Abstract: A TDS pen can only measure the total amount of dissolved salts and minerals in water; it cannot detect the changes that are more worth paying attention to, such as organic matter and particulate matter. Low TDS does not mean good water, and high TDS does not mean bad water. To judge water quality, you need to also look at turbidity, UV254, COD and TOC together.
Keywords: Is a TDS pen accurate, what TDS value is normal, does low TDS mean good water, difference between a TDS pen and a water quality testing pen
One-sentence answer: Measuring TDS alone is not enough. TDS only reflects "how much salt and minerals are dissolved"; organic matter and suspended particles are basically non-conductive, so a TDS pen cannot measure them. To find out whether the "greasy stuff" in the water is increasing, you need a multi-parameter testing pen that can measure turbidity and organic matter, such as the Water Detective 9 parameter version.
What exactly does TDS measure?
TDS is "total dissolved solids", mainly minerals and salts such as calcium, magnesium, sodium and potassium. A TDS pen actually measures the water's conductivity and then converts it into a TDS value. So anything non-conductive is invisible to it.
Three common misconceptions
| Misconception | Actual situation |
|---|---|
| The lower the TDS, the cleaner the water | RO water has a very low TDS, but if there is a problem with the storage tank or post-filter, organic matter can still increase |
| High TDS means the water is bad | Mineral water and tap water in northern China naturally have high TDS; it is mostly minerals and does not indicate contamination. |
| If TDS hasn't changed, the water hasn't changed. | When there is sediment buildup in the tank, rust in the pipes, or organic contamination, TDS may barely change. |
Besides TDS, what else should you look at?
- Turbidity: whether fine particles and suspended impurities have increased; the national standard sets clear limits for tap water.
- UV254: whether a class of organics invisible to the naked eye has increased, which relates to the conditions under which disinfection byproducts form during chlorination.
- COD, TOC: changes in total organic matter and oxygen-consuming substances.
The Water Detective 9 parameter version measures these together with TDS, conductivity, hardness, salinity and temperature in one go, and displays them on screen as a radar chart, so whichever item is weak shows up as a missing corner; the AI in the mini program then links the 9 items together in its explanation, rather than fixating on a single TDS reading to draw conclusions.
Frequently Asked Questions
Q: Is a TDS pen still useful? Yes. It is useful for checking whether an RO water purifier is working and for getting a rough idea of how much mineral content the water has; it just can't be used to judge whether water is safe.
Q: What TDS level is normal for tap water? Generally around 100-200 ppm; higher levels are common in northern regions, while southern cities such as Shenzhen are often below 100; the national standard limit for total dissolved solids is 1000 mg/L.