T
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Richard Feynman Side Hustles
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This would be cool if only it made sense.
To use an analogy with some metaphors: The sensor is like a sealed room with a screen window that only lets in oxygen. To get a reading, every molecule that enters is smashed to create a tiny spark of electricity. However, because the oxygen is destroyed to create that spark, it creates a suction effect, causing more oxygen to rush into the room to fill the void. This creates a major flaw: if gunk builds up on the screen, it slows down the flow of incoming oxygen. The sensor, which only counts sparks per second, is tricked into thinking the oxygen level outside is low, when really the window is just dirty.

By adding a third electrode to replace the oxygen every time one is smashed, you maintain a perfect balance and eliminate that suction. Because the room stays full, the sensor no longer relies on the speed of the oxygen rushing in; it simply measures the steady state of the oxygen already there. Even if gunk gets on the window, the sensor won't be starved of a reading. It might take a few extra seconds for the levels to settle, but the final number will be 100% accurate because the sensor is no longer emptying its own room to get a count.

  • Coeur
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I still don't get it. The outside is dirty, right? He said in his post "You dip this probe into beer, sewage, or canned food a-stewing". So when you say "when really the window is just dirty" I don't get it - yes it will always be, because that's what it is placed in, no?
A dirty window only ruins the reading if you are measuring the speed of the oxygen passing through it. The three electrode design stopped measuring speed and started measuring balance. Unless the gunk is a total airtight seal (which is rare on the scale of an oxygen molecule), the sensor will eventually reach the right answer, whereas the old version would fail.
  • Coeur
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So dirt as a factor that clogs up the sensor does not play into it at all? It's all just about moving it into different environments to measure?
I think maybe one thing you have to consider is that sensors still require maintenance. Software can measure the length of time the sensor requires to reach equilibrium and send a maintenance required alert and someone cleans it (like if the software expects equilibrium in 10 seconds but the reading settles at 60 seconds, it can calculate the sensor is 80% clogged and requires cleaning). There's also all sorts of techniques that can be used to mitigate gunk depending on how the sensor is being used such as physical wipers, air-blast systems, ultrasonic cleaning systems, and chemical coatings. So as long as some oxygen can get in and an equilibrium is made between the fluid outside the sensor and inside the sensor, you'll get a reading that you can trust.
The permiability of the membrane would still be reduced by stuff on it, but as long as there is any permiability, the inner compartment will reach equilibrium eventually.

The big gain comes from a change in how you interpret the presence of electrons.

The older approach converted oxygen to electrical current, the magnitude of current flow relating to magnitude of oxygen depletion. The assumption built into that approach is that low oxygen depletion levels meant low oxygen levels, but that wasn't the only potential cause, because it ignored variation in the permiability of the membrane.

The newer approach equates current flow to oxygen concentration, as the system doesn't deplete the concentration any longer. The permiability of the membrane in this setup only contributes to a longer initial delay as the inner chamber comes to equilibrium with the surrounding concentration.

The sensor is normally placed in a dirty environment. This change prevented the need to calibrate the sensor for the level of dirt.
This is a much better explanation. Thank you
The current is measuring the rate of the reaction. With the two-terminal design the rate of the reaction is proportional to the rate of diffusion of the oxygen into the area where the reaction is taking place, which is related to the oxygen concentration around it but also can be affect by other things. With the third electrode, the current is proportional to the concentration of oxygen in the area around the sensor directly, which will equalise with its surroundings much more consistently than the rate of diffusion.

(A quick google brings up this document which describes the principle. No idea if this is the company in the story: https://semeatech.com/uploads/Tech_Docs/AN%20161205.pdf )

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I agree. It's not clear how adding a sensor "so that it adds back an oxygen molecule" works. shrug
  • brk
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I think this was primarily about speeding up the measurement time. With just two electrodes you had to wait for the device to achieve equilibrium with the material being measured. If the concentration of oxygen on the probe side of the barrier was higher or lower than the material side you would get false measurements, particularly in low oxygen scenarios because you have oxygem trapped in the probe.

By keeping the state of oxygen inside the probe constant and replacing consumed molecules you now can measure almost instantly.

Yes but how do you do that? that magical third electrode sounds harder to make than the original problem.

Edit: I think I get it now, it's a chemical reaction. By applying a voltage with some polarity to the 3rd electrode you can run the reaction in reverse. Still very hard to achieve because you have to make sure the reactions happen at the same rate with the same efficiency, which is far from trivial. This must be a very high end sensor for all this effort to make sense.

An oxygen molecule does some chemical reaction on the sensor electrode that releases an electron, maybe it's made of iron and turns into rust. If you supply the same current to another electrode to do the opposite reaction, maybe one made of rust that turns into iron, it balances.

The sensors must be consumable with a certain lifetime.

