I just had a thought.
The reason I think I need 5.5psi is that I'm combining the depth requirement for both runs. 3.5psi for the diffuser at 7ft and 2psi for the diffuser at 4ft.
Should I be adding these or am I only concerned about the maximum pressure requirement (3.5psi) ?
The compressor does not know what you have downstream. All it sees is the back pressure at its outlet. If your compressor can provide 5 cfm at 5 psig. The 5 psig is the total back pressure on the compressor from whatever you choose to put downstream and still provide 5 cfm. If you have 1 deep and 1 shallow diffuser, with the deepest diffuser requiring 5 psig, when you add the shallow diffuser you will have to sacrifice cfm to the deepest one to maintain no more than 5 psig total back pressure to still get 5 cfm.
...clear as mud!
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