Water (H2O) is essential to life on Earth, and more than two-thirds of the planet’s surface is made up of water in its solid, gas, and liquid forms—the only chemical compound on Earth that naturally exists in all three physical states. The properties of liquid water help dissolve molecules, which enable chemical reactions to occur, yielding friendly conditions for life. Vital substances like metabolites and nutrients are able to travel through liquid water microscopically as well as globally. Water also greatly assists enzymes in catalyzing chemicals, which is also essential for life. Pure water freezes at 32°F (0°C) and boils at 212°F (100°C), although impurities can alter these figures.

This wide temperature range as a liquid also helps preserve life on Earth since typical fluctuations in the weather do not have cataclysmic effects on the planet, which is mostly covered with water.The human body is also mostly made up of water, and we depend on water to grow food and survive. The global modern industrial infrastructure requires water for a wide range of things, from cooling down machinery to the use of water for transportation and shipping. An uncountable number of organisms live in bodies of fresh water, seawater, or brine, whether they are puddles or oceans. Water is indeed one of Earth’s indispensable resources in creating and maintaining life.

Water is one of the most important aspects of life on Earth, and yet so many of us take it for granted. Without water, life on Earth would cease to exist. Plants, animals and human beings all require water. It is, in a sense, life’s battery. Water can also be detrimental, though, if not kept clean and potable.

The dichotomy of its existence…it is a life giver and also a life taker. It is for this reason that humans have been working on methods of obtaining pure water since the beginning of human existence. Archeological evidence shows that people have been attempting to purify water and give it a pleasurable taste since prehistoric times.

Early humans believed that the purity of water was determined by taste; if the water had a pleasing taste, then it was pure. As a result, the method of purifying water in ancient times was to add herbs or flowers to the water, but this was obviously not a true method of purification, nor was it enough to quell the search for one.

In 2000 B.C., Sanskrit documents about medical concerns were created called the Sus’ruta Samhita. The Sus’ruta Samhita declares that “impure water should be purified by being boiled over a fire, or being heated in the sun or by dipping a heated iron into it, or it may be purified by filtration through sand and coarse gravel and then allowed to cool.”

Inscriptions have been discovered on the walls of the tomb of Amenophis II in Thebes. The inscription depicts the Egyptian method of purifying water in which they siphoned the water through a series of wick siphons. The inscription has been dated 1450 B.C.

The Bible even has evidence of possible methods of water purification. In Exodus 15:22-27, Moses and the Israelites came upon Marah and found that the waters there were bitter. Moses was guided towards a tree and told to place the tree into the waters of Marah. Moses did as he was instructed and the waters of Marah were sweetened thereafter. While it is unclear what type of tree this was, or if any type of filtration process was used, the evidence points to at least a concern about water quality.

Another early method of filtering water was developed by the famous Greek doctor, Hippocrates, in the 3rd century B.C. Recognizing that boiling water did not remove suspended solids, Hippocrates used a cloth bag to strain the water after boiling it. This method was later called “Hippocrates’ Sleeve.”

In 1627, Sir Francis Bacon compiled 10 experiments in A Natural History of Ten Centuries. He was led to believe that water could be filtered through sand when he read about a successful experiment purifying seawater in this manner.

In the late 1600s, Lucas Antonius Portius, an Italian physician, wrote about the multiple sand filtration method. The method had 3 pairs of sand filters consisting of downward and upward flow.

Following these discoveries, sand filters and rainwater cisterns were developed. La Hire, a French scientist, proposed in 1703 that all households should have a rainwater cistern along with a sand filter. 100 years later, the first municipal water treatment plant was installed in Paisley, Scotland.

In the 20th and 21st centuries, water quality has become even more important and numerous methods of water purification and filtration have been developed.

Additional information on current methods of water filtration and purification are available in the Education Center and What is TDS? section.

When we use water, we generally add contaminants to it, such as soap, food products, and chemicals, which must be removed before the water is used again.Close to 3/4 of the Earth’s surface is covered with water, but less than 1% is suitable and available for drinking using conventional water treatment. Ice cubes float because ice is less dense than water. Water freezes in a lattice-like formation, which creates buoyancy and allows ice to float.

Hardness in drinking water is caused by calcium and magnesium—two non-toxic, naturally occurring minerals in water. Excessive hardness makes it difficult for soap to lather, leaves spots on dishware, and reduces water flow. Water is the original health drink. It contains no fat, no calories, and no cholesterol. Because 60% of an adult’s body is water, it is essential to replenish the water you lose through breathing, perspiration, and excretion. For most people, this equates to approximately 8 cups (2 liters) a day.

