When troubleshooting the waterflow alarms, we need to look at what is involved.
Building's Fire Sprinkler System
From at least one point of view, the building's fire sprinkler system is a very long air pressure tank. The quantity of air inside the sprinkler system, though, isn't affected very much by the dissolved air that is carried in by the water. That's really a very small quantity. It's the large quantity of ambient air that has to be let into the system during maintenance that is the real issue.
The sprinkler people often have to drain the water out of the system in order to work on it. If they didn't, they'd make a real mess. In order to drain the water out, ambient air (the stuff we all breathe) has to be let into the system to replace the water. When the maintenance is finished, unless there is a way of bleeding out the air at the top of the sprinkler system, the air has to stay.
The air gets compressed (pressurized) by the water.
You noticed that the pressure gauge reads about five pounds less than before the backflow preventer was tested. That means that the air is five pounds less compressed than before the testing.
If the city's water pressure goes up, it pushes more water into the sprinkler system. Then the air is compressed more; its pressure becomes higher to match the water pressure.
City Water into the Building
The water pressure on the city's water system is always changing. It's affected a little bit by the actual water level in the water tower, but other causes change the pressure a lot more.
During the day, when people in the neighborhood are using more water, the water flowing in the city's underground distribution system causes the pressure to be reduced. The opposite is true late at night. Less water use in the neighborhood means higher city water pressure.
Cities often have pressure reducers in various places on their systems. The reducers keep the pressures on the system a little more stable. If these pressure reducers are adjusted, the pressure can change.
Construction in the area can affect city water pressure. These pressure changes can last from hours, to days, to even weeks.
Whenever the city water pressure goes up, there is the potential of pushing water past the backflow preventer, and then past the waterflow switch.
The waterflow switch is a water-movement sensor. It senses the movement of water going into the sprinkler system.
One purpose of the switch is to turn on the outside bell or horn whenever there is water flowing in the sprinkler system. It shows the firefighters that water is actually flowing, and it shows them were to connect their hoses.
Another purpose of that switch is it activates the building's fire alarm system.
See: What is a Waterflow Switch?
for description of how the waterflow switch works.
It could be that the waterflow switch is bad and needs to be replaced.
The waterflow switch has a paddle across the entire inside of the sprinkler system's waterpipe to sense water movement, and an air damper to slow down the activation.
If the air damper inside the switch assembly is dried out, the switch usually activates quicker. I've never seen where a dried-out air damper slows down the switch, but that doesn't mean it can't happen.
The pivot inside the waterflow switch assembly could be corroded so the switch stays on for a long time after a short bump in the city's water pressure. I've seen that problem.
The water pipe for the sprinkler could be corroded so the paddle has difficulty returning after a short bump in the city's water pressure. I've seen that also.
The backflow preventer is a water system separator; it separates the city water system from the building's sprinkler system.
It prevents the water, that has been sitting in the sprinkler pipes for years, from flowing back into the city's drinking water system.
It also prevents the water from flowing back from compressing the air, so the air pressure in the sprinkler system stays at the highest peak city water pressure.
There's always a slim possibility that the backflow preventer is marginally good. I've never seen one that is that way, but being a technician, I have to keep an open mind.
Look at the pressure gauge. The pressure should be higher than right after the preventer test. If it's not, the gauge could be bad, or there could be other problems. If the pressure on the gauge is higher one day, and lower the next day, you can assume there is a leak in the system.
- The leak could be water flowing backward in the backflow preventer. Remember, it was tested a couple of weeks ago, things can change in a few weeks.
- The leak could be water flowing out through pinholes in the sprinkler system. You'd probably see water damage, though.
- The leak could be air escaping through micro-cracks too small for water to escape. Usually, that would happen over months.
15-20 Minutes of Alarm
15-20 minutes of sounding the outside alarm means that the waterflow switch was activated for 15-20 minutes. In order to sound the alarm that long, the city has to push a lot of water past the waterflow switch. The water really wasn't flowing that long, or else you would notice the spiled water.
From your description, it sounds like there was a very short-term influx of city water into the sprinkler system. When that happened, the paddle on the waterflow switch was pushed aside. Rather than returning once the water stopped, the paddle stuck in the aside position for 15-20 minutes. If that's the case, the waterflow switch needs to be replaced, and the pipe, at least where the switch is located, needs to be examined for internal corrosion while the switch assembly is being replaced.
There might be something else causing the alarms, but that is where I'd start looking.
Call the sprinkler company. Find out if they have other ideas.