Wanted: Silica-Gel Dessicant Sachets

I think I've found a source.... but they're not cheap!!!

about 50 pence each and you have to buy 100 at a time... I only want a handful...

Any ideas?

http://cgi.ebay.com/100-Packets-Of-SILICA-GEL-DESSICANT_W0QQitemZ170098737052QQihZ007QQcategoryZ43479QQssPageNameZWDVWQQrdZ1QQcmdZViewItem

You might also try googling "surplus dessicant" - I remember seeing some in an electronics surplus catalog eons ago.

Many thanks Bhankiii, I have just scanned ebay and found some at a realistic price, trouble is its from Malaysia, so it will take a while....

I couldn't believe the price most companies want for these humble little sachets!! 50 pence + each.... !!

John.

uh huh - like I'm supposed to know what a "pence" is.

Ok well here I go Pence = penny 100 = £1.00 = $1.92 approx at any time this will vary up or down depending on the degree of speculation in the market. Any the wiser.

http://cgi.ebay.ca/200-BRAND-NEW-Packets-Of-SILICA-GEL-DESICCANT_W0QQitemZ160102066294QQcategoryZ43479QQrdZ1QQcmdZViewItem

you can also buy it bulk for less than $10/pound in 5 pound lots. In tons it is quite cheap

Yes, I may be in position to help you out and the source is India. The sachet, depending upon the confined volume of the product to be protected, will vary in its sachet quantity (say 1 gm takes care of 1 cubic foot volume - as a rule of thumb). The cost may vary from Rs. 5 to 8 Rs., (about 45 Rs is equivalent to 1 US $)depending upon the quantity need. This sachet content not only keeps away Humidity but also keep corrosion.tarnishing away for a quite long time and product rmains shiny.. Let me have your feed back to help you out.

Hi Dr Thanki, I'm only after say 20 to 30 at the moment, as the volume is small, about 500 cc at the most and the sachet will be in a semi-sealed metal box, also the instrument would need to be calibrated every year, so part of the calibration would be to re-activate the sachet or replace it...

From what I've read for a 500 cc volume all I would need is 1 gram sachet...

Anyone have a handful I could nick? beg or borrow? Okay even pay for then!!

John.

go to a local computer builder. A lot of the drives and accessories come packed in sealed bags with one of those sachets. They just toss them. Chacnes are they will be able to give you a few.

They will of course be saturated with local ambient vapor

Just do the bake-out thing and you have it.

Yes, aurizon, that was a thought I had... trouble is most of the stores around here would throw them away.... I might have to go around with a begging bowl...

But Hey why not!! I got a new carpet for my hall a few years ago by looking at a carpet store and deciding the only colour I liked was a large 'off cut', so the guy just said take it!!!

John.

Don't know of any place in the UK...but there's a great manufacturer we use here in the US. DESICCARE, www.dessicare.com,

Pomona, CA 800-446-6650
Richland, MS 888-932-0405

Good Luck!

Go to a shoe store. Try on a few dozen pairs of shoes.

Grab the silica gel packs and put them in your pocket.

Bake them out at 100C for 24 hrs.

The sachets are an interesting idea. I'm curious about the design of the capacitors. How did you space the plates? What is the dielectric for the higher values? Are you using boxes with rubber seals? Obviously they are not hermetically sealed or you wouldn't need the dessicant.

S

They are hermetically sealed. Once sealed, with a dry sachet of gel inside, they rapidly go down to the equlibrium dry vapor pressure of the sachet. If there is any leakage through the seals, the sachet adsorbs it to it's max and near the max changes color. You usually have a sealed view port for this, or just routinely change the sachet sfter inspection. The seal where a variable capacitor is about the only place it can leak. Often hermetic fixed caps are solder sealed in a can with glass bead sealed feedthroughs and never opened again.

Hi Aurizon,

The units that he is trying to replace have 4 BNC connectors on them. That makes them hard to hermetically seal unless there is an inner box with some glass insulated feed through wires, or something similar. They are also calibrated up to 13MHz by using a special technique that applies up to 150Mhz to each BNC one at a time with another one shorted, etc. The inner box with the feed thru's would no doubt change the RF characteristics.

Regards,

S

well, if they are passive devices you can seal them with RTV quite easily. The RTV will not cause any trouble at 150 Mhz when used as a thin film at penetration points.

