Monitor pressure to detect leaks?
In order to detect (or confirm) the presence of a leak, it is important to monitor pressure variations in the network. But it doesn’t stop there—equally important is the flow rate. For best results, look for the minimum observable flow rate on a daily curve. This low level is reached overnight when consumption is mainly due to water losses. By observing the curve, you get the minimum night flow (MNF). Then you want to assess the legitimate night flow (LNF): it represents what the MNF would have been without water losses and corresponds to the night consumption by toilets, washing machines and other appliances, etc. The difference between these two amounts gives us the leak rate called net night flow (NNF).
To get a basic idea, we could multiply the NNF by 24 to calculate water losses over a 24-hour period. The problem is that the leak rate is not constant throughout the day: leaks are proportional to the pressure in the network. In addition, the lower the flow rate, the lower the head loss and therefore the higher the pressure: at the minimum flow rate, the pressure is max and the leak rate is too. So, a pressure factor is calculated to give a reliable 24-hour average value when applied to the NNF. This is done by relating the average pressure over 24 hours to the pressure at the minimum flow rate.