In this scene from the novel, 'The Martian', by Andy Weir, how does Mark Watney store hydrogen made from water in the tank?

Question

This is the scene from chapter 25, where Watney describes how he had produced and stored hydrogen, which he produced by electrolysing water.

I need clarification of the bold part.

LOG ENTRY: SOL 529

I’m turning water into rocket fuel.

It’s easier than you’d think.

Separating hydrogen and oxygen only requires a couple of electrodes and some current. The problem is collecting the hydrogen. I don’t have any equipment for pulling hydrogen out of the air. The atmospheric regulator doesn’t even know how. The last time I had to get hydrogen out of the air (back when I turned the Hab into a bomb) I burned it to turn it into water. Obviously that would be counterproductive.

But NASA thought everything through and gave me a process. First, I disconnected the rover and trailer from each other. Then, while wearing my EVA suit, I depressurized the trailer and back-filled it with pure oxygen at one-fourth of an atmosphere. Then I opened a plastic box full of water and put a couple of electrodes in. That’s why I needed the atmosphere. Without it, the water would just boil immediately and I’d be hanging around in a steamy atmosphere.

The electrolysis separated the hydrogen and oxygen from each other. Now the trailer was full of even more oxygen and also hydrogen. Pretty dangerous, actually.

Then I fired up the atmospheric regulator. I know I just said it doesn’t recognize hydrogen, but it does know how to yank oxygen out of the air. I broke all the safeties and set it to pull 100 percent of the oxygen out. After it was done, all that was left in the trailer was hydrogen. That’s why I started out with an atmosphere of pure oxygen, so the regulator could separate it later.

Then I cycled the rover’s airlock with the inner door open. The airlock thought it was evacuating itself, but it was actually evacuating the whole trailer. The air was stored in the airlock’s holding tank. And there you have it, a tank of pure hydrogen.

According to my understanding, Watney is inside the trailer in an atmosphere of pure hydrogen. How, then, did he cycle the rover's airlock?

Which is the inner door he is talking about?

Answer 1

The first edition of the book supports the "typo" theory. In this edition, it's clear that it's the trailer airlock, not the rover airlock. This section looks like it was pretty heavily edited, so it's not unlikely that something minor got missed.

LOG ENTRY: SOL 529

I'm turning my pee in to rocket fuel.

It's easier than you'd think. Urine is mostly water. Separating hydrogen and oxygen only requires a couple of electrodes and some current. The problem is collecting the hydrogen. I don't have any equipment for pulling hydrogen out of the air. The Atmospheric Regulator doesn't even know how. The last time I had to get hydrogen out of the air (back when I turned the Hab in to a bomb) I burned it to turn it in to water. Obviously that would be counter-productive.

But NASA thought everything through and gave me a process. First, I disconnected the rover and trailer from each other. Then, while wearing my EVA suit, I depressurized the trailer and back-filled it with pure oxygen at one fourth of an atmosphere. Then I opened a plastic box full of urine and put a couple of electrodes in. That's why I needed the atmosphere. Without it, the urine would just boil immediately and I'd be hanging around in an piss-based atmosphere.

The electrolysis separated the hydrogen and oxygen from each other. Over time, it reduced the urine to a really gross sludge as it pulled the water out. Now the trailer was full of even more oxygen and also hydrogen. Pretty dangerous, actually.

Then I fired up the Atmospheric Regulator. It doesn't even recognize hydrogen, but it knows how to yank oxygen out of the air. I broke all the safeties and set it to pull 100% of the oxygen out. After it was done, all that was left was hydrogen. That's why I started out with an atmosphere of pure oxygen. So the regulator could separate it later.

Then I opened the inner airlock door and had it evacuate the trailer. It pumped all the air in to the airlock's holding tank. And there you have it, a tank of pure hydrogen.

The final step was to take the airlock's holding tank to the MAV and transfer the contents to the MAV's hydrogen tanks. I've said this many times before but: Hurray for standardized valve systems!

Once I fed it the hydrogen, I fired up the fuel plant and it got to work making the additional fuel I'd need.

I'll need to go through this process several more times as the launch date approaches. I could have done this all at once, but NASA doesn't want me to run low on water until we're close to launch. They'd rather I electrolyze urine over time because I've already “used” that water.

If I survive this, I'll tell people I pissed my way in to orbit.

(emphasis mine)

Answer 2

An airlock has an inner door and an outer door. The normal procedure is that you enter the airlock, close the inner door, depressurize the airlock chamber, open the outer and exit into vacuum. Same thing as in a submarine, except with vacuum instead of water.

Luckily for Watney, in the rover the pumps normally move air to a holding tank instead of the interior of the vehicle. So, when the airlock is full of hydrogen, he can pump it to the holding tank. When he does it with the inner door open, it pumps the whole atmosphere in the rover into the holding tank.

Note on language: 'Cycle' here means 'automated sequence of operations'. In a washing machine you have a rinse cycle, which means alternately filling with cold water then spinning a few times.