> Going from a larger diameter to a smaller diameter only results in a change in pressure if there's flow
There isn't a question of how much water can flow, the question was only if you need metal to have the high pressure needed for filtering.
> In fact, the pressure ends up lower in the smaller diameter section according to wikipedia.
You are probably seeing the pressure applied to surrounding pipe itself without taking into account the increase in velocity from the conservation of kinetic energy.
> Putting your thumb on the front of a garden hose increases pressure inside the entire hose because it reduces the flow rate, not just where your thumb is.
I think you are missing the point, if you want more focused you can reduce the flow rate since you still have the same kinetic energy of the water.
This is all irrelevant to the main topic though, apparently industrial PEX can go over 1400psi
There isn't a question of how much water can flow, the question was only if you need metal to have the high pressure needed for filtering.
> In fact, the pressure ends up lower in the smaller diameter section according to wikipedia.
You are probably seeing the pressure applied to surrounding pipe itself without taking into account the increase in velocity from the conservation of kinetic energy.
> Putting your thumb on the front of a garden hose increases pressure inside the entire hose because it reduces the flow rate, not just where your thumb is.
I think you are missing the point, if you want more focused you can reduce the flow rate since you still have the same kinetic energy of the water.
This is all irrelevant to the main topic though, apparently industrial PEX can go over 1400psi