Adafruit HUZZAH32 - ESP32 Feather max ambient temp

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MotorDad
 
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Adafruit HUZZAH32 - ESP32 Feather max ambient temp

Post by MotorDad »

Does anyone know the max ambient temperature that the Huzzah32 - esp32 Feather can tolerate during operation? Espressif lists the operating temperature range for the esp32 as -40°C to +125°C. While Silicon Labs rates the serial to usb converter for +85°C they do list the absolute maximum as 125°C operating and 150°C storage.

I had one of those wild shower thoughts and the more I think about it the more I believe it's possible to build a sensor I can put through an autoclave. I have high temp 3.6 volt batteries that operate up to 145°C and with the board powered through the battery pin the usb is doing nothing since it's not connected or providing power. I have to see if I can get a wifi signal through the chamber walls so I can get data out and not worry about an sd card.

Probably just the wild ramblings of a mad man.

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adafruit_support_mike
 
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Re: Adafruit HUZZAH32 - ESP32 Feather max ambient temp

Post by adafruit_support_mike »

I see a couple of likely problems:

One: most autoclaves have a metal chamber, which would act as a Faraday cage to block any radio signals.

Two: most autoclaves use pressurized steam for better thermal transfer. That has a strong chance of making chips explode.

IC packages are made of filled nylon, which is moderately hygroscopic. Many ICs are shipped in sealed bags with dessicant and a humidity-indicating card. If the card shows exposure to moisture, you should bake the chips a little below 100C to drive off the moisture. If you don't, trapped steam can make the packages crack open in the reflow oven.

An autoclave would also have a reasonably good chance of driving steam into the boundary between a chip's metal leadframe and the plastic package. That would also have the potential to do damage.

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MotorDad
 
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Re: Adafruit HUZZAH32 - ESP32 Feather max ambient temp

Post by MotorDad »

Yeah, Wifi may be an issue. most large autoclaves use a jacket, so I'll have have two steel boxes to overcome. That said, the door seals are a rubber or silicone leaving only the hinges to potentially conduct a field. the door may be open to radio while closed to steam

The pressure I'm not too worried about. Autoclaves have a very slow vent speed for sterilizing liquids, so the pressure drop at the end of a cycle is not sudden. I recently ran an unprotected SHT40 temp and humidity board through for 20+ mins at 121°C and 15-20 PSI with no issues. plugged it in to my huzza esp32 and worked great. That's what got me on this track of running the whole set up through.

I would print an enclosure out of polycarbonate with an o-ring seal to protect the board from the steam. There are variations in temperature throughout a large autoclaves chamber. I have used mechanical and chemical devices to get a very rough idea of their patterns, but to map it out with multiple temp sensors over a given period of time as the unit pressurizes, the load soaks up heat, then as the unit vents. that gets me excited and thinking. sure, I can modify one autoclave with a bunch of sensors, but a system I can just move from one to the next would get so much more data.

All this rambling and I still don't have any idea what the huzza is rated for temperature wise.

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Re: Adafruit HUZZAH32 - ESP32 Feather max ambient temp

Post by adafruit_support_mike »

Doped silicon basically stops working at about 150C.

The underlying principle is called 'Fermi energy', which is roughly the 'you must be at least this tall to ride' sign for electron orbitals. If an electron has less than the Fermi energy for a given orbital, it can't exist in that orbital at all. Electrons can have more than the Fermi energy for the orbital they occupy as long as the excess energy is an integer number of photons (quantum physics). Technically there's no upper limit to the amount of excess energy an electron can have, but statistically most electrons will occupy an orbital whose Fermi energy is close to the amount of energy they actually have (all the lower levels are filled by electrons with less energy).

Fermi energy gets more complicated when you put many atoms close together, like in a crystal. The electron orbitals overlap and combine mathematically. Sometimes they add together, and sometimes they cancel each other. The result is a new set of orbitals around more than one nucleus, each with its own Fermi energy, and some energy levels that simply aren't possible for electrons around atoms in those positions. The energy levels where electrons can exist are called 'bands', and the ones that can't happen are called 'band gaps'.

For some energy levels, it's easier for electrons to hop from one nucleus to another than to stay localized around any single nucleus. Those values are called a 'conduction band'. Atoms below that energy level that do stay localized live in what's known as a 'valence band'.

In metals, the Fermi energy of the conduction band is lower than the Fermi energy of at least one electron in an atom's valence band, so some electrons are permanently non-localized. The shiny appearance of metal is a cloud of electrons in the conduction band.

In semiconductors, there's a relatively narrow band gap between the valence bands and the conduction band.. about 1.1V worth of energy for silicon.

Electrons don't know the difference between 'electrical energy' and 'mechanical energy', and all materials warmer than absolute zero transfer energy from one atom to another. The rules about how that happens are involved, but can be treated as if there were particles called 'phonons' moving from one atom's valence electrons to the valence electrons of another atom. Electrons gain or lose energy through light as photons, or mechanically as phonons.

The amount of energy associated with 1 degree Kelvin is known as Boltzmann's constant: about 1.4e-23 Joules per atom. If we apply that energy to an electron, we get 1.4e-23J/1.6e-19C, and since 'energy per unit of charge' is known as 'Volts', 1 degree K has the same effect on an electron as about 86.2uV of electrical excitation. When you get up to room temperature (300K==26.85C==80.3F), the 'thermal voltage' of an electron is about 25.8mV.

Some thermal collisions have enough energy to kick an electron out of the valence band and into the conduction band. The empty orbital left behind (called a 'hole') behaves like a particle with positive charge, so we treat holes as quasiparticles like phonons. Electrons in a plain silicon crystal usually stay in the conduction band for about 1ns before dropping back into an empty orbital in a process called 'recombination'. At room temperature there's about 1 electron in the conduction band per 1e15 atoms at any given time. That value is called the 'intrinsic carrier concentration'.

We modify the number of electrons or holes in a silicon crystal by adding atoms with a different number of valence electrons (dopants). Silicon atoms have four valence electrons, aluminum has three, and phosphorus has five. Adding aluminum to a silicon crystal effectively creates a permanent hole, while adding phosphorus effectively creates a permanent electron.

We typically dope silicon at a concentration of about 1e-10, which makes the dopant atoms about 1e5 times more common than the intrinsic carrier concentration. When an electron/hole pair forms in a silicon crystal doped with aluminum, the electron sees 100k holes to recombine with, and the hole has a 1-in-100k chance of being the one the electron recombines with. In silicon doped with phosphorus, the hole sees 100k electrons to recombine with, and the electron has a 1-in-100k chance of recombining. Statistically, the ratio of electrons to holes balances out at the ratio of dopants to intrinsic pairs, and the carriers that can't recombine remain available to carry current through the crystal that much longer.. usually on the order of 100us.

At 150C, the rate of intrinsic electron/hole generation becomes a large fraction of the dopant concentration. Instead of having 100k dopants per electron/hole pair, we end up with maybe 1-10 dopant atoms per pair and carriers that last maybe 2ns to 20ns. The lifespan of unrecombined carriers falls, the crystal drops back to being a mediocre resistor, and transistor circuits no longer work the way they would near room temperature.

Silicon-on-insulator devices can keep working up to about 300C (573K), and devices made from silicon carbide and gallium nitride can go up to about 900C (1173K). The cost is that SiC and GaN have bandgaps about 3x higher than silicon (3.3V to 3.4V), so they require higher voltages to operate. That isn't all bad though, because the increased voltage makes them about 10x faster and smaller than silicon for a given amount of power.

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