Performance of reverse osmosis can be measured by an instrument called a TDS meter. This process uses a conductivity meter immersed in water to measure the solids dissolved in the water. The meter converts the conductivity to a measure known as parts per million (ppm).
A reverse osmosis unit, when functioning correctly, will reduce 75-90% of the TDS on the water. The performance of a reverse osmosis system will vary based on the feed water:
- Temperature
- Pressure
- TDS (total dissolved solids)
When testing the performance of a reverse osmosis system, test both the feed water to the RO system and the product water out of the unit.
Instructions for using a handheld TDS meter:
1. Test the feed water - this meter has a hold button that holds the TDS reading:
This test yielded a TDS reading of 356 ppm.
2. Test Reverse Osmosis product water:
This test yielded a TDS reading of 32 ppm.
Reverse Osmosis Performance Formula
The rejection rate is the percentage of the TDS (total dissolved solids) removed by the RO membrane on your water purification system. For example, a 90% rejection rate means that the RO product water from your RO system contains 90% less TDS than the feed water coming into the system. The formula below is used to calculate the TDS rejection between feed water and product water.
Example: