Frequently Asked Questions

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.

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)

“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. For more information on this topic, please see our FAQ page.

Reasons for varied readings include:

Ions:The nature of charged positive ions (which is what the TDS meters are measuring) is that they are always moving. Therefore, there may always be variances in the conductivity, and thus a different reading.

Temperature:Even with ATC, temperature changes by a tenth of a degree may increase or decrease the conductivity. Additionally, the temperature coefficient (what the reading is multiplied by to adjust for temperature differences) changes slightly depending upon the range of ppm. Our meters and virtually every meter under $500 has a single temperature coefficient, regardless of the range. (The new COM-100 offers three temperature coefficient options, but each is linear once selected.)

Air bubbles:Even a tiny air bubble that has adhered to one of the probes could potentially affect the conductivity, and thus the reading.
Lingering electrical charges:Electrical charges off fingers, static eletricity off clothes, etc. on the meter and lingering electrical charges in the water will affect the conductivity of the water.

Beaker/cup material:Plastic cups retain lingering electrical charges more than glass. If the meter touches the side of the glass or plastic, it could pick up a slight charge. If the plastic is retaining a charge, it could also affect the water.

Volume changes:The amount of water in the sample may affect the conductivity. Different volumes of the same water may have different levels of conductivity. Displacement may affect the conductivity as well.

Probe positioning:The depth and position of the probe in the water sample may also affect the conductivity. For example, if a meter is dipped into the water, removed and then dipped into the water again, but in a different spot, the reading may change

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.

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.

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).

A parameter is the characteristic being measured. A scale is a particular range applied to the measurement of that parameter. For example, temperature is a parameter. Fahrenheit or Celsius is a scale

“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.