3035: Trimix

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Trimix

Title text: You don't want the nitrogen percentage to be too high or you run the risk of eutrophication.

Explanation

Trimix is a gas used in scuba tanks that consists of oxygen, helium, and nitrogen. Trimix comes in some standard ratios between the component gases, depending on the required diving depth, for example 21/35/44 (percentage oxygen/helium/nitrogen), 18/45/37 and 15/55/30 for increasing depth. Helium safely substitutes a portion of the nitrogen to minimize nitrogen's narcotic effects at greater depths, and to ease the effort required for breathing, as merely reducing the nitrogen ratio in an oxygen/nitrogen mix will increase oxygen to levels that, at the pressures that exist at depth, creates other dangers (hence why the Trimix ratios also provide slightly lower proportions of oxygen). This comic suggests that, if the ratio of helium to nitrogen/oxygen is too high, a diver will float away before reaching the water to start a dive, as their tank of air starts behaving just like a sufficiently buoyant helium balloon. Divers rarely mix their own blends, so this would be the fault of the supplier who filled the tank from which the cylinder is filled. It would also be impossible to disproportionately breathe more of the non-helium, changing the ratio, although the way the comic depicts the tank only just starting to float might suggest that it is somehow being adjusted to the 'wrong' ratio as we watch.

The reason a helium balloon creates lift is that it can inflate. The balloon has a thin and expandable layer, weighing almost nothing, that allows the intrinsically less dense gas to take up space at not much more than atmospheric pressure. The total weight of the filled balloon is less than the total weight of the air it displaces, thus creating buoyancy. A scuba tank is made of metal, is heavy and cannot inflate in anything like normal circumstances.^{[citation needed]} Even with a perfect vacuum inside it (if that were possible) it would still weigh more than the equivalent volume of air. You would simply increase that weight if you pumped helium, or any kind of gas, into it. An inflating helium balloon also gets heavier, but this is more than compensated for by the greater increase in volume. The more you pumped into a rigid metal tank, the denser and heavier it would get, and it will never be able to create any degree of additional lift. The comic falsely implies that adding more helium to the system decreases its net relative weight, without taking into account the need to displace a greater volume of air.

The real reason scuba divers need to be careful with the amount of helium is to not get too little (or too much) oxygen for the intended depth and pressure, as well as reducing the troublesome nitrogen.

The title text asserts that a trimix diver whose gas mix contains an excess of nitrogen runs "the risk of eutrophication". The term eutrophication describes the process by which nutrients ("fertilizers") accumulate in an environment, typically a body of water, leading to consequences that are often unfortunate for inhabitants or users of that environment. The human body is an ecosystem, but one that is not typically subject to eutrophication, due to its manner of acquiring and jettisoning nutrients. Moreover, the nitrogen in trimix is diatomic elemental nitrogen, not the "fixed" nitrogen that serves as a component of eutrophication. The diver would not breathe "fixed" nitrogen unless nitrogen-fixing bacteria were somehow incorporated into the scuba gear, a complex feature of dubious utility. Eutrophication could also be understood as the diver dying of nitrogen exposure thus adding nutrients (the dead body) to the body of water.

Randall addressed the question "how much helium is needed to lift a human body" in a What if? article. Helium has also featured in comics 2766: Helium Reserve and 2972: Helium Synthesis.

Transcript

[In a single frame five images of Cueball are shown as a kind of a cartoon event sequence, from left to right, with each image a later time in the process. The first image shows Cueball standing by the shore of a body of water. He is wearing scuba gear, goggles, a regulator, a scuba tank with a small H logo on it on his back, and fins, which almost touch the water.]

[In the next four images he has the same equipment on, but with changes. He is presumably still at the edge of the water, but the water is not drawn in the next four images.]

[In the second image, Cueball's scuba tank is beginning to float upwards, so the end that is not tethered to Cueball moves out and up, as shown with three small lines beneath the tank.]

[In the third image the scuba tank is now floating above Cueball's head and the strings pull his arms a bit upwards. Cueball has turned his head looking up at the tank floating above and behind his head. Again three small lines beneath the tank indicates it is moving upwards.]

[In the fourth image Cueball is now being lifted up, so his feet are now off the ground and he is tilting forward. The tank is now pointing its bottom almost straight up and Cueball is looking down with his arms out to each side as the tank pulls him up. Two lines on either side of the tank indicate that it now wobbles above him as it lifts him up.]

[In the fifth and last image the scuba tank is now pulling a dangling Cueball with his legs splayed high above the ground, above his head's height in the first image. The tank is now turned so it points its bottom to the left with lines on either side indicating wobbling motion. Cueball is floating in a roughly horizontal position as the tank's straps pull him upwards. At this point he yells:]

Cueball: Help!

[Caption below the panel:]

Trimix scuba divers need to be careful not to let the helium percentage get too high.

