Measuring temperature accurately with a thermometer
Chemistry · Key Stage 2 · The School
Why careful reading matters
A reading is only useful if it is correct. A careless reading can hide a real difference between two temperatures. It can also make a good thermometer seem wrong, when the real fault was how it was read.
Getting ready to read
Put the bulb right into the liquid or air. Do not let it touch the sides of the container. Wait for the liquid to settle. Let the reading stop moving before you look. A reading taken too soon shows an old temperature, not the true one now.
Reading at eye level
Get your eyes level with the top of the liquid inside the tube. Do not look from above. Do not look from below. Looking from above or below can change the number you read. This mistake has a name: PARALLAX ERROR.
Suppose a class checks a thermometer in melting ice and water. Melting ice is at about 0°C, so the thermometer should read about 0°C. This thermometer has a small mark for every 1°C. A pupil puts the bulb in the middle of the icy water, away from the sides. The thermometer started in a warm room, so the liquid sinks. At first it shows 7°C and is still moving, so the pupil waits. When it stops, the pupil puts their eyes level with the top of the liquid. The top sits just above the 0 mark, much nearer to 0 than to 1. The pupil reads the nearest mark. They write 0°C, not just 0, because the unit must go with the number. The reading matches, so the thermometer passes this check. The early 7°C was only a reading taken too soon.