An oxygen reduction system keeps a strongroom too lean to burn and logs the oxygen level every minute, because a level that climbs means a leak and a level that falls means something is consuming it. Ours falls, overnight, in a sealed room, at the rate a resting adult breathes, and stops at seven in the morning.
I am the collections manager for a records office, and the strongroom is the most carefully controlled room I have.
How the system works.
The strongroom is sealed, and a nitrogen generator feeds it to hold the oxygen at fifteen percent, which is low enough that nothing in there can sustain a flame. Sensors log the oxygen every minute. The controller adds nitrogen when the level rises, which it does slowly as air leaks in round the door, and alarms if it falls, because falling oxygen in a room nobody is in means something is consuming it, and the only things that consume oxygen are fire and lungs.
Staff going in wear a monitor and are limited to two hours. The door logs every entry.
Between about midnight and seven the oxygen in the strongroom falls.
Not much. From fifteen point zero to about fourteen point six over seven hours, and then it stops falling, and the generator, which has been idle because nothing was rising, brings it back up over the morning.
The fall is steady, and I have plotted it against the room's volume, and it is the consumption of one adult at rest.
A person, asleep or sitting still, uses about a quarter of a litre of oxygen a minute. Seven hours of that in a room this size is the fall on my chart, near enough that the engineer who checked my arithmetic did it twice.
What has been checked.
The sensors. Two, different technologies, both calibrated against a gas of known concentration. Both record the fall.
The door. Logged shut all night, and the corridor camera shows it shut.
Fire. The system's whole purpose is that there cannot be one, and there is no heat, no smoke on the detectors, and no carbon monoxide. A fire consumes oxygen and makes things. This consumes oxygen and makes nothing the sensors can see.
The room. I have been in it, with a monitor, with the nitrogen on, at two in the morning. Fourteen point eight on the monitor and falling. It is a room of shelving and boxes and cold, and there is nobody in it, and I stayed twenty minutes.
The carbon dioxide.
That is the check that ended the argument I was having with myself, because breathing does two things.
I put a carbon dioxide logger in the strongroom in 2022.
It rises overnight. At the rate that a person's breathing raises it in a sealed room of that size, and it stops rising at seven, and the mornings' ventilation takes it away.
Oxygen going, carbon dioxide coming, in a sealed room, in the proportions of respiration, with a door that does not open, from midnight to seven, every night, for at least the four years the logs go back.
What is done about it.
The generator has more work to do in the mornings than the model says, and that is on the energy figures as a known load.
The fire officer has seen the logs, because I showed him, and he read them and asked whether the sealing had been checked, and it had, and he said that the room was demonstrably not on fire and that was the extent of his interest, which is fair.
I have not put it to the trustees, because the sentence I would have to write is the sentence that starts this story, and I have not found a way to write it that does not sound like the thing it sounds like.
What I do.
I manage the collection and I read the logs.
The one thing I have changed is the two hour limit. It is a two hour limit for everybody else and a forty minute limit for me, which the assistants think is caution and is not.
The twenty minutes I spent in there at two in the morning were quiet, with the fans, and the monitor on my chest read fourteen point eight and then fourteen point seven, and I was breathing slowly, which is what fifteen percent makes you do.
About fifteen minutes in I noticed that I was breathing in time with something. Not the fans. A slow, even rhythm, at the rate a sleeping person breathes, from the far end of the room, and I had matched it without meaning to, and when I held my breath to be sure, it carried on.