Capacity of desiccant

[Julesomega]

I have been transferring my radio equipment into an unheated steel shed, packing items in strong polythene bags each with a sachet of silica gel, and wondering how long the desiccant will remain active before it needs recharging. Information on the 'adsorption' capacity is scant on the web, even after I discovered that 'absorbtion' is no longer the preferred term.

I managed to find figures ranging from 3-20% adsorption of water vapour, which seems a bit vague so I stuck one of my new sachets on the car dashboard for a few days hoping it would help with the windscreen steaming up overnight. After several days without using the car it had increased from 100 to 130g, and leaving it on a radiator all day brought it down to about 105, so I put it in the microwave with the invariable half-cup of cold tea for double the usual time. It then weighed a bit below 100g and continued to reduce on the radiator.

After several more tea sessions it was heading towards 90g, and each time I take it out I can feel the moisture. Holding it against the window gives a patch of condensation, and after several patches have been made the weight will have fallen again.

There will be people on here who know about moisture rheology, and may know why the weight keeps falling; it's now down to 90g, representing 144%. I presume the supplied 100g nominal sachet was filled in a normal indoor humidity environment, but I can't believe the academics quoted would have made this mistake. Maybe some of the gel is transpired during drying, but I have seen a quoted drying procedure of ovening at 120° with comments that the granules can be heard disintegrating!

If we lose desiccant each cycle, the capacity per sachet will be decreasing. I will continue measurements for a few more cycles. I just measured a sachet that spent the night in the car and it had gone from 130g to 160g = 23%

Also it looks that drying may benefit from heating gently followed by transfer of moisture to a cold surface (condenser)

[Refugee]

That is a very interesting experiment.
I have been looking into recycling gel packs for a gadget rescue chamber for things that have been recently dropped into water by mistake.

[ColinTheAmpMan1]

The silica gel desiccant used in laboratories can be quite good at drying things and can be regenerated by heating in an oven at something like 120° Celsius (Centigrade for the old-fashioned). It can be used and regenerated many times, but it isn't expensive anyway so can be replaced after a few cycles. It is usually "self-indicating" and has a blue colour when dry, but is colourless when it has absorbed its moisture limit. I'm afraid I can't offer any advice on what the process of hydration/dehydration involves, but you might be able to find out. Personally, it just "does what it says on the tin" and I haven't needed to bother about the whys and wherefores when using it. Of course, this isn't very useful information if you are using the sachets of silica gel that you get with various items, as that stuff isn't "self-indicating". You might be able to buy some self-indicating silica gel from a pharmacist, but you would probably have to explain why you want it to justify the pharmacist ordering some for you. You might even be able to buy some from one of the big chemicals suppliers - it isn't particularly hazardous - but you might need to have an account.

Colin (retired research chemist).

[Refugee]

It is easy to get if you are up for 100 pouches.
https://uk.farnell.com/Search?storeI...&st=silica+gel

[eaglework]

I bought a couple of the 500g or maybe 1kg desiccant bags for use in cars that are suffering from condensation. They have a coloured indicator patch - blue being good and pink indicating time to 'recharge'.
I find that once it has indicated time to recharge they need at least 10-15 mins in a 800w microwave (usually in 4/5 minute cycles) to remove all the absorbed moisture. An hour or so on a domestic radiator can help to complete the recharge process as they do seem to come out of the microwave with some residual moisture possibly trapped in the fabric covering.
They seem to last around a week in a car but that is very dependent on the temperature and humidity levels.

[Dave Moll]

Originally Posted by ColinTheAmpMan1

120° Celsius (Centigrade for the old-fashioned).

Or 248° Fahrenheit for the even more old-fashioned.

[Julesomega]

Good to see that others have been using the stuff in similar applications, and this leads on to a second question: what size sachet to use for our radio equipment?

Assume we are packing it in a damp environment, then we might be looking to reduce the humidity from say, 80% to 40%. Can someone work out the mass of the stuff needed per unit volume? An oscilloscope might be about 0.05m^3.

The third question may be answered by measurement: I was hoping to measure a typical rate of ingress of water vapour into an impervious bag, with the open end folded over and tacked with sealing tape. When completely sealed it could be a fit and forget sachet.

[G6Tanuki]

Yes you need to bake silica-gel desiccant at a reasonably-above-the-boiling-point-of-water [373 Kelvin] for a while to dry it out; I store my sachets in a few "Kilner" jars [the type with the rubber-ring seal and a springy-wire to retain the lid] when not in use.

