Need help with formula for calculating the amount of ventilation required.

magneto

Arachnoknight
Joined
Sep 27, 2011
Messages
196
I'm building a terrarium for a bearded dragon and have been recommended to use this formula to calculate the amount of ventilation to put in.

My problem is that I do not understand it. I have a huge problem with math and I have tried very hard to to understand this, but it just doesn't click. The numbers just fly around in my head, only pausing long enough to mock me. I use a calculator but either there are steps missing in this formula, or more likely I'm stupid, or my calculator is malfunctioning as I end up in the millions every time.

Are there any kind people here that would be willing to help me? My terrarium is 200 x 70 x 80 centimeters or 79 x 27.5 x 31.5 inches. And it is for a dry climate lizard so the calculations should be for the 4%

The formula is as follows: For rain forest terrariums, the vent should be approximately 2% of the wall surface, for drier environments, about 4% of the wall surface. Thus, adding the surface of all four walls, (do not count on top and bottom), and then multiply by 0.02, alternatively 0.04. The result is divided equally between the openings you are planning to make.

Example: A terrarium that is 50 x 50 x 100 centimeters or 20 x 20 x 40 inches for total wall area of 15,000 square centimeters or 2325 square inches. Multiplied by 0.04 the total vent area is 60 square centimeters or 23.5 square inches.

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For anybody who might be interested, here is the article it comes from. Pardon any typos or strange wording as I Google translated it from Swedish. I cleaned up the worst parts put i'm sure i missed a bunch:

Regarding the ventilation in terrariums, I have a simple calculation based on the % of the wall surface. Sometimes difficult to apply when building in glass, but when you use the discs are easy to modulate as you want it, even so you can vary the ventilation.

The first time I did this calculation for roughly 30 years ago, I had a room that was good enough humidity in itself, about 50% relative humidity, and therefore I have revised the model once or twice. Unforeseen external factors do anyway because I always now recommend that you plan for the regulation of the surface by a pusher / pull door.

I did the calculation model from the very beginning as a supplement in an article about the placement of the vent openings, which was often, and in many cases still are, completely insane placed so as to create features instead of ventilation.

Either way, the model is as follows; rainforest terrarium for approximately 2% of the wall surface, the drier environment about 4% of the wall surface. Thus, adding the surface of the four walls, do not count on top and bottom and then multiply by 0.02, alternatively 0.04. The result is divided equally between the openings that must be addressed.

Example; terrarium 50 x 50 x 100 cm for total wall area of 15,000 square centimeters. 0.04 multiplied by the total vent area of 60 square centimeters. Divided in two openings will be two holes 10 x 30 cm, divided in three becomes three holes 10 x 20 cm.

I recommend that you take up the holes in the sides at different heights, but never less than half the height of the terrarium as low openings can create drafts. Of the vent openings near the bottom it will definitely be a draft. Very long, narrow cages can have a breakdown in the three openings instead of the normal 2. This takes up the third opening above the slide glasses. Terrariums built on the hill built on the same principle - no opening less than half the height ( I understand that no one is building a 250 cm high terrarium with 40 x 50 cm bottom).

Roof ventilation are always a bad option, although it can sometimes be the only possible, such as when to build simple glass. Heat rises and brings moisture, so a roof opening is really nothing other than a discharge hole for heat and moisture.

My own recent terrarium is built to stand in a corner of the back and left end against the wall, so the left vent opening instead placed far to the left above the slide glasses.
 

The Snark

Dumpster Fire of the Gods
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Aug 8, 2005
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Going to have to ask you to slam on the brakes for a few minutes here. Essentially, you have things bass ackwards.

Okay, example. I have a room X by Y by Z. I need R humidity and T temperature. How much ventilation is required? So corralling this into something I know pretty well, this is a standard patient room at a hospital. No problem. Building code states XX number of Cubic Feet per Minute. XXX for relative humidity, XXXX for augmented humidity, XXXXX for ambient temperature, XXXXXX for desired mean temperature. Therefore your HVAC unit must supply A, B, C at D. Simple.

First, you haven't mentioned augmented air flow. A fan or blower. So you want to rely on natural convection. Natural convection utilizes -normal- air flow. So first, what are you covering the ventilation holes with? Keep in mind, unaugmented air flow is restricted around 90% with normal mesh window screen.

