Monday, May 25, 2020

Should I be measuring water quality in TDS (total dissolved solids) or EC (electrical conductivity)?

Hi there,

I would like to ask you with regards to measuring TDS. I have did some reading and searches but am still confused. Would need your help in checking my understanding and also some qns.

Im looking to get HM Digital COM-100 which has 4 modes, ┬ÁS, mS, ppm 0.5 (NaCl) Scale & 0.7 (442™) Scale. Actually I am interested in using the 442 mode. Based on the specification it is calibrated by 1413 microsiemens solution supposedly KCl i think. Im thinking of getting a Eutech 442 300ppm or 50ppm solution to recalibrate the meter.
  1.  Which mode do i set to when I'm recalibrating with say 300ppm 442 solution?
  2. Another solution which is from HM digital is a 342ppm NaCl solution.

The specification noted that 342ppm NaCl = 478ppm in 442. Does it mean that i can calibrate at 442 mode with 478ppm or just mean that when reading at NaCl mode it will be 478?


Hi Ron,

You may want to read a previous post:How are total dissolved solids (TDS), electrical conductivity (EC) and salinity different

Incidentally, you did not mention what kind of water sample you are testing as that will affect the testing mode you want to use. But I suppose you have a rationale for choosing the 442 mode.

First and foremost, I am not a fan of using TDS for water water quality testing as it is a calculated value and it's relevance rests on the assumption that you have a certain composition of substances in your water sample. I invariably measure EC for all my water quality tests because it is a measured value (this is technically not correct since voltage is the measured value but the correlation between the 2 is strong enough for my purpose) and I DO NOT know what are the substances in my water samples. To me, it only makes sense to measure TDS when you are only dealing with a rather homogeneous water sample which does not fluctuate much in its composition.


TDS (ppm) = conversion factor * EC (uS/cm)

The key lies in what conversion factor (CF) to use. If you are dealing with a sample predominantly made up of sodium chloride (NaCl) e.g. seawater, then the typical CF is ~0.5.

The 442 measurement mode refers to a sample made up of 40% sodium sulfate, 40% sodium bicarbonate and 20% sodium chloride. Some users claim that this composition closely resembles natural waters though I believe natural waters vary too widely to follow a simple ratio of 4:4:2 throughout. Anyway, if you are using the 442 mode, the conversion factor is ~0.7.

(In actual fact, the CF is not constant throughout all concentrations of a substance. in general, as the concentration increases, the CF also increases.)


342ppm of NaCl = 342 / 0.5 = 684 uS/cm in EC

and 684 uS/cm in EC = 0.7 * 684 = 479ppm in 442


I suppose the easiest way to calibrate is to follow the supplier's instructions as they are. I have no experience with your particular model of TDS tester and do not know what kind of calibration standards are required.

But I guess all of us want to get the most bang out of our buck and go for a more economical calibration solution. The next easiest way is then to get an equivalent calibration standard that has the same EC. 

For example, if the TDS needs to be calibrated with 479ppm 442 solution in 442 mode, you can substitute with a 342ppm NaCl solution to used in the same 442 mode.

Again, the way to get around all this hassle of TDS and composition is to use the EC mode (uS or mS depending on your range) and calibrate in the same EC mode using calibration standards certified in uS or mS.
Figure: A typical TDS/EC meter calibration standard. If you try applying the formulas above, the conversion factors are higher than 0.5 for NaCl and 0.7 for 442.

Concerns about drinking tap water in a residential home in Singapore

Hi Mr Chen,

I've been reading your blog after stumbling across it while trying to find out more about water quality in Singapore.

I’ve always drank tap water neat, since childhood and never had any issues but ever since having a kid of my own, I've always questioned somewhat the quality of drinking water in my home. Perhaps because my other mummy friends are using fancy water dispensers with filters themselves and over time, that has made me question my own practice at home.

At the moment I’m using cooled water from a thermal pot (that boils fresh water and keeps it at 98C) for my daughter’s drinking needs. Apart from that, cooking etc i use tap water neat.

I recently dug up some old concern of mine (not sure why) - my toilet tap water intermittently tastes like soap..sometimes even smell like soap. IT disappears after a while, and then recurs perhaps a few months later for a short period of time. I’m not quite sure. The other water taps in my home doesn’t carry that taste/smell.

As such, I've asked PUB for help and they’ve taken water samples for testing and I’ve not heard back yet.
Before they left, the plumber did ask me if i boiled water for my daughter before use and when i said i did, he advised to continue doing so.
This made me think - is our tap water not safe then?

I know you have written articles about our excellent quality of tap water here in Singapore but I’m wondering if the piping used during construction works/ old pipes leading out from the original source itself - could they leach particles/chemicals/trace metals that over time, may be accumulated in the body and start causing harm?

I’m not sure also if any plastic pipes are used at all and over time, BPA/BPS/other endocrine disrupting chemicals could potentially be a concern too?

I”d love to hear your thoughts on this.

Thank you very much!


Hi Jennifer,

I was very occupied in the past few weeks, hence the late reply. 
  1. There are concerns about keeping water heated continually. As some chemical reactions are more likely to occur given the continual supply of energy. These reactions include those that form trihalomethanes (THM), a class of suspected carcinogens. There is of course still controversy over these concerns. Personally, I boil my water once and keep it hot in a thermal flask with no further heating.
  2. As for your intermittent soapy taste/smell from a single tap in your house, are you the only one who notices it? Do other family members experience this soapy taste/smell? If no, can it be due to a medical condition in you?
  3. PUB is obligated to test your tap water if there are complains. However, it never shares its test results, except to tell you whether the tap water is safe or not. In your case, since there is no further reply from PUB, then the water ought to be safe.
  4. You may want to read one of my previous posts - Ok, water from the water treatment plant is clean... do I still need water filters?. In short, PUB is not the only entity responsible for your tap water. Depending on your type of residence, the town council or building management committee may come into play. Depending on how they do their maintenance and accounting for unforeseen events e.g. dead maid in the water tank, there is a chance that your tap water may not be as clean as you like. Perhaps, your plumber was addressing this point when he made that remark.
  5. Properly certified for drinking water and/or food, plastics (including those in piping) should not contain BPA, BPB, plasticisers. But of course, if you are referring to plastics from decades ago, then it is anybody's guess whether these chemicals are present. Then again, if plastics are so old, they should be replaced anyway.
  6. Finally, yes, some chemicals can accumulate in your body and cause harm when a certain threshold has been reached e.g. lead, mercury. Google up on "bioaccumulation" for more information. 
Good luck!
Figure: Mercury/ quicksilver/ hydragyrum (Latin). Its chemical symbol is Hg which came from it Latin word. Mercury poisoning can result from exposure to water-soluble forms of mercury (such as mercuric chloride or methylmercury), by inhalation of mercury vapor, or by ingesting any form of mercury.