A DIY Temperature Controlled Incubator

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A DIY Temperature Controlled Incubator<br>Or... just drill a hole in a mini-fridge<br>CasualPhysicsEnjoyer<br>Jul 30, 2026

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Thank you to David Jordan for the ideas and bioreactor. All mistakes mine. No products here are an endorsement, except David’s reactors :)<br>One point of an experiment is to make natural phenomena happen repeatably.<br>Another point is to test models of the world, by seeing what happens when the experimenter changes a variable. In this case, it is important to only change one thing, and keep constant everything else that could effect the experiment. This allows you to be more confident the thing you are varying is actually causing the effect that you want to measure.<br>It's not possible to control everything though. What makes things harder is that some factors have varying importance. Some things are less important to control, like the physical location of where you do the experiment. And some things are more important to control, like the humidity of your biological samples.<br>And so it seems to be the job of the scientist to decide what exactly needs to be controlled. But this is hard because a-priori there needs to be some science done to figure out what actually matters. Also, some systems have a really large sensitivity to initial conditions, and may be chaotic. This is a philosophical problem that I want to write more about.<br>But anyways. If you are dealing with biology experiments, one thing that is definitely important to control is temperature , because biochemical processes might change as temperature varies. Which means that the experiment would become invalid otherwise.<br>But figuring out a way to keep the temperature constant is hard. Especially for independent scientists that are working out of their rooms and have less access to controlled environments. This is a real engineering challenge. But given the heatwave in the UK, it’s important to solve it. Here is an attempt to do so.<br>I'm trying to do experiments with algae in the bioreactor that David Jordan me, so I needed a way to keep the temperature constant. Whilst the bioreactor could do some degree of temperature regulation, it failed above 28C because it was just too hot.<br>So last week, I got this mini fridge (Subcold Pro 15L Mini Fridge) and tried to figure out a way to use it to ‘keep the temperature of the environment’ constant, using the fridge as an insulating and cooling layer.

Making sure it fits:

The obvious solution is to stick just stick bioreactor in a fridge. But the bioreactor has wires and requires power. Soto do that I needed to drill some holes in the side. I am not a DIY guy so I asked claude to recommend me a drill, and appropriate drill bit such that I could make a 10mm clean hole safely without cracking the plastic of the fridge. I found a corded drill from Screwfix to be the cheapest, along with a conical shaped drill bit for variable hole size.

Here is me drilling:

Hole came out pretty clean!

Also, the fridge only does a ambient minus 18C cooling, which means that if its 30C then it will cool to 12C, but if its 24C it will cool to 6C. So to get it to constant temperature I used an Inkbird, which is basically an automated plug that only turns on the fridge once it gets above a set temperature point, which you can set. Useful. So I put the ink-bird temperature probe in the fridge, attached the fridge plug to the ink-bird.

This solution worked pretty well, the system fluctuates between 23C and 24.5C. This fluctuation is fine because the bioreactor has its own feedback loop. The main thing I really wanted was a heat shield so that I could do experiments in the summer even if my living room is at 30C.

Parts list

Subcold Pro 15L Mini Fridge — the enclosure (already owned). Thermoelectric; runs ~18 °C below ambient. product page

Subcold 12 V AC replacement adapter — mains power lead for the fridge (bought 2026-07-25, £14.99). Must be the 12 V version, not the 9 V Classic one. Subcold cables

Inkbird ITC-308 — dual-stage plug-in temperature controller with two switched sockets (cooling + heating); holds a fixed setpoint (algae target ~24 °C). UK-plug version. ~£30–35.

Small heater (optional, for the ITC-308 heating socket) — e.g. reptile heat mat or low-watt tube heater, so the setpoint still holds when the room is cooler than target.

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