Positive pressure
From UroWiki
What is positive pressure?
Positive pressure comes when you have more air flowing into an enclosed space than you have flowing out, meaning that the pressure of excess air will cause 'leaks' out of the space, but as a bonus will help keep dust out. Negative pressure, conversely, is when more air is being sucked out of a space than is in it - so any gaps will be sucking in air like a vacuum cleaner. In a terrarium enclosure, it is preferable not to have any fans at all (neutral pressure) in order to help stabilise your temperature gradient; but if you find you need to add a fan to help with ventilation and airflow, you will want it blowing inwards at the cool end to create positive pressure, help manage dust and create a stable airflow pattern.
How should I set up my enclosure with a fan?
Preferably, you shouldn't. An enclosure that's properly setup should have adequate airflow and good natural convection. However, this isn't always the case, in which case this guide is for you! The best place to set your fan is high up in the cool end, in or near the roof of your enclosure, so there is no danger and no need for a guard. However, a dust filter on the outside is a must. You should bear the following points in mind when considering adding a fan to your enclosure setup:
- Computer fans suck at sucking
- Blowing out will simply draw air from the warm end to the cool end and mix them, destroying your thermal gradient.
- Blowing in is best
- However, aside from the point above, it's also good because it creates positive pressure inside the terrarium - this means that all the other gaps and holes will 'leak' air (since more is coming in than is going out), preventing dust from entering through these, and also forcing any existing enclosure dust out. Negative pressure will, contrarily, encourage dust to flow in from all the other gaps.
- Fresh air will affect temperatures
- Make sure you take this into account, checking temperatures and thermal gradient both with and without the fan.
Important note: a lot of the ’noise’ of fans comes through low-frequency contact vibrations, which may disturb your lizard, so make sure it is either seated firmly to your enclosure, or else use silicone pads or similar to dampen the vibrations. A voltage regulator or other fan controller will let you fine-tune the amount of airflow into your terrarium which will help with the thermal gradient and airflow management.
Step-by-step guide
Disclaimer: this guide assumes basic electronics and DIY knowledge.
| Here's my power source - a generic AC multi-adapter. You can buy these at any hardware store. Notice the important features - polarity switch, and volt change switch. You can toggle this between 3, 4.5, 6, 7.5, 9 and 12 volts.
Please do make sure you know the voltages and ampereges of your target fans (they're not always the same fan-by-fan, either) before linking up a live circuit. The rest of this assumes some amount of basic electronics knowledge. If you're unsure, do your research first or ask the help of somebody with proven skill. You can also see the other end, showing a standard 9V connector as well as five other pin and barrel type connectors. These multi-adapters are extremely handy for random devices missing their own power bricks, and I recommend everyone keep at least one lying around. |  |  |
| Here's the fan connector, from a standard PC fan. You may have two wires or three (the third is for the computer to control the spin rate, measured in RPM), and you may have the standard Molex connector like this or a smaller, three-pin connector. It's irrelevant. One is live and one is neutral; please make sure you know which is which. In this case, everything's colour coordinated - you'll notice a crocodile clip adapter for the AC inverter, which makes testing connections really easy. |  | |
| The white thing is a dust filter - essential when the fan is blowing in, so as not to suck all the dust in the room into your terrarium! Remember to clean it regularly. Also, a handy tip for those new to using computer fans: when placing your fan, there will be two arrows somewhere around the outside; one indicates the direction of spin, the other indicates direction of blow.
The fan in use here is a 120x120x38mm Antec Tricool at the top of the cool end of the terrarium. Being 38mm deep means it moves a huge volume of air while still spinning quite slowly, which means that it is really quiet - always a bonus. The white cable leads to a useful three-speed selector switch. |  |  |
| Here's the mechanical switch, a cheap thing from a generic DIY shop, and easy to find. The plastic bit is the end of the junction box.
On the back of the switch, the wires go onto pins. When the switch is ON, the pins are joined, and the current flows. When the switch is OFF, the pins are disconnected, and the current can't flow, which means no power to the target device. A little bit of soldering and some hot glue for insulation, and the job is complete. Couldn't be simpler. |  |  |
| Here's the junction box. This particular example is fairly big for the task at hand, but it was one that was just lying around - there should be a wide selection of shapes and styles in your local DIY store. With your circuit wired to the switch (the cables will run out of the back end of the junction box), close it all up and put it somewhere handy. Bingo! Instant, good-looking control box! Safer and neater than bare wires (or in this case, crocodile clips) lying everywhere. |  |  |
