1. What is TDS?
  • “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.

2. 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.

3. 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.

4. 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.

5. 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.)

6. 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).

7. 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

8. 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.

9. 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.

10. 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.

11. 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.

12. 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.

13. 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.

 

14. 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.

 

15. 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.

 16. Should I expect pinpoint accuracy with my PH-200?

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

 17. 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.

18. 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.

19. How do I achieve better negative measurements with my ORP-200?

How do I achieve better negative measurements with my ORP-200?

20. 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.

21. There is a white residue on my cap or sensor. Is this normal?

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.)

22. 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.

23. 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.

24. 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.”

25. 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.