We can consume water not only by drinking water but also through food and other beverages. Through the processes of evaporation, condensation, precipitation, and infiltration—the hydrologic cycle—the total amount of water on Earth remains constant. The availability of fresh drinking water, however, continues to diminish as demand continues to increase.

Yes. While EC and TDS are often used synonymously, there are some important differences to note. EC, when applied to water, refers to the electrical charge of a given water sample. TDS refers to the total amount of substances in the water other than the pure H2O. The only true way of measuring TDS is to evaporate the water and weigh what’s left. Since this is near impossible to do for the average person, is it possible to estimate the TDS level by measuring the EC of the water. Every digital TDS meter in the world is actually an EC meter.

All elements have some electrical charge. Therefore, it is possible to closely estimate the quantity of TDS by determining the EC of the water. However, since different elements have different charges, it is necessary to convert the EC to TDS using a scale that mimics the charge of that water type. The following are the most common water samples, and for the COM-100, each has its own conversion factor:

KCl:Potassium Chloride is the international standard to calibrate instruments that measure conductivity. The COM-100 is factory calibrated with a 1413 microsiemens solution is the default mode is EC-KCl. The KCl conversion factor is 0.5-0.57.

442TM:Developed by the Myron L Company, 442TM simulates the properties of natural water (rivers, lakes, wells, drinking water, etc.) with a combination of 40% Sodium Bicarbonate, 40% Sodium Sulfate and 20% Chloride. The 442 conversion factor is 0.65 to 0.85.

NaCl:Sodium Chloride is used in water where the predominate ions are NaCl, or whose properties are similar to NaCl, such as seawater and brackish water. The NaCl conversion factor is 0.47 to 0.5.

Measurements in EC (µS) do not have a conversion factor, but do require the correct setting for the proper temperature coefficient.

“Dissolved solids” refer to any minerals, salts, metals, cations or anions dissolved in water. This includes anything present in water other than the pure water (H20) molecule and suspended solids. (Suspended solids are any particles/substances that are neither dissolved nor settled in the water, such as wood pulp.)
In general, the total dissolved solids concentration is the sum of the cations (positively charged) and anions (negatively charged) ions in the water.

Parts per Million (ppm) is the weight-to-weight ratio of any ion to water.
A TDS meter is based on the electrical conductivity (EC) of water. Pure H20 has virtually zero conductivity. Conductivity is usually about 100 times the total cations or anions expressed as equivalents. TDS is calculated by converting the EC by a factor of 0.5 to 1.0 times the EC, depending upon the levels. Typically, the higher the level of EC, the higher the conversion factor to determine the TDS. NOTE – While a TDS meter is based on conductivity, TDS and conductivity are not the same thing.

Any of HM Digital’s TDS meters can be used to test for salt (up to the maximum range of the meter). Salt is a part of Total Dissolved Solids and therefore will be part or all of the reading. If you are first filtering the water, and then adding salt, simply use the meter as you would under any circumstances. If there is only salt in the water, and the reading is 2500 ppm, then the it is 2500 ppm (mg/L) of salt. If you are starting with tap water and filling a pool, for example, prior to adding salt, then first test the level of your tap water. Therefore, if your tap water is 200 ppm, and your pool needs to be 3500 ppm of salt, then add 3300 ppm of salt. (A small portion of the tap water TDS may be salt.)

Water softeners do not remove TDS. Instead, water softeners work through a process of ion exchange. As water flow through the water softener, it will pass through a resin, bed of small plastic beads or chemical matrix (called Zeolite) that will exchange the calcium and magnesium ions with sodium ions (salt). Therefore, the TDS level will remain virtually constant (there may be minor differences).

The best thing to do is use a TDS meter, which will automatically do the conversion. EC meters do not use conversion factors because there is no conversion. To convert to TDS, if you do not wish to use a TDS meter, you will need to determine which conversion factor you want to use (NaCl, 442 or KCl) and do the math.

The best thing to do is use an EC meter. If you know which conversion factor your meter uses, you can do the math. Most HM Digital meters use the NaCl conversion factor, which is an average of 0.5. Therefore, if you are using a TDS meter with the NaCl conversion factor, multiply the reading by two, and this will get you a close approximation of the EC level.

Most HM Digital TDS meters use the NaCl EC-to-TDS conversion factor, which is an average of 0.5. Some HM Digital meters, such as the COM-100, have selectable conversion factors, so you can choose which one you want to use. For specific meters, please contact HM Digital.

