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Strain Gauge 8.3 × 10⁻⁶ Impossibleness - How?

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I wanted a rude username:
An all-purpose ape asks an interesting question, replicated here in full:

> While disposing of my old concrete patio at the local landfill, the attendant signaled me to pull my SUV/trailer forward onto the 60 ton truck scale indicating 7304 lbs (3320kg). I walked off the scale to visit the cashier, noticing the display value dropped by 185lbs (84kg) reflecting my body-weight and accurately, to within 1/2% - in the context of the scale's capacity that division is 8.3e-6. A strain gauge and circuit configured to supply an 5vd ADC input over a variation of 60 tons would go from 5v down to .000004v. 24bit A/D converters have that resolving power, but the analog signal conditioning / amplifiers etc... seem impossible to me; wind was blowing, my engine was running, resonance, microwaves from nearby phones, nearby behemoth landfill tractors scraping the earth. Is this a feat of DSP or perhaps optics instead of resistance based strain gauges? Perhaps a highly segmented under-weldment with 100 individual strain gauges? I am looking forward to comments!


0.1% accurate or better plus resolution beyond so that sort of short term measurement is entirely plausible. Probably electronics tightly integrated in the (sealed and shielded) load cell + heavily damped and slow system.

With precisely one single datapoint, I’m not sure how anyone could generalize about the measuring system.

It could have been an unreproducible fluke that his “measured weight” was so close to that of his bathroom’s scale.

Here's the specifications of one 60-ton truck scale:

10000 increments of 12lb, so basically 4-digit accuracy -  easily reachable with a 16-bit ADC.

I have noted the same thing.  Two partial explanations come to mind but don't fully explain it.

1.  Many of these segment the platform into two or more segments, weighing the front and rear axle(s) separately and then adding the results.  Splitting it left-right is the other obvious move.  Cuts the dynamic range by a factor of two or more.

2.  Low bandwidth/long time constant.  Watching the display settle it isn't too long, on the order of a second, but definitely enough to eliminate engine vibration and many other high frequency noises.


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