If he has to deliver voltage to any active device inside he will have to make sure he does not disturb the impedances, but that will not be diffucult at that frequency.

He can even tin the edges of the box and solder seal the whole thing, if he does not want future access. The pressed centers of the BNC connectors press out easily and can be repressed with either RTV or silicone grease to prevent vapor ingress. You can get away with a droplet at the inner and outer area.

Oy.. You two..!!!

who are you calling 'he'.... I am here and reading this you know.... I've got a name....

John.

This may be childish , but wrap them in nappies (the ones with absorbent gel) ?

Maybe buy a bunch of granules from a hardware shop , and make your own sachet from a tea-bag.

Try contacting companies who import electic motors and other componets, they will often get hundreds of these sacks to dipose of, ask if you could put a bin at their factory/wharehouse and say you will take them away, you will probably need to give a cash payment as a perk to the guy helping you out. But most of these sacks are disposed of anyway, even if they get wet you can dry them out. You just have to find the right size to suit, you could always re-package the crystals. This kind of set up is quite common in the UK and it will be a perk for one of the guys on the factory floor, if it is just small scale. The sacks can be used for dehumidifying most things from electrical goods, to stores and even vehicles that have a tendancy to over condensate.

Hi Standards guy... these are a range of standard capacitors we are designing for sale to compete with the Agilent 16380 series and others...

The top values of 1 and 10 microFarads are made using polystyrene dielectric, as mica gets way to big at those values and we have found some capacitors that we have measured as having a temperature coefficient of only -20 ppm!!

For the intermediate values we are using mica 100, 10 and 1 nFarad with an air trimmer if needed.

For the lower values we are using air spaced capacitors, 100, 10 and 1 pFarad.

We have had some excellent results so far with tempco's less than +20 ppm for the mica and the air ones we are testing at the moment.

I would have liked to hermetically seal them in nitrogen, but that gets expensive, so I'm looking at normal sealing techniques with a silicone RTV adhesive.

Accuracy at 1kHz is +/-0.02%

If you're interested in selling or using these or just the data email me!! john@marchdesigns.com

John.

The cost is in making the sachet really. Making sachets and packing them in small quantities is time consuming and quite tricky (you have to ensure the desiccant does not get exposed to moisture in the atmosphere during filling and you have to make sure they are stored in airtight containers until required for use) (or you have to dry them out in an oven) (so use bags/pouches made of cotton - not woven plastic) - postage is also a big factor (silica gel is heavy)

If DIY - don't call then sachets - call them drawstring pouches - then you can pick them up cheaply on ebay - There's someone called Toycrazy - ebay 190100350513 at the moment) - then get some desiccant in bulk (a relative term) from someone like GeeJay chemicals at Sandy, Beds. www.geejaychemicals.co.uk.

Hi Horace, yes I had a look at Geejay, Sandy is just down the road from us (10 miles). But as we only need a tiny quantity say 50 grams, its not really worth buying in bulk.

According to the manufacturer's of the sachets even when they are sealed they only have a shelf life of 6 months before they need re-vitalising - cooking at 100 to 120 *C for an hour or so. Dry heat of course!!

A potentially free source of silica-gel is from a local refrigeration/air conditioner service company. Usually they have a pile of old air conditioners in the back yard. Once a defunct unit has been evacuated of its refrigerant, the liquid line dryer can be removed from the unit. Each dryer contains a few ounces of silica-gel.

Bear in mind that any adsorbent removed from a refrigeration unit might have some refrigeration oil associated with it that will have to be solvent stripped with several successive washes in fresh solvent prior to bakeout, even so you may be unable to make them fully oil free.

The effort will be better spent in finding driers that have only been wetted by adsoprtion in other applications

Hello Electroman,

Silica gel is not even close to the best option for your application. Why do you think they're so cheap? The ridgid structure allows for only a 30% by weight ratio of absorbtion of moisture. There is a much better "dessicant" to use. I'm in Canada, but if you're interested send me a brief email and I will email you a spec sheet with a much better alternative that will eliminate any corrosion. gsmiljan@h2ocontrol.com

Hello Guest - I am interested in 'better' desiccants and I would like to know what it is - maybe activated alumina?

Try http://www.silicagel.net for distributors, and http://www.geejaychemicals.co.uk/ in the UK

we can supply Silica Gel sachets.

Specify the weight and no. of bags.

Cotton bags or paper bags will do.

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