TDS meters are a handy new way to evaluate water quality. Inexpensive, reliable and straightforward to use, these must-have digital devices measure contaminant levels in seconds. But while TDS meters offer a glimpse of what may be lurking in your cup, they don’t tell the whole story. Let’s take a closer look at their capabilities, limitations, and how they fit into your comprehensive water safety plan.

First, it’s important to know what TDS really is: Total Dissolved Solids. TDS in water refers to the amount of total dissolved solids in your tap water.

After water falls to the ground as rain, it dissolves the minerals present in the rocks and soil it passes over or through. As it dissolves these minerals, they remain in the water at varying levels of concentration. This is a very natural process and it helps make water taste “right” by slightly raising the pH of the water. The most common minerals found in water are calcium, magnesium, and sodium.

Some areas of the country have higher levels of these dissolved minerals in their water making it “hard” and other areas have lower levels. According to the Water Quality Association, water in the 120 mg/L (or ppm) and above range is considered hard, with 180 mg/L being considered very hard. Water in the 80 to 100 mg/L range is ideal, meaning it’s not too hard and has the right amount of dissolved minerals for optimal taste. As the TDS level goes below 17 mg/L, (as the minerals are removed) it becomes “soft”, with a lower pH and more acidity.

In addition to inorganic materials like minerals, TDS also features a small amount of organic material from sources such as leaves, sewage, and waste as well.

Ironically, a TDS meter does not initially measure TDS, which is where much of the confusion arises. TDS meters, also known as TDS testers or indicators, are digital or analog meters that measure the electrical conductivity of water. Based on that conductivity, the meters estimate what the true TDS level might be.

As stated previously, TDS is essentially anything dissolved in the water other than the pure H2O. Because hydrogen and oxygen conduct virtually no electricity, and most other elements do, the conductivity measurement is a fair indicator of the overall water purity.

In reality, a TDS meter measures the electrical conductivity of water or, in other words, the total amount of mobile charged ions found in water. If an element is dissolved in water and can conduct electricity, it is called an electrolyte. Salt, for example, is an electrolyte. Sugar, however, is not and will therefore not register on a conductivity or TDS meter. This is one reason why a TDS meter is only an estimate of the true TDS.

Additionally, different types of water will contain different quantities of certain dissolved substances. For example, seawater will naturally have far more salt than freshwater. To accommodate this difference, TDS meters will incorporate conversion factors, typically ranging from 0.4 to 1, to convert the conductivity to the estimated TDS level. As a general rule of thumb, the higher the conductivity, the higher the conversion factor.

Although the measurement on a TDS meter is an estimate, it is reasonably accurate. When simplicity, accessibility and cost are taken into account compared with a precision analytical balance, a TDS meter is more than sufficient for most applications.

A common question that arises is: “At what TDS level should tap water be?” There is no right or wrong answer to this question because the TDS levels of tap water fluctuate widely from location to location and from day to day. Pipe age can affect TDS levels, as can precipitation (or lack thereof).

Neither the U.S. federal government nor any of the states set minimum TDS levels on water; however, maximum levels are set. Some of these levels are enforceable, while others are only recommendations. The U.S. Environmental Protection Agency includes TDS on its list of National Secondary Drinking Water Regulations (NSDWR) and sets a limit of 500 mg/L; however, the NSDWRs are nonenforceable guidelines.

Besides drinking water, TDS affects anything that consumes, lives in or uses water. For example, high levels of TDS will result in excessive scaling in pipes. On the other hand, low levels of TDS may be unhealthy for plants and fish.

The measurement of TDS in water is also extremely important for certain pharmaceutical, manufacturing, industrial, medical and agricultural applications, to name a few.

With a number of economically priced, accurate and easy-to-use TDS meters available on the market, there is no reason not to use one.

A TDS meter does not measure contaminants. It will not tell you if your water is healthy. The TDS meter reading simply refers to the amount of total dissolved solids in your water, it will not tell you what those solids actually are and if they are harmful for your health.

For example, you could have a high TDS meter reading but no harmful contaminants in your water. Some healthy minerals like potassium, magnesium, and calcium can actually cause your TDS meter to spike. So, any water filter with a remineralizer could cause a high reading on your TDS meter, even though the remineralizer is adding good minerals that you want.

On the other hand, you could have a low TDS level even if several harmful contaminants are present in your drinking water. Your water could have heavy metals like lead, or other contaminants like pesticides, pharmaceuticals, or hexavalent chromium. The TDS meter will not pick these contaminants up.

This is why you shouldn’t rely on a TDS meter to determine your water quality. You need to understand not just how many dissolved solids are in your water, but what those solids are and if they’re harmful for your health.

TDS meters aren’t a substitute for water testing, but they can play an important role in monitoring your home water quality.
Since most dissolved solids are likely to be minerals, gardeners benefit by knowing what they’re watering plants with. Higher TDS levels are beneficial for most flowers and vegetables. Fans of alkaline water and its many health benefits are also happy to know what they’re drinking.

Similarly, checking levels in fish tanks improves the habitat and keeps testing costs under control. TDS concentrations above 300 mg/L may reduce water clarity and decrease photosynthesis in sensitive aquatic plants. It can also contribute to irregularities in aquarium temperature and digestive problems in fish. Aquarium filters only remove Total Suspended Solids(TSS).

Once tested, aquarists can test tap water with a meter before adding it to the tank, blending it with purified water to achieve an ideal TDS balance while saving more expensive test strips only to troubleshoot specific substances. It’s a money saver.

Finally, it’s not unusual for shallow wells to become contaminated with solids in areas prone to flooding. A quick check with a TDS meter can tell you if floodwaters have infiltrated your well and further testing is required. Meanwhile, you can keep your family safe by switching to bottled.

Meters can also evaluate municipal water any time there’s an unexpected change. A water main break, for example, may cause a sharp rise in dissolved solids that can carry potentially dangerous contaminants with them.

If you can only afford a water test or a TDS meter, choose the test. But for less than $20, a meter offers enough peace of mind to be a worthwhile investment.

If you already have a TDS meter, you’re a step ahead in managing your water safety. But because older, less sensitive models detect fewer dissolved solids with low electrical charges, you’ll benefit from an upgrade to more advanced technology if your meter is more than two years old.