Summary: The most direct way to test filter effectiveness is to measure tap water and purifier outlet water separately at the same time and compare the difference between them; when the difference keeps shrinking and falls below half of what it was with a newly installed filter, replacement should be considered. Judging by TDS alone easily leads to misjudgment, so it is best to also check organics and turbidity.
Keywords: how to test filter effectiveness, when to replace a water purifier filter, water purifier outlet water testing, RO water testing
One-sentence answer: Whether a filter should be replaced cannot be judged by date alone, nor by TDS alone. Use the Water Detective 9 parameter version to measure inlet water (tap water) and outlet water separately, compare the removal effectiveness of TOC, COD, UV254, turbidity and TDS, and record it in the mini program; when the difference shrinks noticeably, that is the signal to replace the filter.
Why TDS alone is not enough
TDS reflects dissolved salts and minerals. The TDS of RO reverse osmosis water purifiers is inherently low, but when the activated carbon after the RO membrane or the pre-filter fails, organics and particulates may increase first while TDS changes little. For water purifiers such as ultrafiltration and activated carbon that inherently retain minerals, TDS can hardly be reduced much, so TDS simply cannot be used to judge whether they are working well.
Which items to check for different filters
| Filter type | Primary role | Key indicators |
|---|---|---|
| PP cotton/pre-filter | Traps sediment, rust and large particles | Turbidity |
| Activated carbon | Adsorbs organics, odors and residual chlorine | UV254、TOC、COD |
| 超滤膜 | 拦细小颗粒、胶体 | Turbidity, UV254 |
| RO反渗透膜 | 去除绝大部分溶解物 | TDS、电导率，同时看TOC、COD |
检测步骤
- 先用纯净水校准检测笔。
- 放掉管道里的存水，分别接一杯自来水和一杯净水器出水，静置片刻。
- 先测自来水，再测净水器出水；换水前用要测的水冲一下探头。
- 在小程序里看两次结果，AI会对比分析去除效果。
- 每个月对比一次，看差距有没有在缩小。
Filter replacement signal
- The gap between the outlet water and tap water narrows to less than half of what it was when the filter cartridge was new.
- The turbidity or UV254 of the outlet water begins to rise noticeably.
- The TDS of the RO system's outlet water gradually increases.
Frequently Asked Questions
Q: The newly replaced filter cartridge gives higher readings instead? New filter cartridges may contain protective substances such as glycerin; rinse according to the instructions for a period of time before testing.
Q: If the purifier's outlet water TDS is close to 0, does that mean there is absolutely no problem? Not necessarily. Low TDS only means fewer dissolved salts; organic matter and turbidity should also be considered, and the storage tank and piping may also cause secondary contamination.
Q: How many water purifiers can one pen test? It can test any number of purifiers; the mini program saves records separately by time and location.