After a week or so, they were making a big difference, so I moved one to the boot. This got saturated quite quickly as there is a lot of condensation in there. I brought that one inside and weighed it - it was 1600g, which I believe is the capacity (1000g dry, can absorb 600g water, according to the website). So I put it on a radiator in the house. It gets properly hot twice a day, but after 2-3 weeks it is still damp to the touch and weighs around 1300g. So I have got about half of the moisture out. Has anyone used these, and is it normal for it to take this long, or is there a quicker way? I have now moved it to the airing cupboard, which is not as hot but hopefully a drier atmosphere.
A radiator or airing cupboard will probably be inadequate.
We used silica gel packs at work for shipping sensitive components and equipment about inside air-tight plastic bags, and these packs always needed to be first put in a reasonably hot oven for a couple of hours, usually at about 100C. This was fifteen years ago, but I don’t think the chemistry has changed much.
The weight trick is a very good indicator if the crystals don’t have the colour-change dye in them.
Thanks Richard. Part of the bag material is a kind of PVC so I would be wary of putting it in an oven, but I will see if I can find a way to get some proper heat into it for a while. You can’t see the crystals, so weight is the only way to check progress, but it’s pretty foolproof as a measure, I would have thought.
"I said I would report back on how we got on with the Halfords Air Dry Car Dehumidifier.
It has worked really well in the Mazda with front and rear screens both kept clear despite the cold, wet and damp weather. The instructions say it will be effective for 2 - 4 months but after a few weeks the weight had increased to about 1.5Kg (max is 1.6Kg according to the instructions) i.e. it had absorbed 0.5Kg of water. This is hardly surprising given we have a slight leak around the hood. Leaving it on the radiator to regenerate was a waste of time and in the end I stuck it in the oven a few times on a low heat for a few hours each time. Jayne kept wanting to cook food which meant I couldn’t do it all in one go. In the end I got it down to just under 1.1Kg at which point I got bored and put it back in the car.
It has been slightly less successful in the Mercedes E-Class Estate although it has definitely made an improvement. Two reasons I think. Firstly it’s a much bigger space than the Mazda and secondly I use a windscreen cover on frosty nights which, still wet/damp, gets left in the boot when not in use so I’m probably not helping myself.
I bought something like these a few weeks back in Costco made by a company called ThoMar. The packaging warns they can take a few weeks to dry out. Personally I plan to dry mine in an oven on a low heat if they look like taking forever. The PVC should be perfectly ok at 100 degrees c.
I have been shaking mine about too, trying to redistribute the crystals to speed up the drying. A couple of days in the airing cupboard hasn’t made much difference. I’ll try the oven on a low heat when I get chance. I need to choose my time carefully though. She still hasn’t forgiven me for the time she found the steering yoke from the Triumph Sprint in the freezer when I was changing the head bearings. Thanks to all for the input.
I bought similar ones off ebay but the bags containing the crystals have Velcro fasteners and they say to regenerate them spread the crystals on a baking sheet and dry in the oven, I tried putting them on the radiator in the bags but it didn’t seem to do much. It seems they need some proper drying out !
More expensive at £16.49 but they dry out over night. I think I may have spoiled one of them by leaving it plugged in too long. The crystals have gone clear so I think they’re working but I can’t tell because they don’t go pink anymore. (Did a quick Google - did heat them too much but they still work! Phew!)
I’ve also got a couple of the radiator ones but they don’t get used as much because they’re not as easy to use as the plugin ones.
From the Googling:
Reactivating/ drying of silica gel
Silica gel products can be dried and reused "as new".Orange silica gel: Dry pellets are orange and turn dark green when near saturation.Blue indicating silica gel: Dry it's dark blue and with increased water uptake it becomes more pale and pink. The most efficient method to reactivate silica gel is with heat. When heat is applied to saturated silica gel, the beads will restore to the previous moisture absorbing capability and return to the original orange colour. Unlimited regeneration cycles are possible.
Silica gel has a very high melting temperature, 1600°C. However, it will lose its chemically bound water and hygroscopic properties if heated above 300°C. In addition, there is a new class of indicator gels, incorporating organic dyes that are heat sensitive and their colour indicating dye will be affected above 125-150°C . Therefore, it is not recommended that orange indicating silica gel is heated above 120°C. Also, if heated above 120°C through several reactivation cycles, the material may disintegrate into powder. This in turn, reduces the adsorption capacity of the material and may eventually lead to the loss of indicating colour within the crystals. The minimum heat necessary should be used when removing moisture from silica gel; ProSciTech recommends regeneration at between 105 °C and 120 °C. This will prevent the silica gel from deteriorating and it may be re-generated numerous times.
Blue indicating silica gel has a higher tolerance and may be dried by heating it to 150°C. Lower heat for regeneration requires longer drying times, but the degradation of silica gel is reduced.
