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Frequently Asked Questions

TDS and Conductivity

  • What is TDS?
  • What should the TDS level of my water be?
  • How do I care for my TDS meter?
  • Why do I experience different TDS readings in the same water with the same meter?
  • How can I get the best possible TDS readings?
  • Can I use the cap as a receptacle for testing?
  • Are TDS meters really conductivity meters?
  • Is pinpoint accuracy always necessary when testing for TDS or conductivity?
  • Don’t you need the minerals in your drinking water?
  • How do I use a TDS meter to test for salt in salt generator pools?
  • Can a TDS meter be used for testing water softeners?
  • What is the difference between a parameter and a scale?
  • Is "EC" a parameter or a scale?
  • What's that little µ symbol on my EC meter?
  • How do I convert from EC (µS) to TDS (ppm)?
  • How do I concert from TDS (ppm) to EC (µS)?
  • Which conversion factors do HM Digital meters use?
  • pH and ORP

  • How often should I calibrate a pH or ORP meter?
  • What level should I calibrate my pH meter to?
  • What level should I calibrate my ORP meter to?
  • Should I expect pinpoint accuracy with my PH-200?
  • Should I expect pinpoint accuracy with my ORP-200?
  • Can I use the cap as a receptacle for testing?
  • How do I achieve better negative measurements with my ORP-200?
  • How do I care for my PH-200 or ORP-200?
  • Do the pH and ORP sensors need to be stored wet?
  • There is a white residue on my cap or sensor.  Is this normal?
  • What should I do if I experience unusual readings?
  • Why won't the PH-200 or ORP-200 stabilize in distilled water?
  • My PH-200 won't calibrate.  How do I fix this?
  • What should the pH or ORP level of the storage solution be?
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    TDS AND CONDUCTIVITY (EC)

    What is TDS?
    Please see the What is TDS? page.
    What should the TDS level of my water be?
    There is no specific level nor 'good or bad' 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 for drinking water.
    Besides drinking water, a TDS level is specific for each application and particular usage.  Though humans generally 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.
    Can I use the cap as a receptacle for testing?

    No.  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.

    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.
    What's that little µ symbol on my EC meter?
    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.
    How do I convert from EC (µS) to TDS (ppm)?
    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.
    How do I use a TDS meter to test for salt in salt generator pools?
    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.)
    Can a TDS meter be used for testing water softeners?
    No.  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).
    How do I convert from TDS (ppm) to EC (µS)?
    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.
    Which EC-to-TDS conversion factor do HM Digital TDS meters use?
    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.
    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?

    pH AND ORP

    How often should I calibrate a pH or ORP meter?
    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.
    What level should I calibrate my pH meter to ?
    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.
    What level should I calibrate my ORP meter to?
    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 milli-voltage 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.
    Should I expect pinpoint accuracy with my PH-200?
    Yes, within the stated guaranteed accuracy, and if properly calibrated.
    Should I expect pinpoint accuracy with my ORP-200?
    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.
    Can I use the cap as a receptacle for testing?

    No.  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 pH/ORP sensors should be stored wet, in the proper electrode storage solution.

    How do I achieve better negative measurements with my ORP-200?
    Low ORP (negative) water can be high in minerals that can adhere to the platinum band on the sensor.  If you are finding the ORP-200 to be sluggish or unresponsive in negative ORP water, you may need to clean the platinum band.  To do so, please follow the instructions here: ORP-200 Cleaning Instructions for Negative Measurements
    How do I care for my PH-200 or ORP-200?
    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.
    Do the pH and ORP sensors need to be stored wet?
    Yes.  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. 
    There is a white residue on my cap or sensor.  Is this normal?
    Yes.  The white residue is salt deposits from the storage solution, which is very high in salt concentration.  Simply wipe the salt off with a tissue.  Never touch the sensor glass with your fingers.
    What should I do if I experience unusual readings?
    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.
    Why won't the PH-200 or ORP-200 stabilize in distilled water?
    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.
    My PH-200 won't calibrate.  How do I fix this?
    Occasionally, after usage without regular calibration or in wide ranges, the PH-200 may mis-calibrate to the wrong value.  If this happens, perform a "master reset."  Instructions for the master reset can be found on page 5 of the user's guide, marked as "For Advanced Users Only."
    What should the pH or ORP level of the storage solution be?
    You will not be able to get an accurate read on the storage solution, since there may not be enough, there is a sponge in the cap, and the solution value will quickly become corrupted by other liquids.  You should not attempt to calibrate to the storage solution.  The sole purpose of the storage solution is to keep the electrodes saturated.  For calibration, always use a laboratory-certified pH or ORP buffer.

     

    Education Center

    Our Education Center is your resource for all things water.  This knowledge base includes numerous articles on water, water quality and water filtration.

    Did You Know?
    • Each day almost 10,000 children under the age of 5 in developing nations die as a result of illnesses contracted by use of impure water.
    • Water with 1000 ppm of salt (or more) is considered saline.
    • In just 16 hours, U.S. water utilities produce as much potable water as the oil industry produces oil in a year.
    • 60% of an adult's body is water.  Therefore, it's essential to replenish the water you lose through breathing, perspiration and excretion with good quality water.