In Scribes’ Descent, Mallory descends a deep mine called the Bioprison. It’s encased in an indestructible black barrier stone that’s segmented by horizontal slabs into chambers. Doors in the slabs allow only a few to pass. Mallory is one of those privileged people. Well, privileged might be a stretch. If you’ve read the book, you’ll know why 🙂 And you may have wondered about this passage in chapter 20:
Sergeant Hallinger clicked on his holovisor and displayed air pressure and elevation:
1125.224 hPa: -893m
He frowned. “No matter what room we were in, I always got 1006 hectopascals, which is sea level. Why does this show the right depth only now?”
“Aha!” Mallory snapped her fingers. “That’s because we kept shutting doors behind us, blocking out the weight of air from above. The space below must vent to the surface. This door knew that and opened slowly to protect our eardrums from a pressure spike.”
To understand this, let’s look at the Bioprison layout in more detail. Most small chambers are airtight and stay at atmospheric pressure: 14.7psi—roughly matching the pressure in the middle ear. But the first vast chamber, called the Myophos Grotto, is deep down and vented to atmosphere, so its pressure is higher.
About units: in the book, I give pressures in hectopascals (hPa) because Daishon uses the metric system. But in this article, I’ll also provide the equivalent in pounds per square inch (psi).
At 893 meters below sea level, air pressure is 16.3psi:
But why does air pressure go up when you go down?
Air is pressurized by the column of air molecules stacked above it. On a mountaintop, because there is less air above you, pressure is lower. Deep below sea level, more air is stacked above you, so pressure is higher.
Human ears pop when the air pressure changes quickly. When Mallory opens the door to the Myophos Grotto, air pressure rises from 14.7psi to 16.3psi. A 1.6psi increase may not sound dramatic, but that equates to water pressure at the bottom of a 4-foot pool. That’s why the barrier door slides eases open, letting the heavier air from below pressurize her chamber gradually. The pressure spike is still great enough to pop her ears, but not enough to rupture her eardrums.
But… if you shut a barrier door to make a chamber airtight, you no longer have the weight of the whole atmosphere bearing down on that chamber–so is that air still at 14.7psi? Yes, but when you seal the chamber, the air is already pressurized–the weight of atmosphere has already crammed a certain number of air molecules into that volume. The pressure remains because the walls of the container keep the molecules trapped in the same space.
Quick Update: during National Novel Writing Month, I’ve added over 25,000 words to book two of the Scribes Series. I have at least 25,000 more to add, and I’m lighting up my keyboard when I’m not at local sales events. To those who met me at one, thanks for stopping by and subscribing!
See you next month,