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The strangest thing

2 November 2018

I've written a few columns on helium before, because it's a damned interesting element and it is becoming increasingly precious.

However, I was quite surprised to read this week of how a helium leak in an MRI machine caused the failure of a bunch of iPhones.

Apparently there was a leak in the system which captures the helium gas released when liquid helium undergoes a phase transition from liquid, so as to cool the machine, As a result of this leak, high levels of helium occurred in some parts of the hospital and some of the iPhones exposed to this helium-rich environment, simply stopped working.

"How can this be?" you might ask.

"How can a chemically inert gas screw up an electronic device?"

Well if you haven't read about it already, it's all down to exactly how the MEMS (micro electro mechanical systems) devices in the phone actually work.

Quite coincidentally, I just published a video talking about MEMS gyros on my RCModelReviews Youtube channel earlier this week.

Now (as outlined in that video), MEMS devices rely on mechanical movement for their functionality. This movement is converted to a tiny electrical signal that is then used to measure rotational movement, acceleration or even time intervals.

Although failure of an iPhone's gyros or accelerometers would not render the device useless, failure of the oscillators used as clock signals would -- and best advice is that this is exactly what happened in the case of the iPhones.

An interesting twist to this story is that Android phones were not affected by this helium leak -- why was that?

Well apparently most of the Android-based phone manufacturers are using quartz crystal oscillators to create their clock signals, only Apple uses a MEMS device for this system-critical function.

I was quite surprised to discover that the Apple user documentation for the iPhone actually warns about exposing the handset to helium-rich environments -- as unlikely as that might be in the real world.

So what's the prognosis for all those dead iPhones?

Well I'm not too sure. Apparently, there's a good chance that they'll start working again when the helium has had a chance to diffuse out of the MEMS devices -- but that really depends on whether those devices are using chips which have had all the atmosphere evacuated (ie: have a working vacuum inside them) or whether they're the controlled-atmosphere type, where air is replaced with a low-pressure inert gas with known characteristics.

In the former case it is unlikely that the helium would simply leave the cavities inside the device because no other gas could permeate inside to replace it. In the latter case then it is quite possible that time will heal the wound (so to speak).

But here's something very interesting... I recalled watching a video a year or so ago in which the guys from Periodic Videos stuck an iPhone in a bag filled with helium to demonstrate how poor this gas was as a conduit for sound. Here is that video:

I guess that during the time of that brief experiment, the helium didn't have time to permeate through the seals of the MEMS clock and thus upset the phone's operation. However, this is quite surprising, given that the experiment used a near-100% concentration of the gas.

Was this an older iPhone without a MEMS-based clock?

And more importantly -- just how concentrated was the helium component of the air in that hospital, especially given that the leaking helium itself would have rapidly risen to roof-height and dissipated.

It's all a bit strange really.

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