Imagine that instead of a CWST, you had a 30m high, vertical soil pipe. It would contain about 50 gallons of water, not dissimilar to a decent water tank.
You fit a 1/2" fill valve right at the top, and a 22mm tank connector a few inches up from the bottom.
When you first opened an outlet, (provided that it was level with, or below the tank con) you would have great pressure - more or less 3 bar. But as the volume of water in the column descended (because the 22mm tank con was emptying your tall, thin CWST faster than the fill valve could replenish it,) the pressure would drop lower and lower. If you had an appliance that needed 2.5 bar to work, it would stop working fairly quickly - when just 1/6th of the water had been used.
The very lowest point it can go is the level of the tank connector, which is why I was taught to measure head from that point.
as you get higher the air becomes less dense so this means less pressure????????
