Drying camera with desiccant better in the fridge?

[symmetal]

Still having icing trouble with my ASI071 after 4 days in airing cupboard in sealed container with loads of desiccant. I'm thinking would it be better putting the sealed container in the fridge at around 4C as that would make the air release more moisture, to be absorbed by the desiccant. 30% humidity at 4C contains less water than 30% humidity at 25C. When cooled to -15C there should then be less moisture to freeze on the sensor. Does that sound reasonable?

Alan

[JamesF]

Might the moisture not just condense out on the sensor/window as it cools and not actually be taken up by the desiccant?

(No idea, btw.  It's an interesting question.  I hope someone has a correct answer.)

You're sure the ice is forming inside the camera, rather than moisture from the air inside the OTA freezing on the outside of the sensor window?

James

[symmetal]

1 hour ago, JamesF said:

Might the moisture not just condense out on the sensor/window as it cools and not actually be taken up by the desiccant?

(No idea, btw.  It's an interesting question.  I hope someone has a correct answer.)

You're sure the ice is forming inside the camera, rather than moisture from the air inside the OTA freezing on the outside of the sensor window?

James

The ice is in focus so is on the sensor. The camera has a dew heater built in to warm the sensor window. It doesn't start forming (being visible) on the sensor until -2C or so I hoped being in the fridge at 4C the desiccant would still absorb it while it's in the air and before it starts condensing on the sensor.

Shining a torch in the camera it's very difficult to see any ice or moisture present. It's only really visible when taking flats, or having stacked and stretched the images. Small halos around bright stars also indicate there's a problem.

The desiccant works down to -20C or so when used in the freeze drying process so being in the fridge shouldn't inhibit it working.

This previous thread shows the images. I started a new thread as the question was a bit different and more people might look at it. 

Alan

Edited October 29, 2019 by symmetal

[almcl]

7 hours ago, symmetal said:

after 4 days in airing cupboard in sealed container with loads of desiccant. 

How did you condition the desiccant before putting it in the sealed container?

I ask because with enough desiccant that has already absorbed moisture, in a sealed container it will maintain the %RH at which it is conditioned rather than drying the air.  

In the museum I worked in we would sprinkle water on the desiccant packs in sealed textile display cases to maintain 60% RH.  This may be what yours is doing if it  hasn't been conditioned recently.  It may be worth drying it out a bit? 

[Dr_Ju_ju]

A quick way to re-condition the desiccant tablets is to zap them in a microwave oven for bursts of 2\3 minutes.....

[symmetal]

8 hours ago, almcl said:

How did you condition the desiccant before putting it in the sealed container?

I ask because with enough desiccant that has already absorbed moisture, in a sealed container it will maintain the %RH at which it is conditioned rather than drying the air.  

In the museum I worked in we would sprinkle water on the desiccant packs in sealed textile display cases to maintain 60% RH.  This may be what yours is doing if it  hasn't been conditioned recently.  It may be worth drying it out a bit? 

They are left just above a tube heater in the airing cupboard and after a week the FLO sachets are a golden brown and the Pingi 250g bag is blue. I put three temp/humidity meters there (two are mechanical hair hygrometers and one electronic) and they all read 37C and between 21 and 26% relative humidity.

The sealed box was then kept higher up in the cupboard at 27C. Little change in camera icing.

For the last day I've kept the box in a cooler room of the house at 19C. I put the hygrometer that read 24% at 37C in the box and after an hour it reads  27% at 19C. This seems to confirm pretty much your stating that the desiccant just maintains its conditioned relative humidity. This is good as it means the box at 19C has around a 1/4 of the total free air moisture than if the box was at 37C. A rule of thumb is the RH rises by a factor of 2 for every 10C drop in temperature assuming conservation of the absolute moisture.

 

At 27C and 25% RH (where the camera was stored for 4 days) the dew point is 6C so probably explains why the camera experienced icing .

At 19C and 27%RH (where the camera currently is stored) the dew point is -1C. This is better and the ice may not form so quickly.

If I stored it in the fridge at 4C and the RH stayed at say 30% the dew point drops to -12C. Woohoo!

I'll try the camera in the fridge and see if the RH stays low. My current rise of only 3% RH for a temp drop of 18C suggests it will. 

I'll do a test with a Pingi bag in the microwave as Julian suggests then seal it with a hygrometer and see what the RH is compared to my airing cupboard heater method. 

I'll also test if over time a dry desiccant bag will partially dry out a 'wet' bag if they are sealed in together and whether they both then reach the same state or will the 'wet' one always stay wet until it's forcibly dried. This would save me having to open the camera again to remove the tablets to dry them.

Alan

[almcl]

22 minutes ago, symmetal said:

I'll do a test with a Pingi bag in the microwave as Julian suggests then seal it with a hygrometer and see what the RH is compared to my airing cupboard heater method. 

I have quite good results with desiccant spark plugs (rather different application, I know) in the microwave: 2 minutes on full power turned the content from pretty pink to deep blue...

[Star101]

Looking on with interest as one of my cameras could do with a little moisture reduction. Not a problem at the moment but could move that way in the next few months.

[symmetal]

Camera been in fridge for about 5 hours. Temp 3.6C and RH is hardly changed at 31%. as I'd hoped. A drop of 24C and only an increase of 7% RH. Without the desiccant in the sealed container the RH would be 100% as the dew point would have been reached. I'll leave it another day in the fridge so the air in the camera can reach the same RH and then put the stopper seal in the side of the camera quickly while it's still in the fridge. The dew point in the camera should be -12C so fingers crossed no ice.  When warmed to room temp the RH in the camera should drop to around 8%. Assuming no air leaks.

