The compressor's meter counts every litre of air it makes and the receiver's gauge shows what is stored, and the two are reconciled because leaks cost money. Ours draws air between one and two in the morning at the rate of an impact gun taking wheels off, in bursts of the length it takes, with every line capped and the ring main valved off.
I am the workshop manager at a fast fit centre and the compressed air is my budget.
How the air is measured.
The compressor fills a receiver to pressure and cuts out, and cuts back in when the pressure drops. The controller logs every run: when, for how long, and how much air was made. The receiver's pressure is logged too. Between them you can see every draw on the system, because every draw is a pressure drop and then a compressor run to replace it.
Overnight the ring main is valved off at the receiver and the lines are capped. A healthy system holds its pressure to the morning within a bar. Ours does, on every night but the ones I am talking about.
Between one and two in the morning, two or three nights a week, the receiver's pressure drops in bursts.
Short bursts, a second or two each, five of them in quick succession, then a pause of a minute or so, then five more. The compressor cuts in behind them to catch up and the controller logs the runs.
Five bursts of a second or two is a wheel gun taking five nuts off a wheel. The pause is the wheel coming off and the next one going on. Four sets of five, and then it stops, and the compressor tops the receiver up, and the log is quiet until seven.
The ring main is valved off. The air that is drawn is drawn from the receiver on the compressor's own side of the valve, where there is nothing connected but the gauge and the drain.
What has been checked.
The valve. Replaced. The lines. Pressure tested overnight with the receiver isolated. They hold.
The receiver. Drained, inspected, its safety valve tested and replaced because that was the one honest explanation, a valve lifting in bursts. The new valve is fitted and lifts at the pressure on its tag and at no other, and the pressure never reaches it at night.
The controller log, which the compressor firm's engineer took away and came back with a sentence I have kept: the logged events are consistent with an air tool being operated on the system.
There is no air tool on the system. There is no system, at night. There is a tank with a gauge on it.
The cameras.
The workshop has cameras and they record on motion and they record nothing at one in the morning. The bays are empty, the ramps are down, the lights are off.
The compressor is in a shed at the back with its own camera, because it is the most expensive thing we own, and that camera shows the compressor cutting in to replace air, in the dark, with nothing in the shed but the compressor, and the gauge on the receiver moving in the bursts the log describes.
The pattern.
Four wheels, five nuts each, in the order you do them on a ramp, with the time between wheels that it takes to swap a wheel over. I have timed our fitters and the night pattern is inside their range. A slow fitter. Careful.
And on one night a month, roughly, there is a sixth burst after the fourth wheel, longer, which is the sound a gun makes when it is being used to tighten rather than loosen, and which is the last thing a fitter does before the car comes down.
What is done about it.
The electricity for the night runs is on the bill as compressor overnight, and it is a few pounds a week, and the boss has signed off on it as leak losses.
I have told him it is not a leak. He asked what it was. I said it was somebody doing a set of wheels at one in the morning with the air off, and he laughed, and I did not, and he stopped.
What I do.
I run the workshop and I read the compressor log.
The one thing I have changed is that bay 3's ramp is left up at night.
Bay 3 is the one the night pattern's timing matches, because bay 3's airline is the longest run from the receiver and the bursts have the slight lag of that run in them, which the engineer noticed and I had not.
The ramp is left at full height overnight now, which is against the manufacturer's advice and which I do anyway, because in the summer I came in at half past six and bay 3's ramp was at working height, the height you set it to for wheels, with nothing on it, and the wheel gun was on the floor beside it, out of its holster, on its side, the way a man drops it when he has finished with it, and it was warm.