Jul 182011
 

The other day, I opened a corrugated cardboard box containing some expensive electronic equipment.  The equipment was made by a large, reputable Korean firm.  It was sealed in a plastic/foil pouch.  Outside this pouch, was a bit of foam padding and about 6 grams of desiccant – silica gel.

Most of us wouldn’t think twice about this – the silica gel is there to keep the stuff inside dry.  But does it work? If it does work, how long will it work?

A desiccant is pretty much any substance that will adsorb moisture more aggressively than items it’s packaged with.  Wikipedia defines desiccant as a hygroscopic substance that induces or sustains a state of dryness (desiccation) in its local vicinity in a moderately well-sealed container.

Desiccants are used in all sorts of amazing applications for some really specialized tasks (check out the Wikipedia entry for more information).   They’re also used in basic packaging.  And they’re sold by the barrel.

So what’s the lie?

Well, suppose that a gram of silica gel can adsorb 20% of its mass in water – 10g of silica gel could then capture 2g of water.  Now I can’t find a water vapour transmission rate (WVTR) for cardboard, but I can give an example of a material which many people would think is impermeable:  Low density polyethylene (LDPE).  You’ve seen it everywhere, milk bottles plastic bags, etc.  A film of 5mil thickness (.005″ or .125mm) has an estimated WVTR of .28 g/100in2 in 24 hours (nice mix of metric and imperial).  Well, 100in2 is about the size of a piece of paper.  This device was in a box that’s  20″x20″x10″ – the surface area is 1500in2, and we won’t get started about all the gaps around the tape.  So even if this box was lined with plastic sheet, we’d see about 15 x .28 = 4.2g of water going through it in 24 hours (with external humidity at 90%, temperature at 100F).  Ok, so maybe the conditions are a bit extreme, but you get the idea.  Basically, by the time the box has shipped out of the Korean factory, the desiccant will be equivalent to sand.  Well, not quite.  We opened a box like this in Calgary in February, when the relative humidity is about 10% or less.  The silica gel probably dried out significantly in the process.

And the desiccant manufacturers are happy to sell more!

I’m not dissing desiccant out-right, there are many appropriate uses, but it’s not magic – you need to use it appropriately, or you’re just wasting your money.  In the case of the device we were looking at, the foil/plastic laminate pouch would have saved the device from moisture quite well, at least for a while.  Those pouches often have WVTR down around 5mg/100in2 in 24 hours.   The desiccant, on the other hand, was just there for not eating.

References: http://www.cttgroup.com/ralston/transfer.aspx?type=2&file=Water_Vapour_Transmission_Rates_en.pdf

http://en.wikipedia.org/wiki/Relative_humidity#Other_important_facts

 Posted by at 9:56 pm