The spanner in the works may be the tablets already inside the camera and whether they will release any held moisture back into the camera. I suppose I should have removed them and given them a blast in the oven or microwave first but wasn't sure if the fridge trick would work. I'll report back. 

Alan

Edited October 30, 2019 by symmetal

[symmetal]

Tried again. Ice started forming at -4C.  Opened up the camera and the internal tablets were pinkish so they were setting the internal humidity and not the outside, so putting everything in the fridge was a waste of time. The camera sensor PCB is sealed from the outside air by silicone like sealant between the black vanes and the red front panel. The ribbon cable from the sensor pcb to the rear PCB passes through a slot in the red front panel which has black sealant around the ribbon cable to hopefully make it airtight. The front window panel is screwed onto the sensor panel with six screws and a large O-ring to form a seal. I suspect one of the seals is leaking so the outside air has access to the sensor chamber as the new tablets went pink so quickly.

I've put all the tablets in the oven at 130C for a couple of hours and the opened camera with the sensor exposed in a sealed container with a fresh unopened bag of desiccant. Reading 24% RH at 19C in the container. The same as my airing cupboard drier so it looks like the airing cupboard does dry them well although it takes at least a week to do so.

I'll put the cooled roasted tablets in the camera, seal it up and and try again. If I get icing again after a day or so then there is a leak somewhere. In that case hopefully it can go back for an exchange or repair.

Alan

[JamesF]

That's all rather frustrating.  Sounds like you're getting to the root of the problem though.

James

[spillage]

My 1600 has icing issues last year around November time and due to renovations we had no heating system so I was really struggling to get the thing dried out. I tried everything I could but in the end it had to go back to zwo. Came back a few week later working 100% and has been fine ever since. Not really sure how they deal with moisture at the factory and they did not really give me any suggestions on how to dry the camera out, just advise on drying out the desiccants.

 

Edited October 31, 2019 by spillage

[symmetal]

2 hours ago, spillage said:

My 1600 has icing issues last year around November time and due to renovations we had no heating system so I was really struggling to get the thing dried out. I tried everything I could but in the end it had to go back to zwo. Came back a few week later working 100% and has been fine ever since. Not really sure how they deal with moisture at the factory and they did not really give me any suggestions on how to dry the camera out, just advise on drying out the desiccants.

 

I think they disassemble the camera and re-apply the sealant around the sensor chamber reading some replies on the ZWO forum. Luckily my 1600 has never had icing problems in over two years even cooled to -30C and it's spent weeks on end in high humidity on the pier outside. If it's 100% airtight there shouldn't be a problem but it's a bit hit and miss as to whether they are.

If my 071 is OK after my last fiddling, I'll leave it outside for a few hours in high humidity and try again. I'm not too hopeful but may be surprised. 

Alan

[symmetal]

Cooled to -15C and no ice, but dust spots now on sensor and front glass. Spent ages yesterday trying to clean the glass but didn't do a good enough job. Will have to open it up again to clean the sensor.  Before then I'll leave the camera in the shed with humidity around 90% for a couple of days and then check whether the icing has returned. 

My original query as to whether it's advantageous to use the desiccant in the fridge is a bit more complicated. The reason the RH increased from 24% to 27% when cooling from 19C to 4C is because the desiccant did remove the moisture from the cooling air trying to maintain 24% but the water content of the desiccant obviously increased, therefore the slight increase in RH at 4C.

Removing the cooled box from the fridge and letting it warm to room temp it stayed at 27% RH as while the air RH would decrease with an increase of temperature the desiccant was releasing it's moisure back into the air as the air was getting drier than the desiccant.

To get the benefit of fridge cooling you have to remove the desiccant from the box once it's cooled and then remove the box from the fridge to let it warm back up to room temperature. The RH in the box will then decrease, halving with every 10C rise. At 20C the box RH should be around 8 to 10%

This also explains why the desiccant dried fine in the airing cupboard at 37C and 24% RH. It doesn't need high temperature just an environment with a lower RH. High temperature just dries it more quickly. So a camera with saturated tablets will dry the tablets if put in a container with dry desiccant as they try to reach an equilibrium but as the access air hole plug into the camera is small it will take many weeks I expect. Worth doing if the camera isn't going to be used for a long while. Fairly common around here. 

Alan

Edited October 31, 2019 by symmetal

[spillage]

I have wondered if using one of those vacum bags that you use with a hoover would help as I am sure this would remove most of the moisture.

[symmetal]

9 hours ago, spillage said:

I have wondered if using one of those vacum bags that you use with a hoover would help as I am sure this would remove most of the moisture.

It would reduce the total volume of 'damp' air in contact with the desiccant so the slight increase in the humidity of the desiccant as it absorbs the moisture present would be less.

I tested the camera after having been outside in high humidity for 24 hours and still no ice formed at -15C.  I was going to leave it two days but there is high winds of over 60mph forecast tonight so I brought the camera indoors in case the roll off shed started rolling.

There may be a slight air leak in the camera which could cause the desiccant tablets to eventually saturate but as long as I keep the camera in the sealed container with a big desiccant bag when not in use I should be OK. I'll remove the sealing plug from the side of the camera while it's in the container to hopefully remove any extra moisture the tablets have accumulated when outside. This will take a long time but as things are at the moment it'll be spending most of its time in the container. 

Now to remove the dust spots from the sensor.

I've ordered these to place in the containers and jam jars holding the desiccant tablets/sachets/bags to check their dryness. The absolute reading may not be so accurate but should indicate any changes. They cost very little anyway.

Alan

Edited November 1, 2019 by symmetal