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HM DIGITAL is rapidly becoming the industry leader and premier source for the finest quality water testing instrumentation.

HM DIGITAL's handheld and portable TDS, EC and pH testers, in-line monitors and controllers for reverse osmosis (RO) water filters and water purification systems are superior in construction and performance, yet made available at a very economic cost. HM DIGITAL, Inc. is the only manufacturer of economical in-line dual TDS meters that measure total dissolved solids (TDS) levels of feed and product water lines simultaneously, an effective way to compare rejection rates and gauge reverse osmosis membrane, water filter and water purification performance.

With HM DIGITAL water testing instruments, you'll know if your water is pure.

The following information about TDS and Water Quality is provided by HM DIGITAL:

What Are Total Dissolved Solids?
Total Dissolved Solids (TDS) are the total amount of mobile charged ions, including minerals, salts or metals dissolved in a given volume of water, expressed in units of mg per unit volume of water (mg/L), also referred to as parts per million (ppm). TDS is directly related to the purity of water and the quality of water purification systems and affects everything that consumes, lives in, or uses water, whether organic or inorganic, whether for better or for worse.

"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.

Where Do Dissolved Solids Come From?
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 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 purifications 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.

Why Should You Measure the TDS Level in Your Water?
The EPA Secondary Regulations advise a maximum contamination level (MCL) of 500mg/liter (500 parts per million (ppm)) for TDS. Numerous water supplies exceed this level. When TDS levels exceed 1000mg/L it is generally considered unfit for human consumption. A high level of TDS is an indicator of potential concerns, and warrants further investigation. Most often, high levels of TDS are caused by the presence of potassium, chlorides and sodium. These ions have little or no short-term effects, but toxic ions (lead arsenic, cadmium, nitrate and others) may also be dissolved in the water.

Even the best water purification systems on the market require monitoring for TDS to ensure the filters and/or membranes are effectively removing unwanted particles and bacteria from your water.

The following are reasons why it is helpful to constantly test for TDS:

  • Taste/Health - High TDS results in undesirable taste which could be salty, bitter, or metallic. It could also indicate the presence of toxic minerals. The EPA's rescommended maximum level of TDS in water is 500mg/L (500ppm).
  • Filter performance - Test your water to make sure the reverse osmosis or other type of water filter or water purification system has a high rejection rate and know when to change your filter (or membrane) cartridges.
  • Hardness - High TDS indicates Hard water, which causes scale buildup in pipes and valves, inhibiting performance.
    Aquariums/Aquaculture - A constant level of minerals is necessary for aquatic life. The water in an aquarium or tank should have the same levels of TDS and pH as the fish and reef's original habitat.
  • Hydroponics - TDS is the best measurement of the nutrient concentration in a hydroponic solution.
    Pools and spas - TDS levels must be monitored to prevent maintenance problems.
  • Commercial/Industrial - High TDS levels could impede the functions of certain applications, such as boilers and cooling towers, food and water production and more.
  • Colloidal silver water - TDS levels must be controlled prior to making colloidal silver.
  • Coffee and Food Service - For a truly great cup of coffee, proper TDS levels must be maintained.
  • Car and window washing - Have a washer with a spotless rinse? An inline dual TDS monitor will tell you when to change the filter cartridge or RO membrane.

What should the TDS level of my water be?
There is no specific level nor right or wrong answer to this question.  Generally speaking, for drinking water, a lower level of TDS (purer water) is preferred.  The U.S. EPA, all U.S. states, the World Health Organization (WHO) and most nations put maximum limitations on TDS allowed in drinking water.  These limitations are typically 500 or 1000 ppm, but they do vary.  There is no known minimum.

Besides drinking water, a TDS level is specific for each application and particular usage.  Though humans prefer purer water for their health, fish and plants, for example, require water with widely varying TDS levels, most of which are higher than healthy human drinking water.  If you are using a meter to test the water pertaining to a particular device, object or operation, contact the manufacturer of that object. For example, if you are using the meter to test the efficacy of a water filtration system, contact the manufacturer of that system for preferred TDS levels. If you are testing the water for a pool, plants, fish, etc. contact a specialist for your specific application, or the manufacturer of additives or nutrients.

How do I care for my TDS meter?
Please see the Calibration and Maintenance page.

Why do I experience different readings in the same water with the same meter?

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.

How can I get the best possible readings?
Shake: .Always make sure to shake excess water off the meter before dipping it into a water sample, even if it's the same water.
Stir/tap: .After dipping the meter in the water, always lightly tap it against the side and stir the meter to remove any lingering air bubbles or electrical charges.
Positioning: .When taking the reading, always make sure to hold the meter straight up without it touching the sides or bottom of the glass/beaker/cup. The probes should be suspended as close to the center of the water sample as possible.
Time: .The longer the meter is in the water, the more accurate the reading will be.
Temperature: .25 degrees Celsius is the ideal temperature for conductivity readings, even if the meter has ATC.
Rinse: If switching between very low and very high ppm water, always rinse the probes with distilled water to avoid any build-up.

Are TDS meters really conductivity meters?
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.

Most HM Digital TDS meters other than the COM-100 use the NaCl conversion factor (avg. 0.5).  Some products are available with the 442 conversion factor.

Is pinpoint accuracy always necessary when testing for TDS or conductivity?
Usually not. TDS is primarily about range. For the majority of industries that require TDS testing, such as drinking water, aquaculture, hydroponics, etc. it is more important for your TDS levels to be within a certain range. There are a few industries that do require a precise ppm level, but that level is almost always zero. With the exception of colloidal silver, there is never a time in which someone needs an absolute precise level of TDS in their water.

What is the difference between a parameter and a scale?
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.

Is "EC" a parameter or 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.

Don’t you need the minerals in your drinking water?

Inorganic Minerals
  • It is believed that mineral waters help furnish elements for body metabolism. However, there is scientific proof to suggest that most of these minerals are in an inorganic (dead) form. While they may enter the circulation, they cannot be used in the physiological process of building the human cell.
  • With this in mind, we can see that mineral water may give "dead" or "inorganic" minerals to the body which cannot be properly assimilated.
  • These inorganic minerals only interfere with the delicate and complex biology of the body.
  • The body's need for minerals is largely met through foods, NOT DRINKING WATER." -The American Medical Journal
  • Fact: The organic minerals in tap water represent only 1% of the total mineral content of the water.
  • One glass of orange juice contains more beneficial minerals than thirty gallons of untreated tap water.

 Organic or Bioavailable Minerals

  • Only after they have passed through the roots of plants do these inorganic minerals become organic (through photosynthesis) and capable of being assimilated into our tissues as ORGANIC Minerals.
  • Pure water removes the inorganic mineral deposits in your body. Organic minerals are fully absorbed and remain in your tissues.
  • According to many nutritionists minerals are much easier to assimilate when they come from foods. Can you imagine going out to your garden for a cup of dirt to eat rather than a nice carrot; or drinking a whole bathtub of water for LESS calcium than that in an 8 ounce glass of milk?