Increasing heat vapourises adsorbed moisture and above 105°C is removes all water molecule from silica gel. A porous desiccant like silica gel, removes water from the surrounding air by two mechanisms: multi-layer adsorption and capillary condensation. Multi-layer adsorption is the attraction of thin layers of water molecules to the surface of the desiccant. Since the desiccant is very porous, the surface area is high and significant amounts of water can be attracted and absorbed. Capillary condensation is when the smaller pores become filled with water. Capillary condensation occurs when saturated water vapour pressure in a small pore is reduced by the effect of surface tension.
In a conventional oven, the time of regeneration varies from minutes to hours, depending on temperature and the thickness of the gel within a dish. Although silica gel can be dried in a microwave oven, it is difficult to determine the temperature inside the gel. Particularly the orange gel should not be dried using microwaves since excessive heating denatures the indicator dye. As metal cannot be used in a microwave oven, only glass, ceramic or microwave safe plastic with a high melting temperature should be used to hold the gel.
Place 'used' silica gel beads into a large tray
Use an oven at 105 - 120°C for two hours, or continue heating for 30 minutes after the gel turned orange.
Check the silica gels beads periodically. Caution:the gel beads are very hot to touch.
I bought, from the range, the type that collects the water in the container below the cristals. With this type you can empty the water and feplace the cristals wich are available in refil bags. I had one in the boot and one inside of my Mk1 last winter. They work realy well. But be carefull, if you spill any of the collected water on the seats, as I have just done, it does not seam to dry out. I’ve tried leaving the car open in the sun, Kitchen towel on the seat weighted down with magazines and even washing the area with upholstery cleaner. Five weeks on and it still feals damp but does not actualy make my hand wet.
Just after Sunday lunch is done slide it in the oven [ not swiched on!] by tea time its dry yes it was wieghed before and after .7k reduction] simples !
Foolhandy, thanks for the excellent information. I did a bit of an experiment today. I put two of the bags on a baking tray (with the permission of her indoors) and put them in a fan oven at 75deg C for two hours. One weighed 1361g and lost 35g of water, the other weighed 1459g and lost 51g of water. I make that a loss of roughly 10% of the excess moisture. That’s roughly the amount lost in ~2 weeks on a radiator. There was certainly a huge fug of steam when I opened the oven.
Next step will be to leave them overnight on a low heat. Ten hours should make a big difference. 75deg didn’t do them any harm, but I’m not keen to go much higher than that. The PVC got very hot and soft.
I have tried various things to get the bags dried out. I have kept a record of the weights of the bags after each ‘treatment’ and it seems that high heat is the answer. I’ve done this with two bags, but the numbers are the average of the two, to keep things simple.
Initial weight around 1500g (which is in theory 5/6 of the bags’ capacity of 600g). Left on various radiators, in the sunshine etc until weight was 1410g. Then:
2 hours in the oven at 75 deg C took away 10% of the moisture by weight.
2 hours in the sunshine lost another 2.5%.
12 hours in the oven at 50 deg lost 14%.
One week in a dry room actually put moisture back in! Only a few grams.
8 hours in the oven at 75 deg lost a further 18%
12 hours with the temp up to 80 lost another 20%
They have now lost around 65% of their excess moisture and feel completely dry, so I am putting them back into the car. Looks like 1140 grams is about as good as it is going to get. The most effective thing seems to be overnight in the oven at 80 deg or so. I am not very keen on putting them into a much higher temp than that, to be honest. Four or five sessions like that should have them dried out, although the energy involved seems quite a lot. It might be cheaper to run a small mains dehumidifier in the car when it is on the driveway.
Anyway, that’s what I have found out so far. Roll on summer.
I’ve been experimenting with mine now. I didn’t think it was doing a lot, its been in the mx5 in the garage just to keep the cabin dry over the winter, I don’t usually have a problem with it so was an extra precaution this year. But when I weighed it, it appeared to be at is max absorbed weight.
It does say on the box you can regenerate them down to about 1.1kg I think it is, so looks like you have got as far as you can. Mine is slowly going down just sitting on a radiator, but its very slow, and the rad is only really on for a few hours a day I assume it spends the rest of the time possibly absorbing water back from the room! The window its under was lovely and clear of condensation the other very cold morning when others weren’t!
I did wonder if it would be practical to split the bag open and dry the granules out spread on a tray? I kind of assumed it was perhaps a charcoal of some sort in the bags, but not sure now, maybe just silica gel like they put in the bags that go in with some packaging.
Drying in the oven does seem potentially expensive, maybe cheaper just to get another. Presumably if we get a good summer it will be possibly to dry them at no cost ready for the winter with a bit of planning.
Anyway, I think they work, I’m going to get another one. Much better than the ones that have the crystals that dissolve, which are messy in my hands and I get the impression the water it leaves is very corrosive!
