world being what it is, feels like a little humor is in order. Here’s a story from my misspent youth, when I was a co-op student at a steel mill and had a Very Bad Day.

This story was originally published as a Tweet thread in response to the following:

Tweet from @ElleArmageddo

That thread will probably go down with the Twitter ship and I’d like to save it, so here goes.

During the summer of 1980, my last before graduation, I had a co-op job at Dofasco, a steel mill in Hamilton, Ontario. It wasn’t great, the tasks were mostly make-work, although I kind of liked learning RSX-11M Plus running on PDP-11s. Sure, it was primitive by modern standards (I already knew V6 Unix) but it got the job — controlling the sampling apparatus in a basic-oxygen blast furnace — done.

So, there was this Rolling Mill that turned big thick non-uniform slabs of steel into smooth precise strips rolled onto a central core; this is the form in which steel is usually bought by people who make cars or refrigerators or whatever. The factory had lots of rolling mills but this one was special for some reason, was by itself in a big space away from the others. It was huge, the size of a couple of small trucks.

The problem was, it had started overheating and they didn’t know why. The specifications said the maximum amperage was 400A RMS, where “RMS” stands for Root Mean Square. The idea is you sample the current every so often and square the measurements and average the squares and take the square root of that. I believe I learned in college why this is a good idea.

Um, 400A is a whole lot of juice. Misapplied, it could turn a substantial chunk of the factory to smoking ashes.

The mill had an amperage data trap, to which they plugged in a HP “data recorder” with reel-to-reel tape that sampled every little while, and left it there until the overheat light started flashing. Then they needed to compute the RMS.

Fortunately · They had a PDP-11/10, about the smallest 16-bit computer that DEC ever made, with something like 32K of core memory. It was in a 6-foot-rack but only occupied two or three slots. It had a device you could plug the data recorder into and read the values off the tape out of an absolute memory address. And it had a FORTRAN compiler. It was running RT-11.

Who knew FORTRAN? Me, the snot-nosed hippie co-op student! So I wrote a program that read the data points, accumulated the sum of squares, and computed the RMS. I seem to recall there was some sort of screen editor and the experience wasn’t terrible. (I kind of wish I remember how you dereference an absolute memory address in FORTRAN, but I digress.) The readings were pretty variable, between 200 and 500, which the machine specs said was expected.

Anyhow, I ran the program, which took hours, since the data recorder only had one speed, in or out. The output showed that the RMS amperage started worryingly high but declined after a bit to well below 400A, presumably after the machine had warmed up. My supervisor looked at the arithmetic and it was right. The report went to the mill’s General Foreman, a God-like creature. So they told the machine operator not to worry.

Unfortunately · I had stored the sum of squares in a FORTRAN “REAL” variable, which in FORTRAN (at least that version) meant 32-bit floating point. Which has only 24 bits of precision.

Can you see the problem? 4002 is 160,000 and you don’t have to add up that many of those before it gets big enough that new values vanish into the rounding error and make no changes to the sum. And thus the average declines. So the RMS I reported was way low.

Fortunately · The mill operator was a grizzled old steel guy who could tell when several million bucks worth of molten-metal squisher was about to self-incinerate.

He slammed it off on instinct with a steel slab halfway through. It only cost a few shifts of downtime to remediate, which is to say many times my pathetic co-op salary for the whole summer.

At which point my boss and I had to go visit the General Foreman and explain what DOUBLE PRECISION was and why we hadn’t used it. It wasn’t fun. For some reason, they didn’t ask me to re-run the calculations with the corrected code.

You can’t possibly imagine how terrible I felt. I was worried that my boss might have taken career damage, but I heard on the grapevine that he was forgiven, but ribbed unmercifully for months.

And when I graduated they offered me a job. I went to work for DEC instead.


Comment feed for ongoing:Comments feed

From: John Cowan (Mar 29 2023, at 16:25)

I think it's pretty likely that the reference to the absolute address was done using an assembly-language subroutine. The RT-11 Fortran manual at <http://bitsavers.trailing-edge.com/www.computer.museum.uq.edu.au/RT-11/DEC-11-LRFPA-A-D%20RT-11%20FORTRAN%20Compiler%20and%20Object%20Time%20System%20User's%20Manual.pdf> does not describe any PEEK analogue.


From: Andrew Reilly (Mar 31 2023, at 18:05)

The thing about floating point (pre-IEEE or not) is that the exponent part makes it relative-accuracy (precision) preserving, rather than absolute. So your anecdote is real, provided that there were a *lot* of squares being accumulated, but the bit of set-up about the 400-ness of the numbers is a red herring. Wouldn't have mattered if it was 400, 4 or 0.4.

There are strategies for avoiding the problem, if short floats are all you've got, like sorting the numbers before adding them, or batching the adding process up, or (what I'd recommend for that sort of job:) just not add so many up at once - either window or low-pass filter the accumulation.

However, judicious use of double precision does solve many sins, and is arguably the best answer here.

Oh, and the reason that RMS is important is that it is proportional to power, and it's power that heats things up.

Big machine vs student stories are great. My favourite was a lab at Uni, where we had to learn about phase matching for generators and loads of AC systems. One student team managed to get the phase match exactly wrong (negative), and so when they threw the switch one of the half-ton machines instantly snapped all four half-inch bolts and flipped off the pedestal and rolled across the floor. Luckily no one was injured.


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March 28, 2023
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