TDS (Total Dissolved Solids) levels in water tell you how many dissolved substances, like minerals, salts, and metals, are present. Here’s why it’s important to measure:

1. Water Quality: High TDS levels might indicate your water isn’t as clean as it should be. Too many dissolved solids can affect the taste and safety of the water.
2. Health: Some minerals in water are good for you, but too much of certain substances (like lead or arsenic) can be harmful.
3. Appliances: High TDS levels can cause scale build-up in kettles, pipes, and other appliances, reducing their lifespan.
4. Taste: Water with high TDS might taste salty, bitter, or metallic, which can be unpleasant.

In short, checking TDS helps you ensure your water is safe, tastes good, and won’t damage your appliances.

Some dissolved solids come from organic sources such as leaves, silt, plankton, and industrial waste and sewage. Other sources come from runoff from urban areas, road salts used on the street during the winter, and fertilizers and pesticides used on lawns and farms.

Dissolved solids also come from inorganic materials such as rocks and air that may contain calcium bicarbonate, nitrogen, iron phosphorous, sulfur, and other minerals. Many of these materials form salts, which are compounds that contain both a metal and a nonmetal. Salts usually dissolve in water, forming ions. Ions are particles that have a positive or negative charge.
Water may also pick up metals such as lead or copper as they travel through pipes used to distribute water to consumers.
Note that the efficacy of water purification systems in removing total dissolved solids will be reduced over time, so it is highly recommended to monitor the quality of a filter or membrane and replace them when required.

Common water filter and water purification systems: carbon filtration Carbon filtration cleans water by passing it through activated carbon, which traps bad tastes, smells, and harmful chemicals like chlorine. It's commonly used in home water filters and treatment plants. While great for removing certain impurities, carbon filters often need to be paired with other filters to fully purify water. Reverse osmosis (R.O.)Reverse osmosis works by forcing water under great pressure against a semi-permeable membrane that allows water molecules to pass through while excluding most contaminants. RO is the most thorough method of large-scale water purification available. Distillation involves boiling the water to produce water vapor. The water vapor then rises to a cooled surface where it can condense back into a liquid and be collected. Because the dissolved solids are not normally vaporized, they remain in the boiling solution. Deionization (DI)Deionization is a method for clearing water by taking out minerals and salts, making it very pure. It works by passing water through special materials that trap the unwanted particles and replace them with pure water. This super clean water is important in places like labs, medicine, electronics, and cosmetics because even tiny impurities can cause issues. The material that cleans the water needs to be refreshed or changed after some time to keep the water pure.

Any of HM Digital’s TDS meters can be used to test for salt (up to the maximum range of the meter). Salt is a part of Total Dissolved Solids and therefore will be part or all of the reading. If you are first filtering the water, and then adding salt, simply use the meter as you would under any circumstances. If there is only salt in the water, and the reading is 2500 ppm, then the it is 2500 ppm (mg/L) of salt. If you are starting with tap water and filling a pool, for example, prior to adding salt, then first test the level of your tap water. Therefore, if your tap water is 200 ppm, and your pool needs to be 3500 ppm of salt, then add 3300 ppm of salt. (A small portion of the tap water TDS may be salt.)

Though there is a close relationship between TDS and Electrical Conductivity, they are not the same thing. Total Dissolved Solids (TDS) and Electrical Conductivity (EC) are two separate parameters.

TDS, in layman’s terms, is the combined total of solids dissolved in water. EC is the ability of something to conduct electricity (in this case, water’s ability to conduct electricity).

The only true method of measuring TDS is to weigh residue found in water after the water has evaporated. You know those spots you see on a glass after you wash it and let it air dry? That’s TDS! That residue has mass, and it’s possible to weigh it, but if you’re not in a lab, it can be tricky thing to do. Therefore, we can estimate TDS levels based on the conductivity of the water since the hydrogen and oxygen molecules of the H2O carry almost no electrical charge. The EC of most other metals, minerals and salts will carry a charge. A A TDS meter measures that EC level and then converts it to a TDS measurement. Since different metals, minerals and salts will be more or less conductive than others, there are different conversion factors that can be used.

ppm (parts per million) is the most commonly used scale to measure TDS (Total Dissolved Solids).


µS (micro-Siemens) is the most commonly used scale to measure EC (Electrical Conductivity).


