
Post-mortem: MRI disables every iOS device in facility - onuralp
https://old.reddit.com/r/sysadmin/comments/9si6r9/postmortem_mri_disables_every_ios_device_in/
======
anowlcalledjosh
See also:
[https://news.ycombinator.com/item?id=18340693](https://news.ycombinator.com/item?id=18340693)

------
Cogito
Description of the issue from the original thread [0]:

The processor in a modern, high volume device typically has its main clock
driven by what's known as a MEMS oscillator. These are barely visible
mechanical systems that resonate at some designed frequency, and include
packaging to convert this resonance into a useful electrical clock signal.
These devices are extraordinarily cheap ways to produce a steady clock, but
they have a number of drawbacks. Most relevant, in order for these types of
devices to function properly, the mechanical resonator must be inside a tiny
hermetically sealed chamber with either a controlled gas inside or a vacuum,
as the gas composition in the chamber can affect the output frequency.

For both cost and physics reasons, these hermetic seals are not perfect, and
are somewhat commonly permeable to small atomic gasses such as helium.

...

For this specific case, Apple devices probably share a common family of MEMS
resonator to reduce manufacturing costs. This clock likely leaks in helium
rich atmospheres, pushing the output frequency outside of the bounds that the
main processors are designed to handle, rendering them non-functional. If left
idle long enough, the devices may begin to function again, but depending on
the concentration of helium which leaked in, this could take anywhere from
weeks to years to occur in natural atmosphere and temperatures.

[0]
[https://www.reddit.com/r/sysadmin/comments/9mk2o7/mri_disabl...](https://www.reddit.com/r/sysadmin/comments/9mk2o7/mri_disabled_every_ios_device_in_facility/e7g5rcw/)

~~~
SilasX
>For this specific case, Apple devices probably share a common family of MEMS
resonator to reduce manufacturing costs.

At risk of provoking a standard flame war...

They had mentioned that Android phones are unaffected, but don't those
generally use even more "economical" manufacturing?

~~~
Cogito
It’s not economical in the sense of cheap parts, rather it’s economical in
that Apple uses the same part in all devices. This can simplify supply chain
and engineering efforts, so makes sense when possible.

The comment is looking to explain why all the Apple devices failed; they were
all affected by the same sensitivity to helium.

Android devices tend to use a multitude of different parts to each other, so
while some may have been affected you wouldn’t expect them all to fail in sync
like the Apple devices did.

~~~
SilasX
The investigator reported that they couldn't find _any_ Android with the
error.

>>There were also a lot of Android phones in the facility at the time, none of
which were impacted.

Somehow, all these commodity vendors all used designs that were resilient
against helium.

~~~
Cogito
I agree that it's interesting they found no android devices that failed. Would
love to see a more comprehensive study, looking at which devices fail at what
levels, etc.

Not that this is necessarily 'Not Invented Here' syndrome for Apple, as I
think they may have compelling reasons to design and procure a bespoke
component, the benefit of using something off the shelf is that it is battle
tested by other people. You get a component tested not just on your use cases
and failure modes, but everyone else's, so that if your needs or environment
changes it's more likely for the component to keep working.

I would also love to know more about how the enclosures used affects helium
sensitivity; is helium pervasive enough that it gets everywhere anyway, or can
some phone bodies etc keep it out.

------
hatsunearu
Wow, that's a really insane and cool phenomena. I should keep this in my
memory in case I encounter it in my career.

Also, if there was enough helium to contaminate the entire atmosphere, was
there not a pretty significant chance of an asphyxiation hazard?

~~~
alxlaz
I don't think there was. It's not like people figured out _today_ that bad
quantities of helium can be bad, and that big tanks have big quantities of
helium, so MRI rooms are routinely equipped with oxygen sensors (see e.g.
[https://www.pureairemonitoring.com/oxygen-o2-monitor-for-
mri...](https://www.pureairemonitoring.com/oxygen-o2-monitor-for-mri-rooms-
pureaire-sells-a-hassle-free-10-year-oxygen-monitor/) \-- not affiliated, it's
just the first result on Google). If the spill had been significant, these
would have definitely caught it.

Besides, an MRI's liquid helium tanks are big, but not _that_ big to cause
trouble if they leak over a period of a couple of hours in a ventilated room.

------
kiesel
So, what could it be that makes iPhones "react" with helium? Helium is an
inert gas and shouldn't really chemically react with anything in there.

Any clues why such an environment doesn't make Android phones behave in such a
way?

~~~
hatsunearu
Basically electronic oscillators rely on a very small piece of quartz crystal
that is cut in just the right shape. Quartz has a phenomena called
piezoelectricity, where if you apply an electric field, it deflects slightly.
Conversely, if you deflect it, it creates a small electric potential. A
properly shaped quartz, when exposed to some circuitry (oscillator circuitry;
positive feedback), it will resonant at its natural frequency, not unlike how
a tuning fork resonates at its natural frequency.

Since the crystal is literally vibrating inside the metal can it's packaged
in, the atmosphere around the crystal influences the natural frequency. Helium
will make it vibrate it faster, since it presents less air resistance to the
crystal.

~~~
alxlaz
Note, however, that MEMS oscillators (which modern iPhones use) and quartz
oscillators are _not_ the same thing. SiTime's MEMS oscillator is made of
silicon, not quartz.