Next, what is the desired CFM? Is there a desired CFM? If the air outside the enclosure is the same temperature and humidity your air exchange will be dictated by the CO2 content in the air. Are you using a CO2 detector and metering? I'm guessing this is unlikely as that will cost several times the enclosure and probably the animal kept inside.

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Okay, all of the above is just to demonstrate how needlessly complex you have made the issue. You aren't making a negative pressure hospital room. How much ventilation do you need? None, maybe. If the ambients in the room are adverse to the desired environment, ventilation is detrimental. Ventilation's primary purpose is to remove CO2, methane and keep nitrogen gasses at a given level.

What you want is to establish what the desired temperature and humidity is in the enclosure and, since it isn't going to be an isolation chamber, augment the room the enclosure is in to some amount then supplement the enclosure with heaters or cooling and dehumidifiers or humidifiers. Then you add ventilation which will defeat your environment modifiers if the room differs significantly from the desired temp and hum. The more air you move through the enclosure to remove harmful gasses, the more augmentation you will need to implement to maintain the desired ambients.

The rule of thumb is, with synthetic environments like animal enclosures, the less ventilation you can get away with, the better. Ventilation introduces problems in the form of organisms. Your enclosure is about 40 cubic feet. If it is absolutely essential to circulate air through it, a 5 CFM whisper fan forcing air through a normal screen with a screened vent 3 times that size will exchange all the air in the enclosure about 4 times per hour. Quite adequate for just about any environment and far more reliable than all the ventilation holes you could make in it. Hook the fan to a timer or temperature controller if you want. Increasing humidity is easy. Decreasing it means decreasing the moisture in the source air.

Use the KISS rule of engineering. Don't make yourself crazy.
 

magneto

Arachnoknight
Joined
Sep 27, 2011
Messages
196
Thank you. I really appreciate your thorough answer.

But I gotta say, and I am not aiming this at you, that this is what's starting to make me resent this hobby. I know that what you wrote, you believe in. And I know that it might very well be, and probably is, the ideal solution in this case. But the fact is that you are the fourth person in a row that in one way or another basically says:

"Pfff...that's wrong/stupid/overcomplicated, you should do this/that instead!"

And that's just regarding ventilation.
I just recently got into the reptile hobby and I have asked quite a lot of questions on different forums and not once have I gotten two people to agree on a single thing. I mean...if the ideal enclosure conditions are so important to the animals we keep, why are there at least 15 contradicting opinions on each and every minuscule detail? I'm starting to realize that the majority of keepers really haven't got a clue what they are doing.

There...rant over. Again not aimed at you Snark. You might very well be right and so far you are the one with the most convincing, well explained and logic answer I have gotten so far. That rant had just been building up and I had to get it out.
 

Biollantefan54

Arachnoprince
Old Timer
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Jul 3, 2012
Messages
1,997
As everyone has said apparently....just give it ventilation...look at other enclosures and repeat, it doesn't need to be that complicated lol
 

magneto

Arachnoknight
Joined
Sep 27, 2011
Messages
196
As everyone has said apparently....just give it ventilation...look at other enclosures and repeat, it doesn't need to be that complicated lol
That was my initial approach to this issue. But then somebody that has given me good advice in the past told me I should use this formula.

What would you do if you where new and inexperienced with a pet that's sensitive to it's living conditions, and somebody with years of experience with said pet told you you should do this/that? I know I would probably try to follow their instructions to the letter. But then I go on the net and find that everybody do it differently and swears it's the only right way for the animal to thrive.

I have realized now that it isn't that big of a deal as long as there is some ventilation. But the fact remains that being new in this hobby is a bloody nightmare.
 

Biollantefan54

Arachnoprince
Old Timer
Joined
Jul 3, 2012
Messages
1,997
Bearded dragons aren't that sensitive, they are frequently suggested as a good first reptile...just give it some ventilation and make sure it can breathe, it's not that hard. The person using the formula is just making everything complicated and it just isn't necessary at all.
 

The Snark

Dumpster Fire of the Gods
Old Timer
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Aug 8, 2005
Messages
9,764
The formula you should use, must use, was set down in stone, or reiterated, by Gerald Durrell. Paraphrased, 'An intimate knowledge of the animal is mandatory. Each one writes it's own rules. Even two animals of the same species may desire different environments. Always put it's health before any preconceived notions of the habitat we think it might want and let the health be the guide.'
 
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