Substrate Cooling

[Takka]

Now that summer's upon many of us, I thought I'd share my solution for keeping critters cool.

I use an aquarium chiller to cool water that's then pumped through a therapy mat beneath the enclosure. It's a bit of a DIY project, but it's worked very well for me. This is my Google Doc breaking down the parts and performance.

With this I keep my Banana Slug's substrate at 55F (13C) year-round, despite the ambient ranging from 70-80F (21-27C).

 

[viper69]

How much does those chillers cost?

I use central AC, not everyone has AC.

 

[Takka]

How much does those chillers cost?

I use central AC, not everyone has AC.

The whole setup is ~$500, and you can see it all priced out on the spreadsheet linked in OP. The aquarium chiller itself is ~$300.

 

[Wolfram1]

@l4nsky

nice setup, soil is a little low though

not sure if it matters for slugs

 

[Takka]

@l4nsky

nice setup, soil is a little low though

not sure if it matters for slugs

Thanks!

Yeah this species doesn't dig, and the shallow depth helps with the thermal conduction from the pad. Should work well up to about 3", but it saves energy if you don't need it that deep

 

[jennywallace]

Very good idea, looks cool (pun intended ).

 

[Ratmosphere]

Very cool contraption you got there!

 

[l4nsky]

I can't help but wondering if the cooling would be more efficient if the therapy mat was placed into some kind of hood and placed on top of the enclosure. Cool air is more dense than warm air, so the cool air would fall into the enclosure below, cooling the air and substrate as it falls.

Brilliant setup regardless.

 

[Takka]

I can't help but wondering if the cooling would be more efficient if the therapy mat was placed into some kind of hood and placed on top of the enclosure. Cool air is more dense than warm air, so the cool air would fall into the enclosure below, cooling the air and substrate as it falls.

Brilliant setup regardless.

Now that is a can of worms, because this is going to depend on a lot of factors and what kind of environment (gradient and humidity) you want to achieve.

The reasons I chose to cool from below:

• I only cared about substrate temperature, since mollusks spend most of their time on the ground.
• To avoid using air as a transfer medium, which is the case with A/C or such a cooling hood. The problem air as a medium is that at sub-dewpoint temperatures it will dry the air out, and if you want to cool the substrate with such a hood, you're going to need to go well below the dewpoint for any significant effect.
• I keep my substrate a little damp, and because water conducts heat much more efficiently than air, cooling from beneath easily beats out the sinking effect from thermal contraction.
• A thermal gradient where the substrate is cooler than the air is representative of the biomes that these slugs are native to.

Cooling from the top is going to rely on air as a transfer medium, and so depending on the water content of the substrate, it may be much less efficient at cooling a substrate. It will also dramatically drop the humidity, which depending on the inhabitant could be a good thing. It's also an uncommon thermal gradient in the wild.

 

[l4nsky]

Cooling past the dew point doesn't drop the RH. As the space between air molecules (N2, O2, CO2) shrinks due to the loss of heat energy, the RH actually goes up as less water vapor can fit into the space, so the water vapor that is present becomes a larger and larger percentage of the available space. This continues until the RH maxes at 100% (which is the dew point). Cooling under the dew point forces moisture out of the air as the RH can't go over 100%. That water vapor accumulates at a nucleation point in the environment, which creates dew; however, the RH is STILL 100%. When the temperatures start to go back up, that dew will re-evaporate as heat energy becomes available for the process and the inter-molecular spaces in air expand in size. There may be a sub 5% loss of RH depending on the temperature cycle time, but if held constantly cool, there is no loss. You may be thinking of the adiabatic dew point and its relation to rain shadows as compared to the constant pressure dew pointin meteorology.

How much time do your specimens spend on the screen?

