Comment by moralestapia
Comment by moralestapia 2 days ago
This would be cool if only it made sense.
Comment by moralestapia 2 days ago
This would be cool if only it made sense.
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.
I agree. It's not clear how adding a sensor "so that it adds back an oxygen molecule" works. shrug
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.
Yes.
Zinc can do this too. But I like silver, its oxide has decent conductivity.
One of the common arrangements on a basic two-electrode sensor is to have one gold electrode to make contact with the electrolyte, and the electrolyte provides conductivity to a sacrificial silver electrode. With electrolyte exposed to the atmosphere through an oxygen-permeable membrane.
As oxygen makes its way through the membrane, it is consumed by the silver at a steady rate and equilibrium is achieved relative to how much oxygen is in the atmosphere. This generates a steady current which is amplified to move a needle on a gauge, where there are knobs to adjust the meter until it displays the correct amount of oxygen during calibration against a known concentration. And must also be calibrated to display zero when there is no oxygen.
Eventually even if the membrane never gets fouled the oxidized silver builds up in the electrolyte chamber and response deteriorates so maintenance is needed. Remove the membrane, polish the silver, put in fresh electrolyte, new membrane, and re-calibrate.
Adding a third electrode opens up a number of further possibilities.
One of them is the option to use an additional inert gold or platinum contact or a salt bridge as electrical reference against the original gold or silver as the sensor. Plus using a more complex circuit than a plain amplifier, apply controlled responsive current to the sacrificial silver at the same time. So rather than directly amplifying the current produced by different concentrations of oxygen existing in the electrolyte (and waiting for it to equilibrate), instead with 3 (or 4) electrodes the ionic silver concentration in the electrolyte can be maintained electronically in a steady state, and as oxygen permeates, the current required to replace the consumed silver is designed to make a dfferent kind of meter move the needle the same way as above. In this way the oxygen concentration in the electrolyte varies to a much more limited extent compared to waiting for it to be depleted from a high amount to zero before the meter will bottom out.
This can be equivalent to constant-ion electrochemical titration.
Disclaimer: I always like to handle things like this like lives depended on it, because lives depended on it.
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?
It means you do an electrochemical reaction that releases an oxygen molecule, like the original explanation said. It doesn't really matter what reaction it is, but it could for example be electrolysis, where you split 2x H2O into 2x H2 and 1x O2.
The point is this reaction is reversible. In one direction, you end up with fewer O2 molecules than you had before. In the other direction, you end up with more.
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 )
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.
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).
That's very cool and edgy but (Richard) Feynman didn't say this.
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.