DSLR Dry Ice Cold Box
By Christopher Vedeler 2008

Instead of using an electronic peltier cooler that requires a hefty battery and isn't very efficient, I decided to try building a box that would use dry ice instead.  The advantages of using dry ice over a peltier cooler is that it packs a serious punch of cold holding 246 BTU per pound. Compare that to about 15 BTU per hour for a 45 watt peltier cooler. No electricity is needed in the field and no additional wires are needed to the camera. Dry ice provides significant cooling potential if it can be tamed.  Dry ice is very, very cold (-110F (-78C)).  

Below is a picture of my old box with a small chunk of dry ice left inside it.  It was able to drop the temperature inside the box from +73F (23C) down to -34F (-37C) in about 8 minutes.  My peltier cooled box could never do that.  The problem is that -34F (-37C) is actually too cold and the dry ice does not last that long with such exposure.  I had to design a box that would permit the cold of the dry ice to slowly leak into the camera and maintain a cool temperature without the shock or extreme cold inherent with dry ice.

The goal for my cooler is to be able to drop the camera temperature at least 50F (42C) from ambient in less than an hour and maintain that temperature for 8 hours or longer.  Due to the nature of dry ice two separate chambers are necessary for the box to work as intended.  One chamber for the dry ice and the other for the camera.  The camera chamber needs to be able to transfer the cold around the camera as quickly and efficiently as possible to speed the cooling process and thus copper is the ideal material.  It can't however be directly connected to the dry ice chamber or it would cool too fast possibly damaging the camera and use up the reservoir of dry ice too quickly.

As you can see below I added some reflective bubble wrap to the dry ice chamber to help insulate a bit more.  What you can't see in this picture is the 2mm thick hobby foam I hot glued to the back of the copper box to slow the heat transfer from the dry ice chamber to the camera chamber.  

Here is the box assembled.  Notice the two chambers.

Here is my box loaded with dry ice for it's virgin feasibility test.  I didn't put my camera in the box because I didn't know how much of a thermal shock this design would produce and I didn't want to risk my camera.

Here are the results of the first 3 hours of my first test.  It started at 72F (22C) with about a pound and a half of dry ice.  In 30 minutes it was at 1.2F (-17C)!  It reached equilibrium within an hour and held a pretty steady -10F (-23C) for the next 2 hours.  I went to sleep and when I checked it in the morning it was still at -1.1F (-18C) after a total time of 8 hours.  

This is what the ice looked like after 8 hours.  About 1/3rd of the dry ice was still there!  This was very encouraging!  On this picture you can see the 2mm hobby foam I used to partially insulate between the two chambers.

After this first feasibility test I'm now confident that my camera won't be damaged by the cold.  Time to install the camera.  Here are some shots with the camera installed in the box.

Here you can see the circulation fan.  This should help speed the cooling of the camera by circulating the air within the box.

Keep checking back as I hope to run another test with the camera in the next few days.  How cold will the camera get?  How long can it stay that cold?  Inquiring minds want to know.

Update 12/4/2008

I was only able to do a test for an hour and fifteen minutes due to work and family commitments but the results with the camera installed look encouraging.  The results were about as I would predict. Having to cool the mass of the camera and the extra cold loss through the camera adapter opening likely accounts for most of the difference between with and without the camera in the box.

I still achieved quite acceptable results. In reality a delta of 80F is probably too much and would introduce significant frosting / dewing issues even here in Arizona. 50F was my goal and I'm really close with this box.

What I wasn't able to test was the duration the box remained cold and the true equilibrium point. That will have to wait for another test and another supply of dry ice. I have discovered that conventional means of storing the ice (an ordinary beverage cooler) is totally inadequate. I'm going to build a super insulated box with 2" thick foam in multiple layers where needed. I would like to be able to buy dry ice and store it for a week or so if possible.

Here is a chart showing the performance of my new box with and without the camera.

Update 12/10/2008

I was able to run a more complete test.  I decided to install the temperature probe inside the battery compartment of the camera to more closely approximate the actual temperature of the CMOS chip of the camera.  After an initial test this way I was not happy with the speed of the cooling and so removed 1/3rd of the 2mm hobby foam insulation exposing 1/3rd of the back of the copper box to the dry ice directly.  This seemed to do the trick.  I also got a diet scale and measured the weight of the box, the box with the camera and the box with the camera filled with dry ice. 

The weight of the box alone is: 627 g (1 lb 6 oz)
The weight of the box with the camera (Canon 350xt) is: 1165 g (2 lb 9 oz)
The weight of the box with camera and a good chunck of dry ice (70% full) is: 1700 g (3lb 11 oz)

For this test I measured not only the temperature inside the camera, but the weight of the box as the dry ice sublimated.   The chart below shows the drop in temperature with the red line over time and the loss of dry ice in grams with the blue line over the same time.

The ambient temperature was 71F and the camera stabilized at about 17F after 2 hours (a true delta of about 53F).  This test is actually skewed slightly due to the camera being at 66F (point zero on the chart above) when the test was started due to it being cool still from an earlier aborted test.  So the chart above is quite conservative.

The nice thing about measuring the loss of the dry ice over time, it makes it possible to estimate how long the ice would last.  Of course as the dry ice disappears there is less mass to cool and so one would expect the temperature to slowly rise over time as the ice sublimated.  I don't see this as a major problem as from my tests I'm confident it will last long enough and be stable enough for most of a nights worth of imaging. 

Given the results of this test the dry ice should last about 11 hours.  Being conservative I suspect it would be useful for 5 - 8 hours.  After that it would be necessary to refill the reservoir of dry ice for maximum effectiveness.  This would mean that at worst it would be necessary to refill the reservoir twice in a night.  

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