Total Dissolved Solids Meters: Understanding the Science

Total Dissolved Solids Meters: Understanding the Science

At Aquagear, our goal is to remove tap water contaminants that other filters miss, and we rely on science to guide us. We’ve done the research to determine what needs to be removed from water — PFOA/PFOS, microplastics, lead, mercury, chlorine, VOC, trace pharmaceuticals to name a few. We also pay attention to what’s better left in — like calcium, magnesium, and potassium.

What we leave in has generated correspondence from customers who’ve used Total Dissolved Solids (TDS) meters to check what’s in our water – and especially among those who found that their TDS numbers increase after they used our filter! They’d hoped our filters would deliver numbers close to zero and want to know why there’s “stuff” in there. It’s a fair question, and one that has a good answer that we think it’s important for everybody to understand.

Let’s start with a few important basics:

TDS meters report solids in the water based on parts per million (PPM)

When you use a TDS meter on the water that comes out of your tap, you are likely to find a range of between 50 and 1,000. According to NSF International (a 75-year-old international organization whose mission is to protect and improve human health), those numbers are mainly a reflection of naturally-occurring minerals, and do not reflect contaminants.1

Having minerals in water is a good thing.

You can read more about it here, but the bottom line is that the minerals found naturally in ground water are a major contributor to achieving recommended daily allowance levels and countering nutritional deficiencies. Having calcium in water helps support healthy growth and bone development, while magnesium in water helps prevent cardiovascular disease. In fact, the World Health Organization published an extensive study warning against demineralized (low TDS) water.2

Contaminants in water are measured in parts per billion and trillion, not in parts per million

Heavy metals and PFOA/PFOS in tap water cannot be measured by a TDS meter. They are far too small, and are best measured by professional, accredited ISO 17025 laboratories that specialize in calibration.3 To understand why you need such a high level of sophistication, consider the difference between millions and billions and trillions. A single part per trillion expressed as a part per million would be 0.000001.

The EPA’s limit on PFOA and PFOS in tap water is 70 parts per trillion, which would be the equivalent of 0.000071 parts per million.4 The EPA action level for lead in drinking water is 15 parts per billion, which would be the equivalent of 0.000015 parts per million.5 These numbers are far below the testing capability of a TDS meter. Both would register as zero, giving the false impression that neither contaminant is in the water.

If you used your TDS meter on some of the best mineral waters in the world, you’d find that they delivered very high numbers on the parts per million scale. In fact, mineral water is required to have a minimum score of 250 parts per million of TDS. Evian has a TDS score of 340, while artesian water brand FIJI contains 224 parts per million.6

TDS meters are relatively inexpensive, and if your goal in using a water filter is to keep out solids then they’re a good way to gauge effectiveness. But that’s not the same as measuring whether a filter is taking out what’s bad for you, especially when it’s simultaneously keeping out what’s good.

So how do you keep out what’s bad – the tiny contaminants that can do so much harm – while keeping in the good stuff?

Every Aquagear filter includes a stage that leverages an ion exchange process. It infuses the water with minerals while at the same time trapping contaminants. Sometimes the process actually adds minerals to the filtered water depending on what is in the tap water in the first place. So some customers see small decreases in their TDS numbers, some see them stay the same, and some see them increase. Whichever happens in your area, it’s the ion exchange process that makes our filtration technology so superior, and the minerals that we make sure are there that gives you great tasting water that actually contributes to your good health.

We hope that this explainer puts your mind at ease about what Aquagear’s filter does – and does not – do. In case you’re still intrigued by numbers, here’s the most recent data from the ISO 17025 laboratory test of water that’s been run through our filters.

We are thrilled to have customers who are engaged and science driven. If you have any other questions about TDS, contamination removal, or any other aspect of our filter’s performance, please email us at


  1. Total Dissolved Solids in Drinking Water. Retrieved from (ND)
  2. Health Risks from Drinking Demineralised Water. Retrieved from the World Health Organization website. (2004)
  3. General requirements for the competence of testing and calibration laboratories. Retrieved from (ND)
  4. PFAS Contamination of Drinking Water Far More Prevalent Than Previously Reported. Retrieved from Environmental Working Group (January 22, 2020)
  5. Lead Levels Below EPA Limits Can Still Impact Your Health. Retrieved from National Public Radio (August 13, 2016)
  6. ‘Down to Earth’ teaches tips for finding the best bottled water. Retrieved from the Chicago Tribune (August 24, 2020)