Desiccants Ain't Desiccants, Sol

[Astro_Bot]

Silica Gel desiccant is a great way to keep your optics dry when not in use. It can absorb up to 40% of its own mass in moisture. Mostly, it's sold in pre-formed sachets.

I bought a large bag of plain (non-indicating) desiccant sachets about 18 months ago. These were delivered sealed inside a sturdy zip-lock bag, which I then sealed inside another sturdy zip-lock bag.

Shortly afterward, I bought a Desiccant Cap for my OTA that came with some indicating desiccant sachets. These were also sealed in a sturdy zip-lock bag that, again, I sealed inside another sturdy zip-lock bag. These sachets were all bright blue on delivery (I remember that very clearly), meaning they were completely dry.

I stored both sets of desiccant in a sealed, air-tight plastic jar (fitted with rubber seal and clips).

I've been using the first batch of (non-indicating) desiccant until it runs out, changing the OTA's desiccant after each use, and have been religious about sealing those zip-lock bags quickly. To the unaided eye, that desiccant looks "dry" and it feels the same as it always has.

Today I got to the back of the storage shelf to the jar with indicating desiccant, that I have not touched at all, to find that they are all completely pink, meaning they are at least 8% saturated with moisture, probably much more. Now recall these were inside an air-tight jar and two zip-lock bags!

I summise that the original (non-indicating) desiccant is also partly or fully saturated, since it has had far more exposure and less protection, which means I've been putting saturated (and possibly useless) desiccant in my OTA for some time now.

I suppose I shouldn't be so surprised, since H2O molecules are smaller than N2 and O2 that make up most of the atmosphere. That is, if air can get in at all, so can moisture. If a pack of desiccant can't keep the internals of a small, sealed zip-lock bag dry, then what hope a large OTA?

The moral of this story is that despite keeping your desiccant sealed, it may well already be saturated and useless and, unless you can test it, you won't know.

You could weigh completely dry desiccant then weigh your suspect desiccant to calculate how much moisture (up to the 40% limit) has been absorbed. But how do you know if your newly-delivered desiccant is completely dry to begin with? If it was saturated on delivery, your suspect desiccant will appear to be dry!

I realise you can heat desiccant (whether indicating or not) to re-dry it, but unless it's indicating desiccant, as this experience shows, you won't know whether that's needed or whether it's complete.

In conclusion, I strongly recommend using only indicating desiccant and checking it's colour frequently.

[mental4astro]

Sage words, Bot. I've never been totally sure about these desiccants, that is the quality of the product one ultimately gets.

Now if I remember correctly, the indicating dye in these is not a very nice item to our health. Carcinogen comes to mind. But it would require constant exposure to bare hands of the fine crushed dust. But I could be mistaken too.

Mental.

[rat156]

Indeed the Cobalt indicator in silica gel is not safe anymore. There is a new orange one which I think turns blue which we use at work.

http://www.silicagel.com.au/15-kg-orange-silica-gel-beads-self-indicating/

MSDS attached for Blue to pink silica gel.

Cheers
Stuart

[Astro_Bot]

That sounds right - cobalt dicholoride isn't good, but it's locked up in the silica crystals, so, in normal use inside a sachet, you should be fine.

If re-drying, use gloves as recommended, and don't ingest or inhale it, obviously. ;)

Thanks for that suggestion, Stuart. I read about that earlier today - I think the orange one goes green when saturated (although possibly there are other indicator dyes out there).

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If you happen to have both indicating and non-indicating desiccant, and are sure that the non-indicating stuff is dry to begin with (e.g. re-dry it as soon as you receive it), you can simply add one indicator sachet to the rest and assume it'll all change at the same rate. That applies both for in-use (e.g. for a large OTA, I use around 5 sachets) and in storage.

[Astro_Bot]

I've just completed my first re-drying of desiccant sachets and would have to say it's surprisingly easy.

Silica Gel desiccant needs to be heated for a period of time at between 104 and 118 deg. C to dry properly and not damage the silica or the packaging. Too hot, and the silica will crack/crumble and/or the packaging will melt or split. Drying is best done in a fan-forced oven that'll keep an even temperature. (For loose silica beads, a microwave can allegedly be used but beware power settings - it's apparently very easy to overheat desiccant in a microwave).

The Trial Run:

I have a thermometer that reads up to 105 deg. C, which I attached to the front of the top oven shelf with hobby wire, so that I could see it through the glass door. With fan on high, I experimented with oven settings until the thermometer read a steady 104 deg. C - my theory being that the temperature at the centre of the oven would be a few degrees higher - and that happened to correspond to an oven setting of ~115 deg.C, so I figure my oven is fairly accurate.

I put 5 indicating sachets into a small pyrex dish and spread them out evenly. The dish has a lid, but that's not needed until later.

Into the oven they went (lidless). Despite some advice online that drying can take several hours (from 2 hours up to 24 hours!) I found that the indicating desiccant turned completely and utterly blue in under 10 minutes.

I removed the dish, placed the lid on it and let it cool. Silica Gel retains heat very well, and the sachets will be too hot to handle for a while. When the dish is cool to the touch, the desiccant should be safe to store.

The Production Run

I put all of the non-indicating sachets (~80) in my large pyrex roasting/baking dish (also with a lid) and mixed in the remaining indicating sachets (35) - they were 2-3 deep.

Given the larger quantity, I had originally aimed to dry for at least 30 minutes. However, the visible sachets turned blue in under 10 minutes as before. I kept going for 20 minutes to be reasonably sure that the lower layers were "baked".

As before, I removed the dish and put the lid on to let it cool. I poked around gently with a wooden spoon and found that all of the indicator sachets were completely and utterly blue, even the ones buried at the bottom.

Inspection

After cooing, I inspected a sample of indicating and non-indicating sachets, and found no damage to silica beads (all nice and round, with no cracks visible) or to the packaging (the edges of some sachets had slightly curled up, but that was all).

Conclusion

Re-drying desiccant in sachets is surprisingly easy using a fan-forced oven and a pyrex baking dish. No need to spread them out on wire trays or bake for hours - 20 minutes at 104-118 deg. C, heaped 2-3 deep in a dish, worked just fine. :thumbsup: