Measuring Total Dissolved Solids (TDS) With a TDS Meter

22 Aug, 2012  |  Written by  |  under Conductivity Analysis, Dissolved Solids

by Joe Covey

“You can’t always get what you want.” Mick Jagger was not thinking about TDS meters when he wrote that line. Nevertheless, users of TDS meters should consider it good advice.

Why? TDS meters do not measure dissolved solids. They measure conductivity and calculate TDS by multiplying the conductivity by a conversion factor. Two assumptions are at work: all dissolved solids produce conductivity and solutions having the same TDS have equal conductivity. But, to quote George Gershwin this time, “It ain’t necessarily so.” Conductivity comes from ions. Only solids that produce ions when dissolved in water cause conductivity. Solids that do not yield ions do not. And, equal weights (TDS) of different ionic solids rarely make equal contributions to the conductivity. TDS, on the other hand, has nothing to do with ions. It is simply the total weight of all solids in a unit amount of solution. It includes ionic solids, which contribute to conductivity, and non-ionic solids, which do not.

It’s easy to illustrate what happens when the first assumption is not met. A cup of coffee made with tap water has conductivity caused by the ionic solids naturally present in the water. The same cup of coffee with a teaspoonful of sugar has the same conductivity but 400 to 500 times higher TDS (from the sugar). Because a TDS meter measures only conductivity, it makes a huge negative error when sugar is present.

Fortunately, TDS meters rarely produce such large errors because they are used mostly for measuring samples such as natural and treated waters in which ionic solids (salts) are the primary dissolved solids. Non-ionic solids, silica mostly, are also present, but they usually are a small part of the total. Nevertheless, TDS meters still make errors because the second assumption, solutions having the same TDS have equal conductivity, is rarely met.Instead, as the table below shows, salt solutions having equal TDS can have very different conductivity.

Thus, two waters, one rich in sodium chloride and the other rich in sodium bicarbonate, could have the same TDS, but conductivity differing by almost a factor of two. Unless the meter knows beforehand which salts are present and the conversion factor to use, the TDS reading will be in error.

The graph below illustrates the variability of the TDS-to-conductivity ratio (the meter conversion factor) for 25 surface water samples taken from various locations in theUnited States.

The ratios range from 0.51 to 0.83, although most of the values lie between 0.55 and 0.70. Clearly, no single conversion factor works for all samples. Most TDS meters use an average; 0.65 ppm TDS per uS/cm is common. The error in TDS using this conversion factor is typically less than 15%.

The error can be greater, however, as points 1, 2, and 3 illustrate. Sample 1, from the Gila River inArizona, has an extremely high sodium chloride concentration giving it a low TDS to conductivity ratio (see the table above). Sample 2, from thePecosRiverinNew Mexico, contains high levels of bicarbonate salts and calcium sulfate, leading to a high TDS to conductivity ratio. Sample 3, fromEstesLakeinColorado, contains a large proportion of unionized silica relative to salts; it has low conductivity but high TDS.

Some TDS meters avoid the variable composition problem altogether. They assume the conductivity is caused by a single salt, typically potassium chloride (KCl) or sodium chloride (NaCl), and express results as ppm KCl or ppm NaCl.

With the exception of the 5081-C, all current Rosemount Analytical conductivity analyzers and transmitters can be configured to automatically convert conductivity to TDS. To calculate TDS, the instrument corrects the measured conductivity to 25°C using a temperature coefficient of 2% per °C and multiplies the result by 0.65. For the 5081-C, the conversion must be done through the custom curve feature. Custom curve, available in all Rosemount Analytical conductivity analyzers, also allows the user to choose a conversion factor other than 0.65 or a temperature coefficient other than 2% per °C.

22 Responses so far | Have Your Say!

  1. Michael Stajduhar  |  August 24th, 2012 at 11:37 am #

    Good stuff Joe. You present technical information in such a way that anyone can understand. Very interesting. Thank you.

    Michael Stajduhar - Gravatar
  2. Rosemount Analytical  |  September 17th, 2012 at 10:47 am #

    Thanks for your note, Michael. Appreciate your input!

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  3. Rosemount Analytical  |  December 28th, 2012 at 9:28 am #

    Saidulu,
    Thank you for contacting us. A member of our sales team will be in touch with you shortly.

    Thanks again.

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  4. Anil kumar  |  July 23rd, 2013 at 12:51 am #

    Does the TDS determination using TDS meters for the Potable & Raw water in the pharmaceutical application of Quality control testing can be used?

    Is there any standard factor can be applied for these samples ?

    Is the application using TDS meters can be accepted by the Regulatory bodies?

    Anil kumar - Gravatar
  5. Rosemount Analytical  |  July 25th, 2013 at 11:09 am #

    Thanks so much for your questions, Anil.

    TDS meters do not measure total dissolved solids. Instead, they measure conductivity and convert the result to TDS using a simple factor. Some meters used a more complicated algorithm, but typically these instruments assume that the only dissolved solid is potassium chloride.

    If the requirement truly is to measure TDS, there are two approaches. One is to make a gravimetric analysis, for example following Standard Methods 2540B, in which a known volume of sample is filtered to remove suspended solids and then evaporated to dryness. The residue remaining is heated at a specified temperature until it comes to constant weight. The weight divided by the volume is the TDS. Alternatively, the sample can be analyzed for major cation and anions as well as unionized solutes such as silica. The sum of the concentrations is the calculated TDS. The TDS measured gravimetrically and calculated from analysis will not be the same.

    If enough measurements of TDS (either gravimetrically or by analysis) are made and correlated with conductivity, a factor to convert conductivity to TDS specific to the water being monitored can be derived. However, if the composition of the water changes, the factor will likely need to be re-determined.

    Regulatory bodies may not accept TDS calculated from conductivity. It is best to check with the regulator to determine the specific requirements.

    Rosemount Analytical - Gravatar
  6. Abe  |  April 18th, 2014 at 4:30 am #

    Hi,

    I know this is an old article, but I am new to TDS testing and find this interesting. I guess TDS testing is only good for determining conductive particles in the liquid being tested. At least that is some kind of objective analysis. How would a regular joe determine the non-conductive particles in the test subject.

    Is there anyway to make non-conductive particles conduct, and therefore one could simply add the new figure, to the already known conductive particle count.

    Many thanks!

    Abe.

    Abe - Gravatar
  7. Rosemount Analytical  |  April 23rd, 2014 at 11:09 am #

    Abe – thanks so much for your comment. It is important not to confuse TDS calculated from conductivity with true TDS. Actually, true TDS is a bit of a misnomer; TDS depends on the method used to measure it. Two methods are in common use.

    The first method is the most direct. A known volume of sample is filtered to remove suspended solids. The filtrate is then heated until the water evaporates and the residue reaches constant weight. The weight of residue divided by the volume of sample is the TDS. Because the measurement involves weighing a residue, it is called a gravimetric method. The result depends strongly on the temperature at which the residue is dried. Standard Methods for the Analysis of Water and Wastes, a widely accepted manual for water analysis, specifies drying at either 103 -105C or 180C. Normally, the lower temperature yields a greater TDS. Gravimetric TDS measures residue; therefore, it includes both conductive and non-conductive dissolved solids. Of course, substances lost or modified during drying will not be measured. The second method of determining TDS is by calculation. In this method the filtered sample is analyzed for major constituents, conductive and non-conductive. TDS is the sum of the weight per volume of the individual constituents. Generally, calculated TDS will be different from gravimetric TDS.

    The methods described above are recognized, accepted ways of measuring TDS. But neither is particularly convenient to do. Both require a laboratory and a relatively skilled technician. The methods are also time consuming. The gravimetric method, in spite of its apparent simplicity, can take hours to complete, most of the time being spent waiting for the residue to come to constant weight. Although the calculated TDS method would be expected to take even longer because of twelve or more constituents that are typically measured, a complete basic water analysis can be done in four or five hours using modern instrumentation and automation. TDS by calculation, of course, has a major advantage over the gravimetric method: it provides a relatively complete picture of the major components of the sample.

    Inferring TDS from a conductivity measurement is faster and less expensive than either of the above methods. That’s why it is popular. Unfortunately, it’s a bad way to figure out TDS. It only works (sort of) because most of the components in natural and treated water are ionic, that is, they can be measured by conductivity. But, the method falls short in two significant ways. First, it assumes a single factor can be used to convert conductivity to TDS, that is, all salts on a weight per volume basis contribute equally to conductivity. As the blog points out, this is clearly not the case. Second, the method completely fails to detect non-ionic solids, such as silica. Nearly all natural waters contain silica, which the conductivity method will always miss.

    Trying to make TDS-by-conductivity work better by making non-conductive dissolved solids conductive is an interesting idea. Non-conductive dissolved solids can be made conductive, usually by adding an acid or base to adjust the pH. It’s a common trick in certain types of water treatment where a contaminant in its normally uncharged form cannot be removed, but can be removed in its ionic form. Making non-conductive dissolved solid ionic won’t work in the TDS measurement, however. The acid or base needed to ionize the solid is, itself, ionic, so adding it increases the conductivity and must be corrected for. This is a case of a problem for every solution. The only way to determine the contribution of non-conductive components to TDS is to measure them directly.

    I’m not a big fan of using calculating TDS from conductivity. If knowing TDS is important, measure it either by the gravimetric method or by calculation from a water analysis. I’m not trying to de-emphasize the usefulness of conductivity. Conductivity is an inexpensive, simple way of monitoring the total concentration of ions in water. It is an extremely useful measurement in water treatment and in many other industries. It is, however, not a good way to measure TDS. But, if you choose to use TDS calculated from conductivity, understand how the calculation is made and its many limitations.

    Rosemount Analytical - Gravatar
  8. Abe  |  April 23rd, 2014 at 11:44 am #

    Just a note to say thanks for such an interesting reply!

    Best regards.

    Abe.

    Abe - Gravatar
  9. Rosemount Analytical  |  April 23rd, 2014 at 2:35 pm #

    Abe – Thanks again for your interest and let us know if you have any further questions.

    Rosemount Analytical - Gravatar
  10. subbu  |  May 9th, 2014 at 9:56 pm #

    Thanks for enlightenment regarding TDS.
    Could you tell us what are the other common non ionic solids that contribute to TDS other than Silica.

    subbu - Gravatar
  11. Rosemount Analytical  |  May 13th, 2014 at 1:36 pm #

    Subbu – Thank you for your inquiry. The most common non-ionized or poorly ionized dissolved solid in natural and treated waters is silica. There might be others, but I am not aware of them.

    Joe Covey

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  12. Selma  |  March 11th, 2015 at 1:16 am #

    Hi,
    In relation to your response to Abe’s questions, you mentioned that TDS measurements can be calculated from water analysis. How can I go about doing that?

    Selma - Gravatar
  13. Rosemount Analytical  |  March 13th, 2015 at 5:05 pm #

    Hi Selma – thank you so much for your inquiry!

    TDS is simply the sum of the concentrations of the individual constituents. In general, assume the water has the following composition:

    Na+, mg/L XNa Cl-, mg/L XCl
    K+, mg/L XK SO4-2, mg/L XSO4
    Ca+2, mg/L XCa HCO3-, mg/L XHCO3
    Mg+2, mg/L XMg CO3-2, mg/L XCO3
    F-, mg/L XF
    SiO2, mg/L XSiO2 NO3-, mg/L XNO3

    TDS in mg/L = XNa + XK + XCa + XMg + XCl + XSO4 + XHCO3 + XCO3 + XNO3 + XF + XSiO2, where Xn is the concentration in mg/L of each constituent.

    Often in a water analysis, the concentration of Ca+2 and Mg+2 is expressed in terms of mg/L as CaCO3. To convert Mg+2 (mg/L as CaCO3) to Mg+2 (mg/L), multiply by 0.243. To convert Ca+2 (mg/L as CaCO3) to Ca+2 (mg/L), multiply by 0.400.

    Another problem is the expression of HCO3- and CO3-2. In water analysis these results are often expressed as P-alkalinity and T-alkalinity (in mg/L as CaCO3). Converting P and T alkalinity to HCO3- and CO3-2 concentration can be tricky, but in typical natural and treated water, HCO3- is always present and CO3-2 is either absent or present in small amount. Under these conditions, CO3-2 (mg/L as CaCO3) = 2P, and HCO3- (mg/L as CaCO3) = T – 2P. To convert CO3-2 (mg/L as CaCO3) to CO3-2 (mg/L), multiply by 0.62. To convert HCO3- (mg/L as CaCO3) to HCO3- (mg/L), multiply by 1.26. If phosphate or borates are present in significant amounts, the conversion of P and T alkalinity to HCO3- and CO3-2 concentration becomes quite complicated.

    Thanks again. And please let us know if you have any additional questions!

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  14. sheyva kreymer  |  June 15th, 2016 at 11:23 am #

    I HAVE A TDS METER AT HOME. I WAS TOLD THAT IT SHOWS HOW PURE IS THE WATER AND LESS IS THE NUMBER ON TDS METER WINDOW,MORE PURE IS THE WATER.
    I CHECKED THE TOP WATER AT MY HOME WITH TDS HAS NUMBER 40,
    THE WATER FROM BOTTLES HAS ABOUT 28.
    IN OTHER WORDS BOTTLED WATER WITH TDS OF 28 IS BETTER THAN TOP WATER WITH TDS OF 40?

    sheyva kreymer - Gravatar
  15. Rosemount Analytical  |  June 16th, 2016 at 3:46 pm #

    Hi Sheyva,
    Thank you so much for your inquiry! We’re checking in with our experts for a response and will reply to you as soon as possible.

    The Analytic Expert Team

    Rosemount Analytical - Gravatar
  16. Rosemount Analytical  |  June 20th, 2016 at 5:24 pm #

    Hi Sheyva,
    TDS meters estimate the total concentration of ions in water by measuring the electrical conductivity. Therefore, if the TDS number in bottled water is less than the TDS number in tap water, the bottled water contains a lower total concentration of ions. TDS is not a measurement of purity unless purity is defined as total ion concentration. Purity could mean potability, that is, suitability for drinking. If this is the concern, the level of pathogenic organisms and chemical contaminants in the water are important, not TDS.

    I do hope this answers your question, but please let us know if you would like any additional information.

    The Analytic Expert Team

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  17. Suhasini  |  January 12th, 2017 at 11:07 pm #

    Hi i m suhasini i have query about tds when i checked tds by evoporation method it showing 2400 result but tds meter it showing tds result 1140 i m in confusion state plz help me

    Suhasini - Gravatar
  18. Stevie day  |  January 18th, 2017 at 10:57 am #

    Suhasini,
    What is the model number of your instrument and sensor?

    Stevie day - Gravatar
  19. Stevie day  |  January 18th, 2017 at 10:58 am #

    Suhasini,
    Also, what is the cell constant of the sensor

    Stevie day - Gravatar
  20. Rosemount Analytical  |  January 20th, 2017 at 8:09 am #

    Hi Suhasini,
    Although we are not certain about the evaporation method you are using, our TDS is not an actual measured value. We measure conductivity and use a calculation to infer TDS. If you are actually measuring TDS, you can still calibrate (standardize) the unit based on your test method. Without knowing what units you have, in the 1056/1066 you can standardize by:

    Menu > Program > Calibrate > TDS > In Process Cal > wait for process to stabilize > Enter > Take Sample > Enter > Change value to match verified sample > Enter, standardization complete.

    Total Dissolved Solids: Calculated by mulltiplyingg conductivity at 25 degrees C by .065.

    Please let us know if this helps, and if you have any further questions!

    Thanks Analytic Expert team.

    Rosemount Analytical - Gravatar
  21. Lalita  |  January 30th, 2017 at 2:43 am #

    Hi!
    I’m using a TDS meter of Hanna company, model no 96301, range 10-1990 ppm. This meter shows value in two digits. Even in extremely contaminated waters it exhibits 50-70 ppm results. Is this instrument working properly or a conversion factor is required to calculate the results, please help.

    Lalita - Gravatar
  22. Rosemount Analytical  |  February 2nd, 2017 at 11:32 am #

    Hi Lalita,
    Someone with Emerson will be contacting you shortly with some information per your inquiry. Please let us know if you do not hear from anyone, or if you have any additional questions.

    Thanks so much!
    The Analytic Expert Team

    Rosemount Analytical - Gravatar

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