When to reactivate dessicant?

[colin jackson]

I bought several packs of "Eagle Gel type 1 or type 2"dessicant mil-D-3464-E about a year

ago. The store sold it dry and said that it should be redried when it get a bit slushy

feeling when squeezed in the bag as opposed to "rattly" as it was then and still is. The

pack has instuctions for reactivating (250?f for 8 hours).

 

I actually use it with by Blads as they sit for months while I use my Leica stuff regularly but

plan to put in the Leica case for a while to make sure they are dry if I am not planning to

use them for a while.

 

I can't believe the gel is still dry after all this time. I could toast it in the oven to make sure

but I would think there must be some way to tell if it is wet. I have used loose gel before

which changed from white to blue but this is enclosed in brown packs.

 

Does anybody else have experience with this stuff?

 

Thanks

[jordi_rosales2]

Some dessicants made of silicagel are coloured with cobalt salts, so they are rose when dry and blue when hydrated.

If yours is not colored you can heat it for a while 2 hours for instance at 250 ?C, does not matter if it was dry before or not.

 

I assume you live in a very humid country?

[colin jackson]

They are in sealed bags so can't see color. Apparently it was made for use for the military

to keep guns and ammo dry but is sold at Leos in (rainy) Vancouver for camera use. I used

to use the other stuff but it was a pain packing in little pill capsules with tiny holes and the

procedure was messy every time I had to redry which was frequently.

 

As I mentioned, the instructions for redrying are clearly marked but takes 8 hrs. and may

not be necessary.

 

I live near Vancouver, Canada not for from the ocean and my camera guy, Horst, can tell

that some Hassy components internally have been affected by our damp environment.

 

Slightly OT but my Nikon F which was inundated in salt water on the Oregon coast in '69

and washed in fresh water then wiped dry a few minutes later has never needed servicing

and is still working well. Perhaps I shouldn't worry....

[vivek iyer]

"Some dessicants made of silicagel are coloured with cobalt salts, so they are rose when dry and blue when hydrated."

 

Jordy is correct about the Cobalt salts but got the color change wrong. They are blue (if it is a Cobalt salt) when dry and turn pink after getting hydrated.

 

These dessicants are slow types, and take a while to abosrb moisture to get hydrated.

 

Slow heating for longer times are recommended for drying them (250 deg F and NOT 250 deg C).

[jonathan_reynolds]

Forgive my naive question, but does anyone know whether the stuff dehydrates again in a dry atmosphere at room temperature? Or does it always need a higher temperature to restore it?

[vivek iyer]

A higher temperature is required. You need to supply energy to dislodge the water molecules taken up by the dessicant.

[james_elwing]

You could try putting silica gel in a domestic frost free refrigerator, which normally operates at 25-30% RH (ie rather dry). I have had silica gel crystals go from pink to blue in such an environment a number of times, (fridges in a museum for storing film), but haven't taken it any further. Theoretically it beats the heck out of oven heating, as the bags could be made of cotton.

Maybe I should check it out further and let you lot know

[tekkie]

I microwave it when I feel H2O threatened!

[jason_jones7]

Colin,

 

I just had a customer ask me a related question (we use Eagle dessicant bags as a bulk dehydrating agent in the packaging of solid chemicals... I'm a chemist), so figured I'd chime in. In the case of non-indicating (i.e., non-color changing) dessicants, the easiest way to tell if the gel is hydrated is by weight. There should be a dry unit weight posted on the pack. Multiply the unit weight by the number of units to determine the total dry weight.

 

I am not familair with what your relative humidity requirements are, but after speaking with an engineer at the Eagle Chemical Company, learned that if you are looking for no more than 20% relative humidity in a closed environment, the dessicant bag should weigh no more than 111% of the calculated dry weight (i.e., the silica gel will have absorbed 11% of its weight in water). If 40% relative humidity is required, the bag should be no more than 121% of its total dry weight.

 

I was also told that if stored in a well-sealed container which is never opened, the bags have at least a year shelf-life. Open the container and then all bets are off... it all depends on the environment in which the dessicant was exposed and your relative humidity requirements as to whether or not you need to worry about replacing the bag. Its a simple matter to regenerate them, however, so it certainly would not hurt to replace the bag every time the container is opened.

 

To regenerate the gel, simply through the bag in an oven set to 245F for 16 hrs (doesn't matter if you preheat the oven or not). Once regenerated, immediately remove the bag to an airtight container, preferrably a dessicator, until needed. And in case you are wondering, as long as you have not cross contaminated the bag with any other chemicals in the dark room or elsewhere, the silica dessicant bags are non-toxic, as is the silica itself.

 

By the way, you've waited way too long if your bag feels "slushy".