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Discussion

The 'standard' and '2x' sized images had unexpected sizes, so an imagesize parameter has been added to render the image consistently with other comics on this website. See the web archive for more details. --TheusafBOT (talk) 05:15, 9 January 2025 (UTC)

yeah, the image on xkcd.com looks comically large for me, and I think this might be related to this. 172.69.155.86 15:50, 9 January 2025 (UTC)

Not for me. I’m on Safari, and it looks pretty normal. 42.book.addict^{Talk to me!} 16:08, 9 January 2025 (UTC)

I'm on Chrome and it looks much larger than usual. .-. --1234231587678 (talk) 16:50, 9 January 2025 (UTC)

Well, I'm also on Chrome, and it looks totally normal. Normal size on Edge too. RadiantRainwing (talk) 14:35, 10 January 2025 (UTC)

"Comically large"? If anything, the regular size is comically small. Why doesn't randall post comics in HD like everyone else? 141.101.109.167 20:15, 9 January 2025 (UTC)

Because it's stick figures and thus unnecessary. Also, (at least on chrome) the image is so large that it overflows the boundaries. Likely a glitch, but maybe a pun, yeah. Stallman (talk) 04:53, 10 January 2025 (UTC)

XKCD is more than just stick figures, and what exactly are the boundaries? They should be at least 1920x1080. Just how large was the "glitched" image? 141.101.109.167 20:31, 10 January 2025 (UTC)

I thought the "over-inflated" image was part of the pun... - 172.70.210.177 17:48, 9 January 2025 (UTC)

It's a glitch. If it was part of the comic, it would appear oversized on all devices and screen sizes. To me it's looking normal on firefox for android, but oversized on firefox flatpak on a 1366px width monitor. 172.70.140.242 18:34, 10 January 2025 (UTC)

Whatever size it is, it will, by its nature, be comically so.172.68.186.132 09:25, 10 January 2025 (UTC)

I'd question whether you could get a scuba tank to float with any amount of helium. Since you're dealing with a fixed size tank the most lift you going to get would be less than the weight of the air that the tank displaces (lift = weight of air - weight of tank - weight of helium, iirc). A typical aluminum Ali 80 tank has a volume of 11.1 liters which displaces only about 151g of air while the tank itself weighs 14kg StumbleRunner (talk) 07:17, 9 January 2025 (UTC)

Now that you mention it ... I was distracted from the practicalities by the voice of Marvin the Martian running around in my head. Something about an earth-shattering kaboom. 172.71.151.165 07:33, 9 January 2025 (UTC)

Yes of course a SCUBA tank cannot create any float since it cannot change it's volume (except when exploding). This is the reason why it would not work. No matter how much helium is compressed (that it it the problem the gas is compressed!) inside it. I have added this, and are not sure the text beneath this makes any sense. Someone deleted my addition but I hope this was a case of edit conflict they did not care to resolve, rather than they deleted it on purpose!? --Kynde (talk) 08:11, 9 January 2025 (UTC)

I think, on revue, I happened to restore anything you thought lost (albeit in my words, not yet knowing what you'd done) when correcting/removing the awkward misunderstanding that helium is "anti-gravity" in the 'true explanation' bit. (Yes, it's the conceit of the comic, but should not then have been used in the genuine bit of the Explanation. You only become more buoyant if you have less weight in a given volume. For an effectively unchanging volume, more of even a lighter gas is heavier. And you can never have little enough of any gas to make a Scuba-tank buoyant at sea-level air pressures.) 172.70.90.118 11:13, 9 January 2025 (UTC)

I cannot say if it was you who did this, I do not think so, since yo wrote several hours later here, and in both you comments case and the case where something was deleted it is different IP addressees. But the current explanation has taken the gist of what I intended and made it much better, so I'm happy with the current explanation which is much better than my attempt ;-) --Kynde (talk) 08:23, 10 January 2025 (UTC)

Oh, I (above IP, whatever I now appear as) never claimed the fault of wiping out your prior effort, and still don't. I'd gone back to check what you might have lost out on, and but my first reading/writing of it had only ever been later. ;) I haven't taken note of the respective IPs (there's some flexibility of assignment, even moment to moment, but also a degree of geographic clustering that differentiates some of us from all but a few of the rest of us 'anons'), but I like that it at least superficially bears out my innocence in that matter.

However, it's gratifying to know that one's small contribution (amongst all the other worthy ones, equally creditable) should be appreciated. It's by many stochastic improvements that all decent articles are eventuallg formed. :p 172.70.162.164 12:20, 10 January 2025 (UTC)

There is no question here. Boyancy is provided by making the container (the tank in this example) lighter for its volume than the equivalent amount of air. With a balloon, the container volume increasaes as you increase the helium, making the container more boyant. A tank does not expand, and thus gets less boyant as increased helium adds to its mass per volume. The very best that you could do would be to fill the tank with a vacuum, but even then it would still weight more than the equivalent amount of air due to the weight of the tank itself.Geek Prophet (talk) 18:04, 9 January 2025 (UTC)

Trimix is also the name of one of the strongest injectible erectile dysfunction drugs. This was my first thought when I saw the comic title. Even after I recognized my error I half expected a double-entendre on "inflation".Geek Prophet (talk) 18:07, 9 January 2025 (UTC)

When I first found the comic and googled “trimix”, the school Securly filter blocked it because of that exact reason. It was only when I switched to my personal computer that I realised my mistake. 42.book.addict^{Talk to me!} 21:31, 9 January 2025 (UTC)

That's the Scunthorpe Problem, for ya... ;) 172.70.85.70 22:51, 9 January 2025 (UTC)

Won’t breathing too much helium cause Inert gas asphyxiation/suffocation? 42.book.addict^{Talk to me!} 21:55, 9 January 2025 (UTC)

yes. yes it will. However, when breathing at depth, there is the around the same partial pressure of oxygen as in the atmosphere at the surface, so you breathe it just fine as the oxygen will diffuse into your lungs just the same without the risk of oxygen toxicity. The helium just acts as a filler gas, acting to also reduce the risk of nitrogen narcosis by reducing the partial pressure of nitrogen, and is chosen because it is easy to breathe because it is light. consequently, mixtures with low amounts of oxygen are not safe to breathe closer to the surface as there is not a high enough partial pressure of oxygen to support consciousness. (i know i just told you information you could have gotten from simply reading the wikipedia article yourself guess who (if you desire conversing | what i have done) 07:06, 10 January 2025 (UTC)