What does a TDS device tell you?

What does a TDS device tell you?

e all want to know the contents of what we are consuming. Let’s face it, most of us read food packaging labels because we don’t want to fill out bodies with junk. Why shouldn’t this be the same with the fluids we ingest? What’s more, if you’ve invested in a water purification system, you also want to know that it is working effectively. Shouldn’t you go ahead and test it with a TDS (Total Dissolved Solid) device, then? The simple answer is: No. Read on, to find out why and what does a TDS device tell you exactly?

What is a TDS device?

A TDS measuring device, short for Total Dissolved Solids, is a small, handheld piece of apparatus which can be easily placed into a glass of water (filtered or straight from the tap). Then, it gives you a reading, telling you how many of these solids it has detected in your water sample. Simple! Except, it’s not that straightforward.

While a TDS meter does indeed measure the quantity of dissolved solids such as minerals, salts and metals, it is crucial not to equate such solids with harmful substances. What a TDS does not tell you, is what these solids are – and whether you want them in your water. In fact, many beneficial minerals dissolve in water and are therefore included in what your TDS meter results.

What sorts of solids end up in my water sample?

We’ve all heard the term “mineral water”. When water moves over mountains and across rocks, it passes over a great many minerals. The exact quantity and nature of these depends on the type of land it journeys through. If water runs over rocks such as limestone, chalk or gypsum, then it will contain minerals such as calcium and magnesium. This is a completely natural process and, in fact, it means our water tastes as we expect it to, by slightly altering the pH of what we drink.

These minerals are not detrimental to the human body. In fact, they’re completely essential! The average tap water contains around 3mg of calcium per 100ml. Milk, on the other hand, has around 40 times more than this – and we are all aware of the benefits of giving children a glass of milk, “for the calcium”.

TDS Device

What solids can ‘contaminate’ my water?

Of course, not all dissolved solids are beneficial. Some minerals, such as aluminium, chromium and lead, can also dissolve and contaminate drinking water. The process through which this occurs is far from natural – or desirable. Aluminium salts are sometimes added during the water treatment process, and they’re used to remove microorganisms (another “undesirable” and one which, ironically, will not register on a TDS device!). Aluminium can also seep into the water supply from the pipes used to transport it.

Chromium, too, can find its way into water sources, leaching out of paints, dyes and preservatives. It dissolves easily in water and can therefore be detected by a TDS device. Lead, one of the most harmful solids that can contaminate our water supply, will also produce a high reading on a TDS device. Of course, this really is a contaminant we want to know about, but a TDS device is not the piece of equipment best suited to do this.

Phoenix Gravity Water Filter

Should I use a TDS device on Gravity filtered water?

It is NOT effective using a TDS-meter to “measure” water that has been filtered by either activated carbon and/or ceramic.


Carbon and/or ceramic filter elements do not filter out dissolved solids. The use of a TDS device is only advisable in combination with a Reverse Osmosis water filter system and ZeroWater products. 

Using a counter-top water filter ensures the water you drink is safe and free from a range of harmful contaminants, such as heavy metals and micro-plastics. You can find a wide range of home water filtration devices on our shop page to find one that suits your household or office’s needs.

Be careful testing your filtered water with a TDS device, though. As we have seen, the results do not differentiate between harmful contaminants (which are removed through filtration), and naturally occurring, beneficial minerals. These remain in filtered water, and add to the taste and health benefits of your drink.

It’s a simple trap to fall into: assuming that any dissolved solids in drinking water are detrimental to our health. There is no easy way to tell whether a high reading on a TDS device is due to naturally occurring minerals, or toxic heavy metals – the only way to find this out, would be through laboratory testing.

Many of us naively buy a TDS device to assess our drinking water believing a high reading indicates a contaminated water sample, detrimental to our health and therefore to be avoided. The fact is, while these cheap, handheld devices can tell us about the quantity of dissolved solids in water, they do not tell us what those solids are. It’s a grave assumption to make, that the higher the reading, the worse the quality of the water. This simply isn’t true; many sought-after, vital minerals are found in filtered water – and that is precisely why a counter-top water purification system doesn’t remove them! It does, however, remove harmful contaminants. It’s just that a TDS device is not the tool you need to tell you how many.

How do I test if my water filter is working correctly?

If you want to test your filter elements there are multiple ways of doing so;

  • By smell

Most people buy a water filter to remove the Chlorine from their water. If the filter elements are exhausted it means it no longer filters Chlorine and therefore it smells the same as it came out of your tap.

  • By artificial coloring

Drop 3-4 drops of artificial (red) food coloring in your water filter. If your filter elements are working correctly it should be able to remove the coloring. Make sure it is artificial food coloring.

Important Notice!

In any case, make sure to replace your filter elements adequately as specified by the manufacturer.

Many gravity water filters and its filter elements work by adsorption meaning that harmful contaminents adsorp to the surface of the filter element. As a filter element ages it adsorption surface capacity reduces until a point where it has no surface left to adsorp elements.

This means that harmful elements that previously were adsorpted can now easily pass the filter element. In such cases you risk drinking contamined “filtered” water which most probably is more contamined compared to the unfiltered water you deposited.

Happy filtering!

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