What it shows
Water carries dissolved salts that conduct electricity. A TDS meter reads the conductivity and converts it into parts per million, so different sources can be compared.
The science
Dissolved salts such as calcium, magnesium, sodium, chloride and bicarbonate split into ions in water. These ions carry current, so the electrical conductivity of water rises with the amount of dissolved salt. A TDS meter measures the conductivity and multiplies it by a factor, about 0.5, to give the total dissolved solids in parts per million, or mg per litre. It does not detect germs or non-ionic pollutants such as sugar or oil, so a low reading does not mean the water is safe to drink. Readings vary with temperature, so compare samples at the same temperature.
Materials
Cost: under Rs 500. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A pen-type TDS meter (about Rs 300 to 400)
- Ten clean glass or plastic cups
- Samples of tap water, bore well water, RO water, bottled water, rainwater, a pond, boiled water and distilled water
- A marker and labels
- A notebook
- Distilled water from a medical store for rinsing
- A thermometer
Safety
For this project: Do not drink any sample you collect from ponds or wells. Collect samples with an adult and wash hands after handling them. Do not enter water bodies.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
8 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Collect about 150 ml of each source in a labelled cup. Write the place, date and time.
Rinse the meter in distilled water and dry it with a clean tissue before the first test and between samples.
Put the meter in the sample up to the mark and wait for the reading to settle. Record the number in ppm and the temperature.
Test the distilled water first, which should read near zero, to check the meter.
Test each sample three times and take the average.
Make a bar chart of average TDS for the sources, from lowest to highest.
Mix 1 g of salt in 1 litre of distilled water and read the meter, then make a calibration line using 0.5, 1 and 2 g.
Find the dissolved salt in your tap water from the calibration line, and compare it with the common guidance that drinking water is best below about 500 ppm, checking current local guidance.
What you should see
A bar chart that ranks the sources from near zero for distilled and rainwater, to a few hundred ppm for tap water and higher for bore well and pond water, with a calibration line for salt.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What does TDS stand for?
Total dissolved solids.
2Why does a TDS meter work?
Dissolved ions conduct electricity and the meter converts conductivity into ppm.
3Does a low TDS reading prove water is safe?
No, it does not detect germs, chemicals without ions or some pollutants.
4Why does boiling raise the TDS reading?
Water evaporates but the salt stays, so the solution becomes more concentrated.
Take it further
Track the TDS of the same bore well or pond every week through the monsoon and plot a graph.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
Free ebooks for this project
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