TDS and Conversion Factors

EC: There is no conversion for electrical conductivity. (NOTE: The three EC modes in the COM-100 differ only in their ATC programs. The standard EC mode is KCl.)


TDS – NaCl: 0.47 to 0.50
TDS – 442: 0.65 to 0.85
TDS – KCl: 0.50 to 0.57


(NOTE: Most HM Digital meters use the NaCl factor. The COM-100 has the above three modes, which are user-selected. When converting EC to TDS, the COM-100 uses the non-linear scales, as they would occur in nature, thereby giving you more accurate readings than meters that use linear scales.)

Converting between different scales

PPM à µS: The conversion factor of the TDS meter must be known. Once known, the conversion factor should be multiplied by the TDS level. (NOTE: For the COM-100, simply change the mode on the meter. There is no math required.)

PPM à PPT: Divide by 1000 (1000 ppm = 1 ppt)
µS à mS: Divide by 1000 (1000 µS = 1 mS)

The cap is for storage and protection only. For best results, use a larger beaker, cup, glass, etc., so there is a larger volume of water that will be tested. Additionally, to ensure a long lifespan of your product, the TDS/EC sensors should be stored dry.

“EC” is a parameter. It stands for Electrical Conductivity. There are a number of scales used in EC, most commonly micro-Siemens (µS) or milli-Siemens (mS). For example, if a particular application calls for water with “2.0 EC,” this is an incorrect determination. Most likely, the application is calling for an EC level of 2.0 mS. 2.0 mS = 2000 µS.

The symbol ‘µ‘ is not a lowercase U, but the Greek letter Mu. It is the abbreviation for micro, and when used with an S (µS) it stands for mirco-Siemens, which is a scale used for measuring EC.

For best results, always make sure the sensor is saturated in the storage solution solution (included in the cap and the extra bottle). For best results, store the meters standing upright to ensure full saturation. Rinse the sensor in distilled water after each use, especially if testing high TDS water and liquids other than water. For best results, store the meters standing upright to ensure full saturation. Calibrate frequently.

All pH and ORP meters need to be calibrated frequently. For best results, HM Digital recommends calibrating the PH-200 and ORP-200 a minimum of at least once per month. This could vary depending upon frequency and type of usage. For example, if you are testing a wide range daily, for the most accurate measurements, you should be calibrating your meter daily.

A pH meter should be calibrated as close as possible to the level that will be tested. The most common pH buffers are 4.0, 7.0 and 10.0. If you are testing a range, then you should calibrate in the middle of that range.

An ORP meter should be calibrated as close as possible to the level that will be tested. However, a negative ORP buffer does not exist in nature. The reason for this is because the moment it is exposed to air, the millivoltage will begin to change, affecting the reading. Therefore, an ORP meter can only be calibrated to a positive ORP level. If testing negative levels, calibrate as low in the positive as possible. If you are testing a range, then you should calibrate in the middle of that range.

Yes, within the stated guaranteed accuracy, and if properly calibrated

In the positive range, yes, within the stated guaranteed accuracy, and if properly calibrated. In the negative range, obtaining pinpoint accuracy is near impossible, since the meter cannot be calibrated to a negative ORP buffer, and also because a negative ORP will naturally begin to change quite quickly. Therefore, looking for a range of values in the negative will produce more effective testing results.

The sensors need to be stored in the proper storage solution. For best results, always make sure the sensor is saturated in the storage solution solution (included in the cap and the extra bottle). For best results, store the meters standing upright to ensure full saturation.

Yes, it is normal. The white residue you see on the cap or sensor is likely salt deposits from the KCl storage solution used to keep the sensor hydrated. This is a common occurrence and does not affect the performance of the sensor. To clean it, simply wipe the residue with a soft cloth or tissue. Avoid using abrasive materials or excessive force, which could damage the sensor or the cap. Always ensure the sensor is stored in the proper KCl storage solution when not in use to maintain its accuracy and longevity.

For both the PH-200 and ORP-200, it is because your meter probably needs to be calibrated, the sensor is dirty or the sensor needs to be saturated in the storage solution. 1. Rinse the sensor in distilled water. 2. Store the sensor in storage solution (with the meter standing upright). 3. Re-calibrate your meter. 4. For the ORP-200, clean the platinum band using a silver polishing strip.

The pH and ORP sensors react off of conductivity. Therefore, they will not be stabilize in distilled or pure water. If you need to use the meters in distilled water, lightly swirl the meter in the water while waiting for the reading to stabilize. It will begin to stabilize after approximately 30 seconds.