How to Select the Best TDS Meter for Drinking Water in 2023

TDS stands for Total Dissolved Solids and represents the combined content of all substances in a liquid that can be measured in molecular, ionized or micro-granular suspended forms.

Simply put, TDS refers to anything in water apart from the H20 molecule itself. These can include dissolved salts, minerals, compounds, or metals found in a given volume of water.

Because many of these dissolved salts, metals, and minerals are considered to be contaminants, TDS is directly related to the quality of water as well as the quality of water purification systems. And because many of the dissolved molecules in water are salts, the TDS concentration in a given unit volume of water can be given as the total amount of negatively-charged ions (anions) and positively-charged ions (cations) found in water.

To better understand TDS, it is important to understand its relationship with EC and PPM.

Electrical Conductivity measures the ability of water to conduct an electrical charge. A solution is only capable of conducting electricity when it contains dissolved negatively-charged and positively-charged ions. Since charged ions are directly related to the minerals contained in the water, water which lacks the impurities and minerals is therefore unable to conduct electrical charges. Water that is absent of any of minerals is known as pure water.

Electrical conductivity and TDS concentration are positively correlated as the presence of an electrical charge indicates high TDS concentration due to dissolved salts, minerals and/or metals in the water.

However, while it is true that EC is a good indicator of the salinity of water (meaning the presence of TDS), it is still inaccurate when it comes to the ion composition of the water. Also, some elements considered to be fertilizers (like urea) do not add to the EC of a solution.

Parts Per Millions, abbreviated as PPM, is the unit of measurement used by TDS meters to display the concentrations of substances and indicates 1 milligram of dissolved solids per kilogram of water.

A TDS meter converts the EC reading into a ppm reading using a conversion factor.

The relationship between TDS and EC can be displayed as the following equation:

TDS = keEC

Where ke is the correction factor which usually varies between 0.4 and 1.0, and is unique to each salt. It is therefore important to know the type of salt(s) in the solution to enable you to use the accurate correction factor. Results are displayed in ppm.

Water from natural sources, industrial wastewater, and water run-off all contain dissolved chemicals and therefore have a certain level of TDS. When rainwater hits the ground, it dissolves minerals, salts and metals in the soil (such as iron, potassium, and sodium) which become part of the water supply. The chemicals used to treat water are also a major source of TDS as are the hardware and piping used to distribute treated water to us. Mineral springs, salt used for de-icing of roads, agricultural run-off, and carbonate deposits are all sources of TDS.

Most of the TDS found in water include microorganisms such as bacteria and viruses which are organic compounds founds in water. Phytoplankton, which is a natural water flora can also be found in the water supply. Others include salts, heavy metals pharmaceutical chemicals used to make drugs which are deposited into the water supply via human and animal waste.

A TDS meter is used to measure level of dissolved solids in the water supply before determining what methods are needed for proper treatment.

The total acceptable level of Total Dissolved Solids (TDS) concentration in potable water, as set by the Environmental Protection Agency (EPA) is 500 ppm. It is important to note that parts per million (ppm) can also be expressed as milligrams per unit volume of water (mg/L).

It is therefore important to measure the level of TDS in water before ingestion as a majority of water supplies exceed this safe level. TDS levels greater than 1000 ppm are generally considered unsafe for human consumption as a high TDS level indicates potential water concerns, which is a call for concern and investigation.

Usually, high TDS levels are associated with high levels of chlorine, potassium and sodium ions in water which generally have no little or no short-term effects. However, toxic ions (such as nitrate, cadmium and lead arsenic among others) can also be present and these have long-lasting negative effects. That is why even the best water purification systems available today still make use of TDS concentration levels to test the efficiency of membranes and/or filters.

TDS meters are generally an inexpensive means of measuring TDS in water. Most TDS meters are hand-held and similar to thermometers. Because of their flexibility, they can be used to measure filtered water and tap water. It is pretty straightforward to use as once its measurement sensors are placed into the water, it displays a notification immediately which carries information about the mineral content (TDS) of the water. It is advised that TDS meters be given time – around 10 seconds – to appropriately calculate the EC of the water.

However, as previously mentioned, TDS meters do not actually measure the level of TDS in water. What they do is measure the Electrical Conductivity (EC) – the number of positively and negatively charged ions (electrolytes) in water. Because pure water has an EC of zero, anything above zero indicates the presence of dissolved solids in the water. Salt, which is the most frequent dissolved solids in water is an electrolyte and can increase the EC of water. Sugar, on the other hand, is not an electrolyte and will therefore not increase the EC of water. Because of this, TDS is usually considered an estimate of true TDS.

Also, because water from different sources will contain different quantities of dissolved solids or substances, TDS meters make use of conversion factors, which usually range from 0.4 to 1.0. These conversion factors are used to convert EC to the estimated TDS level. As a general rule of thumb, higher EC means a higher conversion factor.

TDS meters can be a very efficient test of Reverse Osmosis (RO) membranes. The procedure for using TDS meters to test the efficiency of RO membranes is as straightforward as the process of using these meters to test TDS concentrations levels in general.

To use your TDS meter to test the efficiency of RO membranes, simply measure the TDS concentration of your tap water or water before reverse osmosis. Once that has been done, save the number you see on the display. Then, measure the TDS for the water after it has gone through the RO membrane.

Now, compare the two TDS concentrations. Normally, the TDS level of the water after reverse osmosis should be about ten times lower than that of your tap water. So, if your tap water gives you a TDS level of 500 ppm (mg/L), then your reverse osmosis water should give you a TDS level of 25 ppm (mg/L) or less. By doing so, you are measuring the effectiveness of the RO membrane at reducing the amount of TDS in the tap water. A reverse osmosis water TDS level which is ten times lower than your tap water’s TDS level means your RO membrane is functioning properly. Read our full article explaining how home reverse osmosis systems work.

The TDS Meter by HoneForest test for TDS, EC, as well as water temperature. It features automatic temperature compensation for accurate reading and an auto-off function after two minutes of inactivity. Factory calibrated and offers a 3 year warranty. Measures from 0-9999ppm and is ideal for drinking water, hydroponics, aquariums, and reverse osmosis systems.

HM Digital's TDS meter measures from 0-9999ppm and is highly accurate at 3% readout accuracy. Featuring an auto-off function and is factory calibrated with 342 ppm NaCl solution. Long battery life and bright LCD screen.

The Health Metric TDS meter tests for TDS, EC, and temperature with automatic temperature compensation. Fully factory calibrated with LCD screen and auto-off function. Ideal for testing reverse osmosis membranes.

The MagicPro TDS meter measures 0-9999ppm and is factory calibrated with a 342 ppm NaCl solution. Meter shuts off automatically after 10 minutes of non-use to conserve batteries. Highly efficient and accurate due to its advanced microprocessor technology.

The Digital Aid TDS measures TDS at 0-9999ppm and tests for TDS, EC, and water temperature. Featuring a fully-lit LCD screen and auto-off function, this TDS meter is factory calibrated. 100% lifetime guarantee.

The Sunny Digital TDS Meter features accurate, reliable testing for TDS, EC, and temperature with automatic temperature control. Automatic shut-off for battery longevity. Factory calibrated and offering a lifetime guarantee.

TDS Meters are useful tools in determining the amount of dissolved solids in your water. While TDS meters don't give you an actual reading of the dissolved solids, it measures the electric conductivity in the water, which is a representation of the amount of particles present. TDS meters are practical for aquariums, hydroponics, water purification, and for testing reverse osmosis systems.