Eric Roy, Ph.D, | Scientific Founder   

***Updated on March 22, 2022***

We get quite a few questions about TDS/ppm meters and TDS measurements. While we love when people take steps to learn more about their water, some people (including journalists from reputable publications - Example #1 and Example #2) have used TDS/ppm meters to draw false conclusions about water quality, which incited fear in people already in the midst of a terrible water quality crisis. In this article, we answer the questions that we get asked the most about TDS measurements and TDS meters. If you're curious about water filters that address meaningful contaminants, check out this recent water filter study by Duke/NC State.

What is TDS? What Does a TDS/ppm Meter Measure?

TDS stands for Total Dissolved Solids which is related to the total charged mineral content of water. TDS can be easily determined by measuring the conductivity of a water sample, which is exactly what inexpensive TDS probes do. TDS meters typically display the total amount of dissolved solids in parts per million of ppm. If you start with deionized water (which has a TDS of zero), and expose it to minerals that contain sodium, calcium, and magnesium... the water's TDS of ppm rises. This is why there's no such thing as deionized water in nature. Depending on a region's geology, natural TDS/ppm levels can vary across the U.S., and this variability has nothing to do with the water quality (except in extreme cases when the water is too salty to drink). 

What Does a TDS/ppm Meter NOT Measure?

Because TDS/ppm is an aggregate measure of charged compounds in water, uncharged things like motor oil, gasoline, pharmaceuticals, and pesticides do not contribute to a TDS/ppm measurement. Most relevant to current nationwide water quality problems, TDS/ppm meters do not detect PFAS in drinking water. For example, the glass on the left in the picture above contains deionized water with Malathion (an organophosphate pesticide) dissolved into it at 100 times high concentration than allowed by the EPA for drinking water, and the TDS/ppm probe reads 000.

Even though these toxic metals are charged when dissolved in water, a TDS/ppm meter does not give meaningful information about their presence or concentration in water. There are two main reasons for this:

• A TDS/ppm meter is a nonselective measurement and cannot differentiate among different ions. A more sophisticated piece of equipment is needed to perform those types of measurements. The value of 184 that was measured using a TDS meter in a prominent Huffington Post Article was not the lead concentration... it was the water's natural TDS level (which is dominated by minerals like calcium, magnesium, and sodium).

• A TDS tester is not sensitive enough to measure toxic levels of lead, chromium 6, or arsenic, even if they are present in a sample. This is because the reading displayed on an inexpensive TDS meter is in parts per million, while things like lead, chromium 6, and arsenic are toxic at part per billion concentrations (1000 times lower). Using a TDS meter to measure ppb lead concentrations in tap water is like trying to use a car's odometer to measure a child's height... it's the wrong tool for the job. For example, the water sample shown on the right hand side of this article's header image has levels that are 100x the EPA limit, and the TDS reading teetered between 000 and 001. 

To reiterate: meaningful lead and arsenic measurements cannot be made using a TDS/ppm meter (or any other handheld device). They must be measured by trained staff in analytical laboratories that use much more sophisticated scientific equipment. Hydroviv Undersink filters are NSF/ANSI 53 certified to remove lead from drinking water. 

Should I Buy a TDS/ppm Meter To Test My Drinking Water For High TDS Levels? 

No. There is absolutely no reason to drink low TDS/ppm or deionized water. If you are concerned about water quality, put the money toward the purchase of an effective drinking water filter that removes harmful contaminants from your water. 

When is a TDS Meter Helpful?

Some types of water filters (e.g. distiller, Reverse Osmosis, high performance pitchers, etc) work by deionizing the water, and for these specific products, a TDS meter is a great tool to test to make sure the product's core technology is functioning properly (similar to how your vehicle's check engine light lets you know that the car's systems are working properly).

Do Hydroviv Filters Lower TDS/ppm?

No. Hydroviv's filters selectively filter harmful things from your water (like lead, chromium 6, pesticides, pharmaceuticals, petroleum products, and disinfection byproducts), and things that make water taste and smell bad (chlorine, chloramines, and sulfur). Hydroviv's home water filtration systems don't remove minerals like calcium and magnesium. In fact, we use some types of filtration media that actually add minerals to the water, so TDS/ppm levels in water filtered through a Hydroviv system are sometimes slightly higher than unfiltered water. 

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