A cinema's ventilation runs on carbon dioxide, because people breathe it out and a full room needs more air than an empty one. Screen 5's sensor reads twelve hundred parts per million at nine in the morning with the doors locked, and the fans ramp up to clear a crowd that is not there.
I am the building services engineer for a cinema, which means the plant room is mine and so is every sensor in every screen.
How the ventilation is controlled.
Outside air is about four hundred parts per million of carbon dioxide. An empty room settles to that. People breathe it out, and a full auditorium climbs past a thousand within twenty minutes if the fans do not move, so each screen has a sensor in the return air and the fans run to hold the reading under eight hundred.
It is the most direct occupancy measure a building has. Not a guess from tickets sold: the actual product of lungs.
The readings log every minute, and the fan speeds with them, because the energy bill is the second largest cost in the building.
Screen 5 reads twelve hundred at nine in the morning.
Doors locked since the last show, nobody in the room, and the sensor reports the air of a room with two hundred people in it, and the fans run at full speed to bring it down, and it does not come down.
It holds at about twelve hundred with the fans at full, which is what a full room does. When there is a full room the fans hold the number, they do not clear it, because the people keep breathing.
Then at about half past ten, most days, it falls to four hundred over twenty minutes, the way a room falls when everybody has left.
What has been checked.
The sensor. Three of them now, two makes, one of them the self calibrating kind that resets itself to outside air every night. It reads twelve hundred in the morning after resetting itself to four hundred at three.
A hand held meter. I have carried one into screen 5 at nine and stood in the middle of the stalls.
Twelve hundred. In my hand. The air I was breathing in an empty room had two hundred people's breath in it.
The plant. If the fans were recirculating stale air the number would be real and the cause would be mechanical, and that was my first answer. The dampers are open, the fans are pulling outside air, and the supply air into that room measures four hundred at the grille while the room measures twelve.
The building. Gas, combustion, a boiler flue, a car park below. Nothing under screen 5 and nothing beside it, and carbon dioxide from a flue comes with carbon monoxide, which the same sensor reads and which is at zero.
The energy.
Full fans for ninety minutes a morning, every morning, is a number on the bill, and it is why this went beyond me. The owner asked why screen 5's plant runs an hour and a half of full speed before opening.
I told him the sensor reads a full room. He asked whether it was broken. I said it had been replaced three times and read the same in my hand. He asked what I thought it was and I said I thought it was two hundred people, and he laughed and then he stopped, and he has not asked again.
What is done about it.
Screen 5's ventilation runs on a schedule in the mornings now instead of on the sensor, which is a step backwards in efficiency and which the controls contractor set up with a note that says sensor unreliable.
The sensor still logs. I still read it. Twelve hundred, nine in the morning, falling to four hundred by eleven.
What I do.
I do the plant checks and I keep the logs.
The one thing I have changed is that I do not go into screen 5 in the morning.
It is on the plant check round and I do it from the projection booth, through the porthole, which shows me the room and the exit signs and the fire door indicators, which is all the check needs.
The reason is the morning I stood in the stalls with the meter in my hand at twelve hundred and it was warm.
An empty cinema at nine in the morning is cold, because the heating is off overnight and the room is big. Screen 5 was warm, the still warmth of a room full of people, and the air had the smell of a room full of people, coats and breath and the sweet smell of the concession stand on their clothes, and when I turned to walk out the seat backs along row F were warm under my hand, one after another, all the way to the aisle.