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Activity for misk94555‭

Type On... Excerpt Status Date
Comment Post #290874 @#36396 I like the idea of an accelerated lifetime test. If I sous vide the sensor in seawater for 2 months at 70°C, that should be equivalent to a year in the field. (Assuming the common rule of thumb that every 10°C accelerates damages by a factor of 2x). I could freeze it a few times along the...
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2 months ago
Comment Post #290874 @#8176 **Corrosion.** That’s an interesting question. The electrodes will be ENIG plated. If ENIG isn’t enough, I could do hard gold plating (like PCIe board edge connectors). The metals involved (from top to bottom) are gold, nickel, copper. Gold is a noble metal, so it shouldn’t react with se...
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2 months ago
Comment Post #290874 Olin mentioned at the end of his answer that he’s doing an immersed capacitive level sensor, and it works. I bet there are vias, and they aren’t causing too many issues. (At the same time Olin’s situation is different, because he’s not trying to hold water with a PCB.)
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2 months ago
Comment Post #290874 @#8176 **Vias.** I can tent the vias with solder mask. I would pick small via diameter (say 0.15-0.20 mm) which would mean that solder mask would plug the vias entirely. That would be the easiest solution, because it’s a bog-standard PCB fab process. If that’s not enough, the PCB fab can plug th...
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2 months ago
Comment Post #290875 @#36396 Every 15 to 40 days these sensors will be retrieved. Every year there will be off-season maintenance. The PCB with electrodes can be replaced once a year, if it’s cheap enough. It’s expected that a quarter of these sensors will be lost each year to storms.
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2 months ago
Comment Post #290875 @#36396 You mentioned platinum-plated electrodes which dipped into drinking water. I noticed that high-end conductivity sensors use platinum electrodes (low-end sensors use stainless steel). I wonder why platinum? Is there something wrong with gold? [Should I post this to the main board as a sep...
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2 months ago
Comment Post #290874 By the way, I’m going to use an [LPS28 pressure sensor](https://www.st.com/en/mems-and-sensors/ilps28qsw.html) to measure depth. Correction for barometric pressure will be done during post-processing.
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2 months ago
Comment Post #290874 @#36396 The purpose of the electrodes is to measure conductivity (salinity). I’d like to cover the range from 0.1 to 50 mSiemens/cm (surface water, and seawater). I can settle for 0.1 to 8 mSiemens/cm (surface water only). The device will be completely submerged all the time.
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2 months ago
Comment Post #288118 On a different note, the way the P-channel MOSFET is drawn in this schematic doesn’t look right to me. The source should be on the left. [Common schematic error. I draw a P-channel correctly from a third attempt myself.]
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12 months ago
Comment Post #288118 @#8176 I’m a bit skeptical that a MOSFET drain is sufficiently protected against ESD “for free”. It's true that drain is more resilient against ESD than gate. MOSFETs with higher avalanche ratings are more resilient against ESD to a drain. Nevertheless, I've seen N-channel MOSFETs in an open drai...
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12 months ago
Comment Post #288118 @#8176 The ESD TVS is a fast Zener by nature. How would you choose the breakdown voltage of the ESD TVS? It has to protect the MOSFET, so it needs to be upstream of the MOSFET. At the same time, it needs to survive a load dump. The voltage in the load dump pulse is around 90V, which is orders of...
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12 months ago