lifecycle of silica gel packs / desiccant pouches

[Nicool 30]

Hi eveyone,

Like many of you, for many years I have stuck a pack of silica gel at the bottom of my housings, to prevent condensation issues. 

This has seen me condensation free for 10+ years, including diving in tropical places (with/without air conditioned closing of the housing), and temperate areas in all seasons. 

My longest-lasting silica gel pack is the one pictured below, which I must have got in a shoe box, it has sat in my Nauticam D500 housing for the last 6 years (and I can't remember, perhaps it was in the previous housing already).

Anyways, yesterday I opened my housing, to find my D500 DSLR and the parts of the housing covered in condensation!!!! What's weirder: I had put back the camera there so that it's would be ready for next dive (I find my housings the safest place to store DSLRs when not in use), and it had been sitting there for the last 2 weeks, the housing itself was sitting on our laundry room, which is slightly moist, but hey, it's been sitting there for many years. Noteworthy: I had NOT vaccumed the housing that time.

I tried everything, found the desiccant pack very moist, so it was time to dry it up and rejuvinate my silica gel. Googling the best way to do it (oven, microwave...) I realized that:

1/ there isn't much good guidance available online. Most of the articles refer to free-flowing silica crystals that you can look at (colour change indicates if they are saturated in water or not), the idea being to pack them in a box, in one's camera bag. But these boxes are too big to fit in our housings  

2/ I am still unclear why that condensation formed at home. INTERESTINGLY I've read somewhere (can't find the link anymore...) that saturated silica crystals (having absorved the max of their water capacity) may release that moisture on their own, without heating. Could it be that my silica pack went on strike and tried to flood my housing on land?!?

 

All-in-one, I am curious what you people do to keep your silica pack doing it's job?

If you do use paper silica bags like I do, how do you heat them up, how do you know when the crystals are dried enough? (apparently, if you heat them up too much, you may impair their water absorption capacity).  

Have a great day,

Nicolas 

[ChrisRoss 150]

I've never used silica gel packs with my Nauticam housing and have never had a condensation problem - in the tropics or otherwise.   Sydney would be a great candidate for that to happen - Hot humid day in November then jumping into water that is still only 16°C.

The capacity of those small silica gel packets is very limited and they are only able to keep the housing dry while it is sealed.  If you leave it outside the housing for a few hours or so it will be saturated by atmospheric water.

If the packet contains 10 gr of silica gel, that can carry about 3 gr of water.  Assuming a 3l internal housing capacity, that would be about 0.025 gr water at 50% humidity so can absorb the water from the housing about 100 times - assuming it is never stored outside the housing.  The actual capacity is probably less as the absorption slows down as the gel is approaches capacity and you pack might only be 2-3 grams?   It will reach equilibrium water content in about 6 hours outside the housing.  All this assumes it is fully re-generated when you take it it of your shoe box which would only be the case if it was sealed inside a plastic bag when it arrived and the shoes release no moisture.

As to why it condensed inside - first your silica gel was most likely exhausted when you closed it up and has been for some time, possibly inside a warm room inside then you put it a cold laundry room.  I don't believe silica gel gives up it's moisture unless you heat it.

If you want to work out how long to heat your silica gel get some indicating silica gel crystals, they change colour when re-generated, otherwise you are just guessing.  You would need to find some sort of porous container to hold the beads that you could pull out and re-generate.

Have a look at this link, you'll see the capacity of the satchels  yours looks like it might be a 2-3 gr satchel by dimensions.  The 2 gr satchel is recommended to protect 1.5l of enclosed space - keep in mind this is designed for once only.  You put a fresh pack into the space and seal, the package gets opened by the new owner, the desiccant removed and is thrown away - this is the design case.  This indicates for you use case - the housing you put a fresh desiccant package in each time you use it - You probably should be re-generating the pack every few dives.  https://www.silicagel.com.au/silica-gel-packets

[Nicool 30]

Thanks a lot Chris, very insightful and I feel a little silly now, for using that 1 pack for 6 years in a row... 

It looks like silica gel can only absorb indeed, while other hybrid gels can act as absorbents and desorbents, in ranges of 40-70% humidity:

• Hybrid gels were developed to be highly effective at both absorbing and desorbing moisture. Hybrid gels are better at maintaining mid-range relative humidities (40-70%) (ArtSorb, Arten Gel, Prosorb and Rhapid Gel are hybrid gels available commercially in several forms including loose beads, sheets, cassettes, and tiles).

It would be a pain to keep re-heating or replacing (also not envrionmentally friendly) silica gel packs every few dives, so re-considering my options there.

I suppose we, underwater photographers, don't need the relative humidity to be pulled down as low as 40%, we just want it to stay below 100%, so I am thinking a hybrid gel shooting for 70% could do the trick, while not wasting its absorption capacity, when surrounding air humidity is only 60-70%.

I still don't understand why my housing condensated in the laundry room, wouldn't be the first time I close it in the warm living room, and then leave it for a few days in the laundry (which goes from warm/moist to cold at night). Only change is this time I hadn't pulled the vaccum, which thinking about it, should cool down the temperature within the housing?

I think you've either been lucky not to condensate in Sydney summers, or you did close your housing at home when it was closer temp to the sea temperature.

[Isaac Szabo 108]

Like Chris said, that little silica gel packet hasn't been doing anything for a long time. I think the key reason why you experienced condensation this time is that you didn't pull a vacuum. Decreasing the air pressure decreases the dew point, so pulling a vacuum is most likely the only tool you need to avoid condensation issues. If you want to be sure, you can do a test by closing up your housing in warm, humid air, pulling a vacuum, putting it in a cold environment for a little while, and seeing if any condensation forms inside. 

[ChrisRoss 150]

3 hours ago, Isaac Szabo said:

Like Chris said, that little silica gel packet hasn't been doing anything for a long time. I think the key reason why you experienced condensation this time is that you didn't pull a vacuum. Decreasing the air pressure decreases the dew point, so pulling a vacuum is most likely the only tool you need to avoid condensation issues. If you want to be sure, you can do a test by closing up your housing in warm, humid air, pulling a vacuum, putting it in a cold environment for a little while, and seeing if any condensation forms inside. 

Yes agree, I always pull a vacuum, the dew point will be at a lower temperature if you reduce the pressure.  I always close my housing in the early morning before heading out so the temperatures will be lower probably not always below sea temperature though.

I suspect a lot of people get fogging by trapping a water drop inside the housing - every time I open my housing I will see a couple of water drops clinging to the o-ring and the housing.  Trapping one drop could be enough to bring moisture content of trapped air up to saturation point.  There's no way around that other than to pull the o-ring, shake it dry and dry the o-ring groove.  Is it possible there was water under your housing o-ring? 

This issue is, I am sure the root cause of people claiming that leaving a housing in the sun causes fogging.  What leaving it in the sun does is to vaporise any trapped water which can then condense when cooled.  If the housing is kept cold the water will be slower to evaporate.  The solution is to remove the water!

[Paul Kay 67]

You can buy Indicator Silica Gel cheaply enough off eBay. It is orange and turns green when it has absobed water. To reuse it needs heating to a 80/90 degrees C (not boiling point) until it turns orange again. You can use a fine mesh (stocking or plankton net or similar) to make sachets. FWIW I keep my housings and cameras in Pelicases whn not in use and use Indicator Silica Gel in with them to ensure no trapped moisture causes problems - it is surprising how long water can take to dry out from the recesses of housing. You can do the same in a housing which may help ensure condensation is minimised.