Comment by brk

Comment by brk 2 days ago

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.

Panoramix 2 days ago

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.

Reply View 2 replies

direwolf20 2 days ago

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.

Reply View | 1 reply
- fuzzfactor 2 days ago
  
  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.
  
  Reply View | 0 replies