“Don't let yourself get attached to anything you are not willing to walk out on in 30 seconds flat if you feel the heat around the corner.”
    Robert DeNiro as Neil McCauly, Heat (1995)

This quote has nothing to do with this blog post. I just like Heat.

Heat makes things expand. This occasionally gets used as a plot device in inflation stories. So today we’ll be taking a look at thermal expansion.

So the question is, how much bigger can heat make things? Short answer: not that much.

Dealing with gasses is simple thanks to the beauty of the Ideal Gas Law.

PV = nRT


V = nRT/P


P = pressure
V = volume
n = number of gas molecules, in moles
R = the universal gas constant
T = temperature, in Kelvin

It has a number of implications, but the important one for our purposes is this: all gasses respond to heating/cooling the same way. Assuming pressure remains constant, an absolute increase in temperature by 10% will cause an increase in volume by 10% whether you’re dealing with oxygen, hydrogen, helium, or methane.

To demonstrate, let’s use our hapless victim from the post Water. She’s currently filled with two thousand liters of hydrogen and oxygen, making her slightly over five feet in diameter (and dangerously explosive, but that’s an issue for a different post).

Let’s assume that water she consumed, and resulting converted gas, was at 60℉ and that gas is slowly heating up to body temperature, 98.6℉. When you convert to Kelvin and run the numbers, and assuming pressure stays constant, she goes from 2000 liters to 2,149 liters. Volume increases by 7.5%, resulting in an increase in diameter of about an inch and a half.

I’m sure that our test subject would find that to be quite significant, but the casual observer wouldn’t notice.

We can push this a bit farther, but there are limits. We have to keep the temperature within a range that will neither freeze nor cook a person. If we start at 32℉ and heat to 105℉, we get a 15% increase in volume and a three inch increase in diameter.

So heat doesn’t provide a significant amount of expansion over any reasonable temperature range. But you shouldn’t feel obligated to be reasonable. Exaggerations of various sorts are central to inflation fiction, so please carry on.

Average: 4.3 (3 votes)
mearsob (not verified)
I didn't get it. : (.

I didn't get it. : (.

mearsob (not verified)
How about wish no. 2? Could

How about wish no. 2? Could you scientifically explain how to inflate a person using helium? If it were possible?

LutherVKane's picture
Short answer: No. There are a

Short answer: No.

There are a number of posts in the forums regarding real life inflation, but nothing anywhere near the extent that I discuss in these blog posts. It simply isn't possible. If it were, then I can assure you I'd be spending a lot more time inflating people and a lot less time writing about it. This is more about examining various aspects of physics that would apply in a fictional world where inflating people was possible. They're thought exercises for writers of inflation stories who might be curious about such things.

If you were to inquire about other hypotheticals, like how much helium it would take to make a person float, then I could write something on that.

mearsob (not verified)
That is exactly what I meant

That is exactly what I meant and that's exactly how I saw it. I was talking about a fictional world where that sort of thing is possible.

LutherVKane's picture
Ah, okay. I've got something

Ah, okay. I've got something like that in the works. I'll see what I can come up with.

NameTaken's picture
Well, that answers that

Well, that answers that question.

Good point

So true in some imaginary worlds. I'll take mine, if that's all right.

Let's take an amateur leisure inflatist who doesn't stretch out too hard in the gym but already has some modest skills to show. She is average height (5'5"), average build, not too strong or resilient. She owns an amateur air pump that completely fills her in 40 minutes.

This lady will become full at 9'2" and slips if she stretches to 9'10". If she just finished filling comfortably full and wants to slip - she'll have to leave her pump purring for eight more minutes, very long.

Another way to slip is simply to roll into a sauna. The heat will make her slip much faster than a pump.

External observer won't notice a change in size, but observer won't miss how she becomes translucent, starts shaking and loses control. Heat matters in my imaginary world - it takes a lot to keep together if you became just a balloon ^_^

* cool office air - 65 F
* hot sauna air - 165 F
* a pump has a flux 0.1 l/s (or a score DAM 30 in Arena)
* uninflated inflatist - 5'5" high correspond to 500 pixels in VSG
* an inflatist when full - 9'2" correspond to 840 pixels in VSG and zero effort, in Arena it is ability score VOL 240
* the same inflatist when about to slip - 9'10" correspond to 900 pixels in VSG and effort 850 kcal ph, in Arena it is ability score VOL 288

Vac Hume
Vac Hume's picture
Great movie!

Great movie!

Boiling liquid gas and clathrates

Most liquefied gases would be cold enough to kill immediately. Even air at room temperature would be problematic. However, how about refrigerating capsules with their own power supply which warm up and slowly release their content? Sulphur trioxide and butane seem like possibilities to me, though toxicity would be a problem there I imagine. Alternatively, some kind of molecular cage for gases could be productive. Palladium can apparently hold fifty times its volume in hydrogen and I think it's non-toxic.

I believe there is  a way to do this.



Bapho's picture
I've probably seen that movie

I've probably seen that movie like nine times. It completely rules.

Inflate123's picture
I like school now.

I like school now.

Actually, this had got me

Actually, this had got me thinking to a story concept I once had but never got around to finishing called The Detonatrix--about a supervillain that can gain weight and explode like a bomb almost at will. One idea I had was giving her the ability to convert some of her 'firm fat' into hydrogen so she could safely (kinda) ignite it to fly. If she converted a pound of fat into hydrogen, it would produce 192 cubic feet/5.047 cubic meters of gas. And yeah, it'd take more than just a couple pounds to make her airborne...

LutherVKane's picture
Amusing coincidence. I'm

Amusing coincidence. I'm currently working on a post about helium and buoyancy, and I came up with a similar idea. A very fat woman dreams of flying and wants to turn herself into living blimp. She acquires a substance that will convert fat into helium. Yes, hydrogen makes more sense since fat actually contains hydrogen, but I was writing about helium at the time.

She calculates how much helium she'll need to lift her body weight and measures out the appropriate amount of elixer. Unfortunately she forgets that each pound of fat converted is one less pound that needs to be lifted and winds up with far too much lift. Hilarity ensues.

Feral Jerky Cloth
Feral Jerky Cloth's picture
The guns of science speak again!

And blast an old story I wrote into oblivion. Heh.