Lifting a Million Pounds of Stainless Steel | Hacker News Reader

Lifting a Million Pounds of Stainless Steel

npr.org

82 pointsstate_machine27 comments

I work at NIST and took a tour of this machine a few months ago. A few bonus facts:

- The spatial distribution of the local gravity field is a significant part of the uncertainty of these measurements. The weights extend into the basement of the lab, and the gravity in the basement is less than that above the surface. They produce gravity maps by dropping things in a vacuum to get a handle on it.

- The drift in the dead weight standard was mostly caused by the individual masses welding themselves together under the immense pressure of the weight stack. The interface has been designed to reduce this effect.

- The same group is also working to count the number of atoms in the kilogram, so that the mass of the dead weight stack will not be 500,000 times the mass of a piece of platinum in Paris, but will be 500,000 times the mass of 6.XX E23 silicon atoms.

More about that piece of platinum in Paris, the International Prototype Kilogram (IPK): https://en.wikipedia.org/wiki/Kilogram#International_prototy...

Also, something I had never heard of, Vienna Standard Mean Ocean Water: https://en.wikipedia.org/wiki/Vienna_Standard_Mean_Ocean_Wat...

It's not in Paris but in Sèvres, 10 km west of Paris (where I live).

The most precise reference for a kilogram in the whole world? It's approximately in Paris, more or less- we care about precise weights, not precise locations. /s

> The spatial distribution of the local gravity field is a significant part of the uncertainty of these measurements.

Interesting! Could you please add a few numbers, just to get the order of magnitude?

Can you expand on this: "the individual masses welding themselves together under the immense pressure of the weight stack."

It's not really even that heavy (~450mt). Is this effect something that is entirely negligible for things which don't require such immense precision? Like, what kind of drift are we talking about...milligrams, micrograms?

Those sound like great reasons to use something sensible like a hydraulic ram to apply 1,000,000 lbf. Force = pressure x area, and neither pressure nor area is difficult to measure precisely. I can't see the upside of using 1,000,000 lbm of stainless steel.

It's much simpler to compensate for dead weight to get as exact as possible measurement. With a hydraulic press you need to compensate for the compression and heat buildup of the liquid etc.

It's easy to measure area but how do you calibrate your pressure sensor?

I think you'll find that this device is used not as a testing tool but as a calibration tool.

I was curious about the tolerance in a mass of this size. Article didn't mention that. Do you happen to know?

There's some reasonable sized photo's of this monster here:

http://www.nist.gov/pml/div684/grp07/million-pound-deadweigh...

This would sound much less impressive if they just said 500 tons. Also, why the dead weight? 500 tons is easily doable with a (smallish) hydraulic press.

The actual title of the article is "How Do You Lift A Million Pounds Of Stainless Steel?" I feel like they didn't answer that question either. It turns out that the weights are in fact lifted by hydraulics.

I suppose it's a different shear force to apply all the weight at once rather than hydraulically.

It's a question of accuracy not maximum force. At some point you want to calibrate the things you use to calibrate the hydraulic presses. http://www.nist.gov/calibrations/index.cfm

Right. Something the parent article completely fails to mention. This is NIST's force standard for large forces, the place you send load cells to be calibrated.[1] If they have to use the whole million pound stack, the fee is $16,489.

[1] http://www.nist.gov/calibrations/force.cfm

>Also, why the dead weight? 500 tons is easily doable with a (smallish) hydraulic press.

Sure, but how do you know the hydraulic press is applying 500 tons? You need to have a universal standard for length (to determine the area of your piston) and for force (which doesn't exist), and consider the friction in your hydraulic system. We can generate a universal standard for the kilogram, then duplicate that log(500) times, and create a very well controlled mass that we can hang from stuff. The level of accuracy that is needed would surprise you.

Because 500 tons of dead weight is going to stay 500 tons.

A hydraulic press would need to be calibrated.

That's why it's "stainless" steel. Don't wanna gather up a bunch of oxygen molecules.

Now they just need to try folding some paper 7 times, squash some playdoh animals and set up a YouTube channel and the thing will pay for itself:

https://news.ycombinator.com/item?id=11694819

Why they don't use SI even for scientific measurements?

Considering the machine was built in the US in 1965, it's not surprising that the machine used imperial units. Based on Wikipedia's table [1], it looks like metrication had not yet begun in a large number of countries by 1965.

Also, if your goal is to build a device that calibrates other machines, and most of the machines in your country (and, likely, most of the machines belonging to the majority of your trading partners) use one system of measurement, then that's probably the system of measurement you build the machine to calibrate against.

[1] https://en.wikipedia.org/wiki/Metrication#Conversion_process

That's an excellent way to express the situation back then. Given the ease of converting results with modern technologies, it's hardly a handicap to use the Imperial units, besides doing so preserves the "feel" of the era it came from. I think that's not a shabby or minor aspect of the machine being brought back to useful life.

My guess is that when it was constructed 50 years ago, the "device" was engineered with the units that were familiar or already organized into formulas to plug data into. In the early to mid 60's, simple extremely limited dedicated calculators were a new thing and hard to get. Not at all like the tech in our world we take for granted, numerical analysis was much harder to accomplish.

Of course today we have vast computation resources right under our fingertips. How hard is it to translate from one set of units to another? No doubt on issuing a report of their test results, the output will be expressed in whatever units anybody wants, no trouble at all...

Correct. Indeed, the NIST uses Newtons now. http://www.nist.gov/calibrations/force.cfm shows that pretty clearly.

The supply chain isn't set up for metric. Steel supply in the US is mostly set up for imperial measurements. If you want 500 metric tons of steel the guys at the foundry will convert it to imperial before the order gets put in. The guys at whatever machine shop is finishing them will convert all the dimensions to imperial before finish machining. The guy doing the final calibration will convert to imperial before doing his job (probably a combination of drilling holes to remove material to get them just under the target weight and then adding precisely weighted chunks of steel for final calibration). Why not just have the conversion be the last step. Saves effort and reduces screw-up potential.

Did they put the tools back in the cabinet when they were done?

That's a serious question.

> why the dead weight?

I assume it's because masses are more stable and easier to calibrate than the gauge on a hydraulic press.

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