I am learning how to install water pipes and I was told that to check if a pipe is leaking water I need to check the water pressure. But I don't understanding how water pressure works inside the pipe.
For example, a water pump is connected to a valve and to a tap like this:
pump ----> valve ---->tap
the water pressure is caused by the pump. Now if I close the valve and blocking the water from the pump to the tap, what will be the water pressue in the segment of pipe between the valve and the tap? i.e.
pump ----> X ----> tap
My understanding is that the pressure should drop to zero as this segment is disconnected from any "source of water pressure" (the pump). But If I connect a meter to the tap before closing the valve:
pump ----> X ----> meter
The reading on the meter doesn't drop to zero. Instead, if there is a water pressure drop, it indicates a leakage. But what causes the pressure when the valve is closed?
I know air pressure inside a sealed system is caused by the Brownian motion of the air molecule does the same thing applies to water?