Water: Temperature, Clarity, Conductivity
Two glasses of water can look exactly the same. Sensors measure three things your eyes cannot always see: how warm the water is, how cloudy it is and how easily it carries electricity.
For parents: a one-page guide — dinner questions and an experiment without a screen.
01Remember
Think about each question, then open the answer.
What does a sensor do?
It turns something from the world — light, heat, sound — into a number a computer understands.
What do you use to measure how warm something is?
A thermometer. The number is in degrees Celsius (°C).
What happens when you stir soil into water?
The water turns brown and cloudy. But some things in water cannot be seen — for example dissolved salt.
02Story
In 1865 a scientist named Angelo Secchi made the simplest tool for measuring cloudiness: a white disc about 30 cm across. He tied it to a rope and lowered it into the sea.
When the disc disappeared from sight, he stopped and measured how deep it had gone. The cloudier the water, the shallower the disc disappears.
A white 30 cm disc is still used in ocean research today. There are also electronic sensors that measure how water scatters light. The idea is the same: how far does the light get?
03Worked example
Imagine a glass of water and a small device with three sensors. Here is what each one measures:
Temperature
How warm the water is. A small chip in a sealed probe gives a number in °C. One well-known chip — the DS18B20 — measures from −55 to +125 °C.
Turbidity (cloudiness)
How “cloudy” water is because of tiny particles that are hard to see one by one. An LED shines through the water, and a light-sensitive part at the side catches how much light was scattered. More particles means more scattered light. Unit: NTU.
Conductivity
How easily a weak electric current passes through water. Salts dissolved in water are tiny charged particles — they carry the current. More dissolved salts means higher conductivity. Unit: µS/cm. Scientists compare it at 25 °C, because temperature changes it a little too.
What they do NOT say
Three numbers are not enough to say whether water is fit to drink. That is why drinking water is checked officially, by rules.
04Try it now
1 · Predict
You dissolve salt in a glass of clear water and stir. Which number will change the most?
2 · Run: the glass with sensors
Move the sliders. Yellow light travels through the glass to the sensor. Watch what happens to the numbers.
3 · Change: a glass that fools the eye
Challenge: make water that looks clear (turbidity under 5) but has high conductivity (over 1000). How?
05No screen: turbidity with a jar
You need: a tall clear jar or glass, a sheet of paper with a thick black cross, a ruler, tap water, a little clean garden soil, a spoon and a small cup for pouring. A plastic bottle works too, but an adult should cut off the top and tape over the edge.
Put the sheet with the cross under the jar. Pour tap water in slowly and look from above. Can you see the cross? Measure with the ruler how high the water is.
Pour the water down the sink. Stir half a spoon of soil into a cup of water. Pour it into the jar and stop when the cross disappears. How many centimetres of water is that?
Repeat with a whole spoon of soil (stir before every pour). Does the cross disappear at a shallower or a deeper level?
Write the numbers down. The cloudier the water, the shallower the cross disappears. This is the idea of Secchi's disc.
06Make your own
Draw your own water station: three sensors, a small computer and a screen. Label what each sensor measures and in which units. Make a table for the three glasses from the simulator: tap water, with soil, with salt.
If you ever build a real station — only with an adult
- The power is off while you connect the wires (the USB cable is not in the port).
- USB 5 V only. Never mains electricity (230 V).
- Only a sealed probe goes into the water. The board and wires stay dry, on a table, away from sockets.
- Tap water in a cup only. Not a pond, a pool or the sea.
- A home sensor proves nothing about whether water is fit to drink.
07What I learned
- Temperature — how warm the water is. Turbidity — how much light particles scatter. Conductivity — how easily current passes when salts are dissolved.
- Salt in a glass of water cannot be seen, but it changes conductivity. Eyes are not enough, so we have sensors.
- Three numbers from a sensor do not say whether water is fit to drink.
A sensor gives numbers, but it does not know whether you are thirsty. And you — how do you know you are thirsty?
Check yourself
1. Which number shows how much light is scattered in water?
2. What changes the most when you dissolve salt in water?
3. A sensor shows three “nice” numbers. Does that mean the water is fit to drink?
It is yours when you can explain to someone what temperature, turbidity and conductivity measure — and why they do not say whether water is fit to drink. No points and no ranking — only a skill.
08I have a question
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·Sources
- Title: Water: Temperature, Clarity, Conductivity (DB-IS-08) · Author: Children's Library, East Accelerator Foundation · Source: original content, 2026 · Rights: CC BY-SA 4.0.
- Secchi disc (1865, white disc about 30 cm, turbidity): Wikipedia — Secchi disk (link only; no text copied).
- Turbidity, light scattered at an angle, units NTU/FNU: Wikipedia — Turbidity (link only).
- Conductivity and dissolved ions: USGS — Chloride, Salinity, and Dissolved Solids (link only).
- The DS18B20 chip (−55…+125 °C, 3.0–5.5 V): manufacturer datasheet (Analog Devices) (link only).
- School link: Bulgarian curriculum “Man and Nature”, grade 5 (in Bulgarian) — “Water and aqueous solutions”; measuring temperature with a thermometer.
- Licence: CC BY-SA 4.0 · Children's Library, East Accelerator Foundation, 2026. You may share and adapt this lesson if you credit the author and keep the same licence. Learning is free forever.