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Comment by seesawtron | Hacker News Reader

This [0] AMA from the research group answers most of the questions I am seeing here in the thread.

"Technologies range from aqueous amines - the water-rich solvents that run through modern, commercially available capture units - to energy-efficient membranes that filter CO2 from flue gas emitted by power plants. Our newest solvent, EEMPA, can accomplish the task for as little as $47.10 per metric ton - bringing post-combustion capture within reach of 45Q tax incentives."

[0] reddit.com/r/askscience/comments/mdouzu/askscience_ama_series_hi_reddit_we_are_scientists/

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Clickable link: https://old.reddit.com/r/askscience/comments/mdouzu/askscien...

$47.10 per ton smells like someone has calculated the cost of the reagents and cited that as the price...

Any estimate of this process cost would never be to 3 Sig fig.

I don't know why $47.10 is any less credible than $47, but here's more detail on what the estimate represents from TFAMA:

"For the $47.10/metric ton carbon capture cost, 48% comes from CAPEX while the remaining is OPEX. The equipment life of a carbon capture unit would be similar to that of a power plant, about 20-30 years. The $47.10/metric ton cost is just for carbon capture, and does not include transportation and sequestration costs."

It is still suspicious to use so exact numbers for a technology that has not yet been tested out at scale at all.

In my experience, the obsession with correct significant figures is an american high school thing. Why should you deliberately have to choose a number that is not your best guess? Ideally you should include confidence bounds, but if you don't, it shouldn't imply that you are confident to that level.

Because it gives an idea of what range is expected for the actual price. If someone gets an estimate of "about fifty bucks" versus "about $51.04", their reaction to seeing a final price of say $62.17 would be different. The first one implies that it's a rough estimate, while the second one doesn't.

"About" is doing a lot of work, too, no?

If someone said "about fifty bucks" and it turns out to be $62.17, I'd immediately understand them to be scamming me.

Really depends on what the context is. Maybe for a taxi ride when negotiating a price ahead of time, but pointing at an object, asking for its price, getting an estimate that's off, then getting the actual price before the transaction is completed doesn't feel like a scam.

Please price estimates for car repairs in dollars and cents and state it's only an estimate all the time.. with variations possible depending on what's discovered that might have been missed it that particular context.

Yep. The weird thing is that their sigfig rules of thumb aren't particularly accurate and after any non-trivial operations (or more than a few operations) just give wrong results.

Tracking precision through calculations requires interval arithmetic, which is a pretty big PITA and not frequently used.

I think the sig figs cult comes out of mistaking a simplified lesson on precision for a useful practice.

Not sure exactly what you mean. It is well known that you should keep additional digits throughout the calculation and then drop non-significant ones for final results.

The number of digits out is only a simple function of the digits in if you're doing fairly trivial calculations.

Checkout https://en.wikipedia.org/wiki/Interval_arithmetic for what it takes to monitor precision through complex calculations (in particular, see the bits about using the taylor expansions of non-trivial functions).

When I was getting my engineering degree too many significant figures would earn you -2 pts.

Measure with a micrometer.

Mark with a chalk.

Cut with an axe.

(Apparent practice at a workshop I used to frequent.)

Old crusty semi-retired engineer I knew said when he was young he liked designing things to close tolerances. But once he was old he found more pleasure in designing things that worked with really shitty tolerances.

-I had a colleague once, fresh out of mechanical engineering at the local univerity college, who as his first project did a locking pawl for a winch drum.

Tolerance? H7.

The fitters threw a fit trying to make that happen.

Clearly he wasn't working in any sort of mechanical capacity while getting that community college degree.

That's definitely where I learned that it was expected.

I look at it as one of many possible conventions for communicating precision. For what it is, it works well. But it's not universal, and people who do and don't use it should both keep in mind that the other group exists.

It hints that someone doesn’t understand these confidence values at all.

Why $47.10 and not $47.1032471?

> I don't know why $47.10 is any less credible than $47

It shows a lack of understanding of either basic science or napkin maths.

For either reality, science or invention, it matters.

https://en.m.wikipedia.org/wiki/Significant_figures

Sig figs are just a crude method to in-band signal some bounds on your estimates. They don't seem super relevant here when communicating an order of magnitude improvement.

I am pretty sure the $0.10 per tree of the Eden reforest project would be far cheaper than $47.10 per ton, to say the least.

That would be 471 trees for $47.10.

That price is per tree planted and there's not guarantee the tree is going to grow. It's great to plant trees, but even if we could reforest the entire Earth on the good places we'll need other technologies to capture carbon.

Does the 0.10$ include land acquisition for the forest? If not, the cost could be significantly higher.

They work with the native communities (that is why around 10% of trees planted are designed as a food source) which allows that 10 cents to cover everything, not just the planting of the trees.

Reforestation only sequesters carbon if the land remains forest forever. If it’s cleared again in the future then that carbon goes right back into the atmosphere.

Doesn't it depend on what is done with all the wood that was chopped down? If used say for construction, that carbon is staying out of the atmosphere.

Or do mature trees not make up the majority of the carbon that is stored in forests?

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