I have about 7 meters of PVC pipe connected to the water meter. Some of this pipe is above the meter level so some air sure gets trapped inside.

Is it possible that the air in pipes expand and contract changing the meter reading? In a 24 hour period, my meter registered 50 liters of water. I checked the pipes many times and haven't found any leaks (the pipes are exposed). At that rate the leaking should be pretty visible, right?

I was watching the meter now (noon, full sun) and I could see it registering some hundred ml in a few minutes, but no drops anywhere.

  • If there is any effect on the meter, I believe it will average out; meters are designed to deal with a bit of sloshing. (I can see mine waver before settling when the washing machine cuts off suddenly, for example). A small drip can add up to a lot of water. If it's a leaking faucet or something like that, dripping right into the drain, it won't be especially visible.
    – keshlam
    Nov 16, 2014 at 15:16
  • Toilets (Water Closets) and water softeners are the two most common places for a leak you can't easily see, in my experience. More generally, if it leaks into a drain, it won't be visible.
    – Ecnerwal
    Nov 16, 2014 at 15:19
  • No drains there, the pipe just has a garden tap and the other end is capped (the house is in construction). I think air trapped in the capped end might be expanding and contracting, i set it up for another test later. All in all, i dont think that more than 3 liters fit in the pipes. Nov 16, 2014 at 15:26
  • 1
    I made another test. I assured myself that all joints were dry, opened the shut off valves and let the system quiet for 1 hour. When I returned it registered 1.8 liters consumed. That averages to 30ml/minute, this is visible, any joint leaking this much would be soaking wet, but they were all dry. I'm calling the water company tomorrow. Nov 16, 2014 at 19:59

2 Answers 2


It is a long time ago since you posted your comment, but I will answer anyway, as I believe it can help many people. I live in Brazil, and here we have had many issues like you described, due to changes in the pressure of water supply company. It makes that much air comes into the piping, and when this trapped air comes to the home meters, it is registered as a normal water flow, and you pay for it. Some small companies here have developed a kind of a valve that blocks this air, and keeps it trapped before your meter, while permitting the water to pass. This is reducing the water bill for almost 50% in some cases.

Please look for "bloqueador de ar". I did not find nothing like in english language.

Hope it helps, and good look! Paulo

  • I don't know if I would agree, water has 200x the density of air and the way meters work here in the U.S. there might be a slight difference ,but I have seen home systems that do not have check valves measure water ussage when the water heater cycles and there is no ussage , because water meters are only certified for ussage in 1 direction water flowing backwards is not counted the same.
    – Ed Beal
    May 16, 2018 at 1:21

Municipal water meters are positive displacement meters as they are the most accurate at low and high flow rates. If the supply is shut off for a prolonged period of time (several hours) it is likely that water in the pipes will be drained by consumers trying to get as much water from the supply as they can. The result is air filled pipes. When the supply is restored and consumers open their taps, that air will then pass through their meters and they will then "pay for the air" as if it was water. The idea of an air breaker as mentioned in Paulo's post that is used in Brazil is great except that is is on the municipal side of the meter and may not be tampered with by the consumer.

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