  • boxed
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Because then it doesn't alter the side of the membrane where it does the reading (plus one minus one equals zero). That makes the measurement more accurate.
Specifically, if you assume a partial pressure of Oxygen and of all other gases on the electrode-side of the diffusion membrane, then you'll only see a certain number of "ionization events" per time, and you're limited in how much electrical signal you get by how fast oxygen can diffuse across the membrane. This is likely driven by maintenance of a partial pressure within the membrane. However if you re-ionize the oxygen that you deionized, then the partial pressure is much closer to equilibrium, and therefore the partial pressures are only dependent on the amount of oxygen outside of the membrane instead of being dependent on both the ionization rate and the recovery rate through the membrane. It probably makes the calculation a lot faster and more closely dependent on the environmental presence of oxygen which is what you want.
You're not really making things clearer.

What does "adds back an oxygen molecule" mean?

  • boxed
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That's an implementation detail no? Are you asking how to add an oxygen molecule, or how this makes the sensor better?
Yeah, how do you add the oxygen molecule, and how do you know when you have to do that?

Elaborate and you'll find the issue with this setup.

How do you add the molecule? Well, you're not just dealing with single-digit numbers of molecules. Have an oxygen tank with a flow meter for example, open the valve to release the required volume of oxygen. The ideal gas law tells you how many molecules you let out.

How do you know when you have to do it? The sensor tells you how many oxygen molecules you consumed, as a proportion of the current flowing. So just let oxygen flow into the tank at the same rate as you're consuming it. Which you know because the device literally measures how much oxygen it is consuming.

I think the real issue is that the explanation in the tweet is from a physics perspective rather than an engineering one, which means it reads like it was implemented with impossible magic.

>open the valve to release the required volume of oxygen

Mega LMAO. I can assure you this is not what's going on, at all. Also, if you release oxygen in gas form into the liquid you're going to run into a zillion other problems.

One of the golden premises of measuring things is to avoid altering what you're measuring, lol.

The issue here isn't the setup, it's with people understanding it.
Enlighten us, then, savant[1].

1: as in, one with detailed knowledge in some specialized field (as of science or literature)

This way you're measuring change in oxygen concentration. As more oxygen comes into the compartment in order to equalize with the outside you consume and at the same time produce more oxygen. You measure the change in rate of oxygen consumption/production. It is always consuming/producing oxygen but the rate changes with the concentration.

At least that's what I assume.

  • spott
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I think of it differently.

Before, you measured diffusion rate of oxygen and inferred oxygen concentration from that (the concentration outside the chamber is always greater than the concentration inside). Dirty membranes etc all changed the rate of diffusion, which caused issues.

After you measure oxygen concentration directly (the concentration inside and outside the chamber are always the same).

Trust me, if we all understood Richard Feynman the first time he said something, the world would be a very different place.
That's very cool and edgy but (Richard) Feynman didn't say this.
  • 0-_-0
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Had to read it 3 times but it makes sense
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It is a neat story about Richard Feynman. What about it reads as "Nazi" to you?
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The complaint is about it being a post on X.

Some people around here really do think that way about Elon Musk.

While I don't agree with the "this shouldn't be on HN" part, people thinking Elon is a Nazi isn't exactly unreasonable. He literally sieg heil'd an audience at Trump's inauguration. Twice. Call it trolling or whatever you like, but he did it and the outcomes for such an act are ... predictable.
I don't recall this "richard feynman" guy ever publicly supporting trans rights
The way people continue to use X is a perfect example of Arendt's banality of evil: that moral wrongs can be enabled by ordinary people, acting through conformity, careerism, convenience, and thoughtlessness.

They don't have any deep ideological commitment or overt cruelty, they just don't care enough. They justify themselves with claims such as "it's the way things are," "not my role," "everyone does it", or that it doesn't really make a difference.

But if everyone on X that doesn't explicitly support fascism and Nazism stopped using it, it would make an immediate difference. X would turn into something more like Truth Social.

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So do you have to be a god tier Nobel Laureates to get this kind of gig where you just learn about a business and then offer random suggestions that might or might not help them and charge obscene fees for the privilege?
  • brk
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You definitely don't have to be god tier anything, you just need to know at least a little more than the companies you are consulting for.

This kind of work has been my primary income for the last 4 years or so. Nowhere near on the same level as Feynman, but I know enough about enough other things that I get a lot of reputational referrals.

>you just need to know at least a little more than the companies you are consulting for.

sometimes (i'd argue often, actually), you don't even need that. simply having an outside/fresh perspective and the fact that you aren't part of any of the existing groups/silos is valuable.

Often the most useful thing is just listening to the right people in the company. I wouldn't be 100% surprised if someone in the company in the story had already had the idea for the third electrode, but it took the suggestion from the high-paid consultant to get it taken seriously.
Probably true, but to get the job in the first place you probably need some sort of showy, impressive credentials.
I think the story sounds fake because they listened to him.

Having the ideas is easy. Persuading and organization to change is not.

Perhaps it’s a cultural difference between the middle of the 20th century and now.

Often the highly paid consultants are there entirely to get the organisation to listen to the right ideas that already exist within the company.
  • leoc
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That's basically what happened with Feynman's involvement in the Challenger enquiry, as he himself freely admitted in his memoir.
  • hoten
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Why would a small company CEO hire a famous consultant only to ignore his suggestions? Absolutely not evidence of it being fake.
Nope! There are consulting companies all over the place filled with bids and not filled with Nobel laureates!

Ergo...

None that offer that level of work life balance though…
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