I wanted to share how I confirmed that the new TDS meter I bought online actually worked. It is possible for anyone to use this method to put an upper bound on the error of this instrument.

Why was I curious?

This story starts from a discussion I had with my dad regarding the correlation between liver disease and low magnesium intake. He had brought to my attention this article.

I did some more reading online and found that Non-alcoholic fatty liver disease (NAFLD) is very common in India and people with it are also at risk of Mg deficiency, all this got me wondering about the quality of water I drink everyday.

A sedentary lifestyle is going to increasingly make NAFLD a risk for many Indians.
A sedentary lifestyle is going to increasingly make NAFLD a risk for many Indians.

Which meter to get?

So I found a 300₹  TDS Meter made by KONVIO NEER. The company seemed to sell water purifiers so I hoped it would work. But mainly it was cheap ¯\_(ツ)_/¯

So the meter arrived within a week and the temperature meter seemed to work properly showing 25.4 Celsius on fresh tap water. You could view it in Fahrenheit too.

The TDS value was shown in ppm units. 1 mg/L is 1 mg/1,000,000 mg or one part in one million. There was also a button to hold the value so that we can remove it from the water to closely observe the result.

It looked pretty old school and the plastic felt cheap.
It looked pretty old school and the plastic felt cheap.

What is TDS?

As per WHO, The principal "solids" that are usually "dissolved" in water are calcium, magnesium, sodium, and potassium cations and carbonate, hydrogencarbonate, chloride, sulfate, and nitrate anions.

Since only calcium carbonate is considered when looking at hardness, a low TDS value indicates that the water is soft. Hard water might be linked to hair fall, I could find no supporting research but ancedotal evidence is plenty. But moderately hard water might help protect you from cardiovascular disease.

Less than 50 ppm is supposed to affect the aesthetics of the water - for some people it tastes flat, weak taste. But almost everyone agrees at high levels of TDS, the water's taste becomes brackish, metallic etc. From around 500 to 900 ppm it becomes unacceptable.

So the goal was to confirm that TDS was somewhere between 50 to 400 ppm.

TDS values at home

So the initial test results showed that my tap water had TDS value of 39 ppm while after it ran through my Eureka Forbes AquaSure Crystal Water Purifier the value was almost unchanged at 36 ppm.

This surprised me until I took at closer look at what my the purifier at my home does.

It has 3 stages, the first stage just does a pre filtration to remove large suspended particles. Total Dissolved Solids won't change there.  

Then the activated carbon filter works to remove lead, pesticides, chlorine, soluble gases (CO2, N2 etc), organic matter etc. It works by having a huge surface area onto which these compounds physically get adsorbed. Taste and odor are improved due to this stage.

But the TDS value is measured using a proxy, that is electrical conductivity. It is inorganic compounds that contribute to making water a better conductor. So this stage does not change TDS either.

Finally the UV-C light is absorbed by bacteria and viruses in the water killing them. This won't change the TDS either. So actually it makes sense, TDS would not change with my current purifier.

But my water tastes great. Not flat at all. This made me suspicious, how could I be sure the instrument worked at all? What if it was just showing me a number.

Verification

Dad talked about going to a lab nearby and ensuring the meter was calibrated, with some more discussion we decided to try and dissolve solids in the water and see if the reading increased.

I tried sugar and it dissolved quickly. But the reading did not change. Of course, sugar does not ionize. So that was nice to see.

I added some salt to the glass and saw the reading shoot up. So that was also heartening. But now I needed to try to quantify and control how much salt I added and what the TDS increase was expected to be. Let's get precise!

So I took a 1 litre water bottle that I have and found that that 1 litre of tap water was showing 37 ppm which meant there was 37 mg of dissolved salts in that one litre.

You can see how quickly it finds the reading as 37 ppm

Now if I can add a known amount of salt into this water, I can predict what the new TDS value of the salt water should be. But the weighing machine I have at home cannot measure weights near 1 gram anywhere near accurately.

So we turn to volume, we know 2.16 g/cm³ is the density of salt. I could find out that 1 US teaspoon is 4.9 cm³, but I don't have labels on the teaspoon lying around at home. So I used a known measuring cup I found lying around and tried to fill it with a tiny spoon.

It took 3 spoons full of water to reach 5 mL.
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It took 3 spoons full of water to reach 5 mL

So I was able to find a spoon with 5/3 or 1.6 mL capacity. I filled it with salt but unlike water, salt crystals won't be packing with 100 efficiency. Assuming it naturally sets into a hexogonal packing structure, we only have 74% of the volume occupied by salt, and the rest is air!

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Does not seem like 26% of it is air right? I am not totally sure myself lol

Well water also seems to take up more volume vertically due to surface tension, so acknowledging these sources of error, we have gotten our hands on $ 2.16 \times 0.74 \times 1.6 = 2.557 g = 2557 \ mg$

So we should now have $2557+37 = 2594 \ mg$ of dissolved solids in our 1 litre bottle. I am pretty sure all the salt got dissolved.

Ah and the moment of truth has arrived!

....

I saw I got 2710 mg. Less than 5% error!

That was not bad at all! I have used science to feel reasonably confident about this meter.

Now considering how I used my eye to ensure if those 3 spoonful of maybe equal water was 5 mL, how it would not be an ideal hexagonal packing throughout, and I still feel skeptical if salt occupies all the space water got to occupy due to surface tension. This is quite an acceptable range of error and I can now trust in the calibration done by the company.

So go ahead, get a TDS meter and comment below what the situation with your water is. I will be interested to get your location and TDS value.