Today’s banner: inner workings of the Mettler H20 analytical balance, a work of art, a demonstration of stunning artisanship, a 1960s-era substitution balance precise to ten micrograms with a range of 160 grams. It’s analog.
We were shocked. We’d heard of this “analog” before, and we’d encountered the occasional example, but rarely in a device of such complexity.
To be sure, we thought we’d been around. We’ve broken, disassembled, reassembled, repaired and even designed and built a few analog devices in our time (more on this in a future post). But this thing made us humble. More so even than the time we disassembled the 1950s sewing machine (which we did in fact get working again).
Here’s another view of the interior of the Mettler H20. To prevent accumulation of dust we’ve placed a glass pane over the open top.
Notice the variously-sized rings all over the place? They each have a different but very precise mass. There are three sets: combinations of the four large rings total zero to 150 grams in ten-gram steps, shown below. Click for a video; note that the rings count in binary (but — analog?)!
Combinations of the four medium rings total zero to nine grams in one-gram steps. Click for video.
Note that this time the rings don’t count in binary. Only ten combinations of weights were needed — a relaxed constraint which permitted the designers to reduce cumulative error by arranging the weight combinations so that no more than two weights are lifted concurrently. (At least, that’s our best guess. We’re still discussing it.)
Now would be a good time to note that because the rings are concentric, the center of gravity of every ring combination lies on the same axis. This is important (recall how levers work? Yeah, we had to think about it too. We forgot most of grade school. This was on purpose.)
Finally, subsets of the four small rings total zero to 900 milligrams in 100-milligram steps. Click for video. Again, no more than two rings are lifted concurrently, although this time they don’t share the same center of gravity. But note how they’re arranged (from back to front: 200 mg, 300 mg, 600 mg, 100 mg). We’re pretty sure this important, but we’re not sure why…
An optical scale provides one-milligram resolution from zero to 100 milligrams. This raises the question: why doesn’t the range stop at 99 milligrams? The rest of the ranges stop one step short. But they all measure by means of mass-substitution — in other words, they’re independent of gravity fluctuations, and probably very robust to temperature variation, and so on. It’s possible that the milligram range relies on a less-robust mechanism, like spring deflection (which measures weight, not mass — yeah, once again, we had to really think about). This extra step may permit calibration: place a standard 100-milligram calibration mass on the pan, calibrate the scale to read 100 milligrams in the optical scale, and then remove the mass; if all is well, the optical scale should read zero.
A fine-adjustment dial provides ten-microgram resolution from zero to 990 micrograms. When correctly dialed-in, the alignment bar at the right lies exactly between the triangles. This cannot be done when anyone else is awake in the building. This scale measures footsteps two floors away. A cat walking nearby will throw the measurement. Exhalation will throw the measurement. Unless the scale is on a massive slab of marble, or directly on bedrock, both of which were common practice, it’s best to weigh things at 3:00 a.m.
Below is another view of the optical scale, with the lights off. Notice the crispness of the image! The absence of pixel-aliasing! There isn’t even an LCD — it’s completely nondigital, with no electronics whatsoever — and the image is rendered with a tiny projector! Not unlike film, which we’ve also heard of (apparently this is how cameras used to work).
In a subsequent post we’ll provide an explanation of the “substitution” principle in the design of this balance. If we have the time, we’ll also assemble a manual, since they no longer seem to exist. For now, here are three more views as we back-out of this wonderful device:
On your way out, observe the Authentic Lab-Applied Stickers and the thirty-dollar price marking in the next photo. Forty-five years ago this balance cost over a thousand dollars, which is equivalent to perhaps twenty-five thousand dollars today.
From the dusty depths of the local university-surplus store, otherwise known as the Toy Store for Nerds, this balance is another gem.
Best — stochastic