You really need both a PSI and a TDS meter. This PSI Meter is critical for system maintenance and problem solving. If you have a problem, the second thing Aquasafe will ask is "what are the PSI readings?". The PSI meter is the only way to check to confirm that the RO membrane is working properly and at what efficiency. It is also the only way you can tell when the DI filter is exhausted. We strongly recommend buying PSI and TDS meters as they are absolutely essential in diagnosing problems, and confirming the system is operating as it was designed.