 

[Takka]

Cooling past the dew point doesn't drop the RH. As the space between air molecules (N2, O2, CO2) shrinks due to the loss of heat energy, the RH actually goes up as less water vapor can fit into the space, so the water vapor that is present becomes a larger and larger percentage of the available space. This continues until the RH maxes at 100% (which is the dew point). Cooling under the dew point forces moisture out of the air as the RH can't go over 100%. That water vapor accumulates at a nucleation point in the environment, which creates dew; however, the RH is STILL 100%. When the temperatures start to go back up, that dew will re-evaporate as heat energy becomes available for the process and the inter-molecular spaces in air expand in size.

Exactly correct until accounting for a thermal gradient. Think of how dehumidifiers work - there's a cool surface that drops below the dewpoint so that the moisture can be collected, and that moisture is then stored unsealed at ambient temperature. While yes it evaporates back into the air, it doesn't evaporate anywhere nearly quickly enough to keep up with the rate of water production ...unless perhaps you spread it across a very outsized surface area.

With such a proposed enclosure cooler, you're going to end up with significantly lower RH and a soggy substrate as the dew drips down.

How much time do your specimens spend on the screen?

About 25% of their time.

 

[l4nsky]

there's a cool surface that drops below the dewpoint so that the moisture can be collected

In your setup with the cool surface being the substrate, by your logic the thermal gradient between the cold substrate chilled below the dewpoint temp and the warmer air above should be actively pulling moisture out of the air via condensation and draining it away via drips. At this point, why isn't the substrate just mud?

Edit: Can of worms indeed lol. I really am interested in cooling enclosures in the future for more temperate species, just trying to better understand the reasoning here.

 

[Takka]

In your setup with the cool surface being the substrate, by your logic the thermal gradient between the cold substrate chilled below the dewpoint temp and the warmer air above should be actively pulling moisture out of the air via condensation and draining it away via drips. At this point, why isn't the substrate just mud?

Well let's think about how cold the cooling surface needs to be for these two scenarios: top vs bottom, and assuming the same target temperature of 55F for the substrate.

I keep my substrate damp because that's what the slugs like, and the benefit of that is better thermal conductivity. The thermal conductivity of water is about 25x higher than air, and to conduct to the surface of my substrate (where I care about the temp) I only have an inch of damp soil to deal with. Granted damp soil isn't going to conduct quite as well as pure water, but in reality I'm achieving a 7F delta between coolant and the top of the substrate, which is pretty good. And that's just talking efficiency because with a target of 55F I'm not even hitting the dewpoint within the tank. Despite running this system for more than a year, the soil hasn't gotten noticeably damper with time.

Now cooling from the top of the tank has to rely on air as a medium as opposed to damp soil. Even with damp soil not being as conductive as pure water, let's be super generous to this scenario for the sake of argument and say it's only 5x more conductive than air. That's still much less efficient from the start, and then you have to conduct through a much bigger medium of probably ~12" of air as opposed to 3" of damp soil. This means your delta is going to need to be extreme to cool the soil to the same 55F - convection really isn't a significant factor at that point... The delta would probably be greater than 20F, which will put the cooling surface way below dew point in all but the most extreme environments. We use double-pane windows in our homes because air is such a good insulator, for better or worse.

Edit: Can of worms indeed lol. I really am interested in cooling enclosures in the future for more temperate species, just trying to better understand the reasoning here.

Happy to talk about it! I went through a ton of iteration on this project and a number of my assumptions were really off - it was a lot of fun.

 

[l4nsky]

assuming the same target temperature of 55F for the substrate.

I think this is where we have a bit of a disconnect as I'm not thinking of the substrate at the bottom of the enclosure being 55°F. I'm thinking more of a modified Retes stack on one half of the cage under the hood with the highest level being the absolute coolest as it's closest to the element.

The falling cool air would also help circulate the internal air volume as it will displace the warmer enclosed air up and out or up to the hood for cooling to repeat the cycle. I asked about the time spent currently on the lid because unless there is some side ventilation I'm not seeing, my concern is since there is bottom cooling, there's no air movement and the enclosure tends towards stagnation. Bottom heating works because hot air is less dense and rises, but bottom cooling would have the opposite effect. One of the warning signs is an animal frequenting the gas exchange barrier (ie fish gasping at the top of an aquarium) and avoiding the lower levels where heavier gases tend to pool.

