RH "requirements" for (COTS) optical media

It might not be significant at all; perhaps it was just an achievable test condition (it's REALLY hard to get to zero percent relative humidity). Folk who operate environmental test chambers have told me that low temperature low RH is the hardest condition to achieve and maintain.

Dumb semi-OT question - would compressed nitrogen or air from a cylinder not be very dry if not completely devoid of water?

Perhaps. I've had no problems with optical media and data loss/retention. And, always thought *dry* was a considerable advantage (cool, dry, dark).

The "5%RH" figure wouldn't have attracted my attention anywhere else. But, *here*, that's achievable. I'd hate to discover there was a tiger behind the *other* door, too!

Hi, Don:-

I doubt optical media (polycarbonate) cares, but polyamide (Nylon) gets brittle if you let it sit in really low humidity conditions (as your environment is).

Some companies dispense and seal a drop of water into the plastic bag with every bunch of nylon cable ties.

In this case, it's probably because nobody thinks that environments that dry are worth checking for (does it have to live in a dessicant cannister?). Las Vegas is supposed to be the driest in the US but apparently still averages over 20%. In the Atacama desert in Chile or Egypt it might get below 10%..

Best regards,

Hi Spehro,

Hmmm... I didn't realize that! So, all those ty-wraps that I diligently removed from the bags they came in (and repackaged in cardboard boxes) are at risk?

I imagined humidity might affect "preprinted" optical media -- causing some sort of delamination? For recordable media, maybe something similar in terms of the dye layer?

I can't recall ever seeing this. Is it *literally* a "drop of water"? E.g., you often see a little sack or canister of dessicant (sp?) in with items; I've never seen a similarly packaged humectant (?).

But "averages" aren't the only issue, are they? E.g., it was 10%RH here when I posted the original question. I briefly checked historical records and see lots of 4's.

And, what sort of time period is involved? E.g., bring them up to high temperatures for a few seconds and they're trash :> How long do they need to *remain* at low humidity levels before there is an effect?

E.g., oxidation is a relatively quick process (depending on temp and humidity). What about the opposite case?

Hi, Don:-

Daily averages are probably a reasonable number for most purposes.

The time depends on how thick the plastic is- could be only hours or it could be weeks.

Best regards,

Unless some chemical wizardry is performed, I'd expect 20 MPa (3kpsi) air cylinders to have any moisture present that didn't condense when compressed. Water, at 20C, has 2.3 kPa partial pressure, meaning a hundredth percent of the molecules are water. You could get more accurate with some fiddling and Van der Walls modeling of the gasses.

In a test chamber, of course, there's a stack of media that have some outgassing of water and other vapors.

The gas supplier here has "technical air", I think it is just a mix of oxygen and nitrogen, H2O content less then 20ppm

Nitrogen should be dry, that's why it is used in tires

-Lasse

Blowoff nitrogen from a LN2 tank is very very dry. I expect a lot of commercial nitrogen is made that way.

Cheers

Phil Hobbs