A teaspoon of water

[AstroJunk (Jonathan)]

I had an icing over issue with my camera a couple of weeks back and ordered a heap of 50g desiccant packs off ebay to build a drying box. It worked a treat and now the camera is, for the time being at least, bone dry.

I happened to notice that the desiccant packs individually fit into 2" eyepiece holders and placed one in my SCT and a into a couple of refractors too. Thought I'd weigh one today and can announce that my SCT is 5.4g lighter in moisture as a result, which equates to just over a teaspoon of water. Got to be a good thing

The instructions that come with the desiccant suggest baking the packet at 100 degrees for a couple of hours to restore them to their starting weight of 50g. I can see them getting plenty of use from now on!

[JTomba (Julian)]

Thanks for the tip!

[Drac0 (Mark)]

Will check these out, cheers.

[AG Hybrid (Adrian)]

Wait - was all that moisture inside the OTA of your SCT? What was your results with your refractors?

[JA]

Originally Posted by AG Hybrid

Wait - was all that moisture inside the OTA of your SCT? What was your results with your refractors?

That's a good question as that much moisture (5.4 g) seems like far too much to be able to extract from the air inside even larger 11 or 14inch SCTs, even in reasonably humid and warmish temperatures. It's Samford, Queensland as Jonathan's location, so there's no doubting a warm and humid climate is possible. Who knows, it could even be water accumulated from a leak.

Perhaps Jonathan could comment....

It would be good to know if the dessicant bag was measured as 50g before installation or whether it was just taken at its nominal specified mass, straight out of its packaging as new.

Whatever it was and given the icing up, it's certainly better out than in

Best
JA

[AstralTraveller (David)]

Hi Jonathan,

Interesting stuff. If I ever get that obs built, moisture control is something I need to consider.

Just ran some 'back of the spreadsheet' calculations (attached as zip because xls is an invalid file type to upload). I guessed the minimum internal dimensions of a C-14 and took the formula for the max carrying capacity of air at temp 0-40C from here. My calcs show that the scope could hold, as vapour, less than half of the water you recovered. I can hazard three guesses as to why.

1) My calcs are wrong. Please, could someone competent check these?
2) The OTA isn't air tight and so you are attempting to dry the whole room.
3) There was some liquid water in the OTA which has since evaporated and been absorbed by the gel.

Whatever the reason it's worth investigating further. I'd be interested to know the max capacity of the gel. (BTW you did weigh the dry gel pack didn't you - not just assume it was exactly 50g?)

cheers,
David



r (m) h (m) vol (m^3) Temp (Cel) sat vap density (g/m^3) Max content (g)
0.18 0.76 0.0774 15 12.76 0.987
0.18 0.76 0.0774 20 17.25 1.334
0.18 0.76 0.0774 25 23.08 1.786
0.18 0.76 0.0774 30 30.50 2.359
0.18 0.76 0.0774 35 39.72 3.073
0.18 0.76 0.0774 40 51.00 3.945

[AstroJunk (Jonathan)]

Hi all,

Yes the packs weigh in at exactly 50g when the pack was first opened. I have to say they are very thirsty creatures and will grab at anything!

Current weights:
SCT 56.9 - It is a closed system running hyperstar, so once it's fully dry it should stop that way. I think david is on the money with micro droplets of water inside the tube. I flocked the inside of the SCT with black felt which no-doubt acts as a sponge too.

The 100m refractor was 59.2 but I did leave the bag out during the day a couple of days back whilst solar observing and it isn't entirely air tight around the focuser.

The manufacturer recommends drying the packs at about 30%. A pack I left on the kitchen top as an experiment has reached 65g after a week - I'll leave it longer, but it does appear to have slowed down in its determination to rid the world of water