Military electronics of the 50s and 60s often came with a small screw-in desiccant cartridge - for a radio [with a hermetically sealed case] of about a cubic-foot volume - something like a R209 or R210 - the desiccant cartridge was about 1/2 inch diameter and 2 inches long. they had a little circular glass window on the end - visible on the front panel of the equipment when in-service: behind this little window was a disc of Cobalt Chloride paper surrounded by a light-blue-coloured annulus. When the desiccant was still doing its thing the centre and annulus were the same colour - when the desiccant was exhausted the centre went light pink, signifying a return-to-workshops was needed.

[Julesomega]

Yes, about 4cm^3 - assembled in a dry workshop and hermetically sealed, that should last forever. I'll get some semi-rigid foam sheet to protect against puncturing the polythene when I stack up boatanchors with all their projecting screw heads

[Glowing Bits!]

I found some rechargeable units on Ebay, they weigh about half a kilo and have a fig 8 radio type socket on them, they need at least 16 hours on the mains to dry out, the indicator normally turns blue when it's ready.
These things have already found a use in the car, the window has finally stopped misting up.
The above said item may be one way of keeping things safe in the shed but they ain't the cheapest things on planet earth.

[Terry_VK5TM]

Just be aware that "plastic" bags are not impervious to water vapour ie water vapour can still get through.

https://www.evaluationengineering.co...re-barrier-bag

[Julesomega]

'grief, looks like we're doomed! I can't afford humidity indicator cards. Maybe it's not so bad if you don't need to store your things at 100°
I know about PET being pervious to carbon dioxide so you need to drink your bottle of Cola within a couple of months. I expect alcohol is even more permeating as it's never ever been bottled in plastic

[dave walsh]

I think I recall you mentioning this steel shed previously Julian Short of spraying the insides with a layer of foam insulation, I fear that you may be up against it-whatever "it" is, from the start with such an inherently cold choice of material

Dave W

[Ambientnoise]

This is indeed a difficult problem and one which I experienced in a professional environment when I dealt with underwater and ship board kit.

One tip which I have, in addition to those above is to use vacuum bags, normally used for clothes. These obviously reduce the available moisture but need protection against sharp edges etc. Available in many sizes, not too expensive.

Whatever you do, frequent inspection is essential.

[Julesomega]

It'll be pretty hot and dry by July, Dave - the challenge is to store some of that dryness
The general rule is to use positive pressure to keep contaminants out - are there any effervescent desiccants?

[John_BS]

Julian: alternative approach: if you're bagging and storing in a shed, do you really need it!

John

[Julesomega]

My wife has already made that suggestion

[Neutrino]

Originally Posted by Julesomega

... it's now down to 90g, representing 144%. I presume the supplied 100g nominal sachet was filled in a normal indoor humidity environment, but I can't believe the academics quoted would have made this mistake. Maybe some of the gel is transpired during drying, but I have seen a quoted drying procedure of ovening at 120° with comments that the granules can be heard disintegrating!

If we lose desiccant each cycle, the capacity per sachet will be decreasing. I will continue measurements for a few more cycles. I just measured a sachet that spent the night in the car and it had gone from 130g to 160g = 23%

Commercial dryers continually repeat the drying of the silica gel with acceptable life. In a microwave there is no control of temperature so I speculate that the weight loss is due to the silica gel getting dryer than it would with reactivation at 120°C.

RS Components sell silica gel at £78.90 for fifty 100 g sachets or £69.48 for fifty 50 g sachets. I do not know how many items you have to store!
https://uk.rs-online.com/web/p/silic...ators/0601057/
https://uk.rs-online.com/web/p/silic...ators/0601063/
The installation instruction manual for these has guidance on how much silica gel is required.
https://docs.rs-online.com/dbfc/0900766b81582c07.pdf

Unless you are using commercial hermetic packaging, it’s probably simpler to err on the large size.

I suggest periodically weighing some of the sachets. This will give you an idea of how often they need reactivating.

A relative humidity below about 50%~55% will practically stop corrosion. If you have any woodies be aware that very low humidity will dry out the wood.

David

[kellys_eye]

Would the use of vacuum packing bags (as used for storing linen under beds etc) be any use?

It wouldn't be difficult to detect if they have been compromised as they 'inflate' a little.

[Julesomega]

Thanks guys for all the suggestions, and David for the datasheet link: this has the hard facts I was looking for.
Based on what G6Tanuki said about military radios desiccant cartridges, my 100g sachets seem a bit OTT for all but the largest and leakiest packages so my next order will be for 10g sachets. I buy at a good price from a reputable eBay dealer in N.Ireland btw.
I shall indeed carry out frequent inspections!