In an RO- systems that uses the ERP- 1000 Permeatepump the RO- membrane always sees the full pressure across the membrane even if the tank is almost up to the same pressure. This way the system does not loose its performance.
In a normal system you would for example have 60 PSI of line pressure. If the tank sees 30 PSI you only would have 30 PSI left for your membrane. In reality, this gives you less than 50% of your normal performance. In a permeatepump system you would still have 60 PSI across your membrane even on 50 PSI tank pressure.