If you have a TDS meter or are thinking about getting one, here is a plain-English guide to what the numbers mean and how to use them with your RKIN water filter.
What Is TDS?
TDS stands for Total Dissolved Solids. It measures the combined amount of minerals, salts, metals, and other dissolved substances in your water, expressed in parts per million (ppm). A TDS meter gives you a quick snapshot of your water quality.
What Do the Numbers Mean?
- 0–50 ppm: Very pure water. Typical of reverse osmosis filtered water (before remineralization).
- 50–150 ppm: Good quality. This is the range you can expect from RKIN RO systems with the alkaline post-filter, which adds healthy minerals back.
- 150–300 ppm: Average tap water in many US cities.
- 300–500 ppm: Higher-than-average. You may notice a mineral taste.
- 500+ ppm: The EPA secondary standard recommends below 500 ppm. Above this, a filtration system is strongly recommended.
Testing Your RO System
To check your reverse osmosis system performance:
- Test your tap water (before the filter) and note the TDS reading.
- Test the filtered water and note that reading.
- A healthy RO membrane should reduce TDS by 85–99%.
Important note for ZIP and U1 owners: If you test from the dispensing spout, your reading will be higher than pure RO water because the alkaline post-filter adds minerals back. This is by design — it is a good thing! To test the membrane itself, you would need to test from the line between the membrane and the post-filter. For most people, just checking that filtered water is significantly lower than tap water is enough.
When to Be Concerned
If your filtered water TDS is above 10–15% of your tap water TDS, it may be time to replace the RO membrane. For example, if tap water is 300 ppm and your filtered water reads above 45 ppm (before post-filter), the membrane may be nearing end of life.
RKIN offers an affordable Dual Function TDS Meter that measures both TDS and water temperature. It is a handy tool for keeping an eye on your system performance.