 

[Takka]

I think this is where we have a bit of a disconnect as I'm not thinking of the substrate at the bottom of the enclosure being 55°F. I'm thinking more of a modified Retes stack on one half of the cage under the hood with the highest level being the absolute coolest as it's closest to the element.

The falling cool air would also help circulate the internal air volume as it will displace the warmer enclosed air up and out or up to the hood for cooling to repeat the cycle.

Ah okay, so different goals. You're definitely going to cool way more air with the cooling surface at the top, but I still know it's going to kill the RH because the evaporation won't keep up. I think an A/C duct would be a cheaper and easier way to go about it - you'll still have humidity problems, but at least no soupy substrate.

I asked about the time spent currently on the lid because unless there is some side ventilation I'm not seeing, my concern is since there is bottom cooling, there's no air movement and the enclosure tends towards stagnation. One of the warning signs is an animal frequenting the gas exchange barrier (ie fish gasping at the top of an aquarium) and avoiding the lower levels where heavier gases tend to pool.

A typical day for a wild Banana Slug, at least where I live, is sleeping on the ground for part of a day, then eating, and then climbing about 10-15ft up a tree to sleep again. My captive slugs follow the same pattern and so it seems fine for them, but that's interesting to think about for other critters.

Bottom heating works because hot air is less dense and rises, but bottom cooling would have the opposite effect.

The role of convection in thermal systems is often really overstated - a decent thermal conductor will move much more heat than convection. Taking advantage of convection for better performance is awesome, but my personal goal isn't to cool the air because the soupy substrate and low RH was just too big of a downside. With that very specific goal, cooling from the bottom has the best deltas and by extension efficiency.

 

[The Snark]

@Takka I have yet to analyze the system in our refrigerator but it seems to work on similar lines. No significant condensation in the refrigerator. The compressor and evaporator un underneath the freezer which is at the bottom. Air from the freezer area is fan forced into the upper refrigerator chamber. Foods in the fridge tend to dry out.
BTW, nifty invention. F minus for that electric plug fustercluck.

 

[Takka]

F minus for that electric plug fustercluck.

Haha well I have a smart plug in there for graphing the current draw for specific components, and then a splitter to combine some of those for the same metrics. It doesn't look like that because I ran out of sockets lol I promise.

 

[The Snark]

Haha well I have a smart plug in there for graphing the current draw for specific components, and then a splitter to combine some of those for the same metrics. It doesn't look like that because I ran out of sockets lol I promise.

Nothing worse than living with a rats nest of wires you're afraid to touch or even clean near. You're organized well enough. Do like I did with my present 5 computers. One dedicated grounded source, bolted to a service table. From it, two sets of plug strips preferably mechanically as well as electrically integrated, one with a current meter. I hot glued mine to the service table. If quality plug strips aren't available it's worth the time and effort to build your own. Use NEMA service boxes and hot glue them together if nothing else is at hand. Ultimately I've got 22 receptacles in two clean banks of 14 and 8. Way overkill at first but now, room to spare and able to connect or disconnect anything without concern of an OOPs - reboot and hope... Your friends will think you're nuts at first.
My motivation was a friend who built a horrendous CNC controller system for his mill, lathe and 3D printer. Ultimately covering a 2 foot by 3 foot plywood board screwed to a wall. Eight power supplies alone plus the normal electric circuits.

PS Your delta calculations bring back nervous jitters of the administration at the hospital wanting to incorporate alternative energy into the hot water and HVAC systems.

 

[Takka]

Ha well I try not to over-engineer everything. It's really not a rats nest, but I've definitely got that and the "friends thinking you're nuts" covered by many of my other projects:

 

[The Snark]

@Takka Yummy! I especially like the splices. Looks like what's inside the dashboard of my car. How many times did you revamp that little jewel? PoE was still a wet dream when I was active in the trade. To give you a clue of my antiquity, when I first got in on communications one of my jobs was an in house phone system upgrade to DTMF.