Brief description

A thermometer is an instrument used to measure temperature or temperature gradient. It is essential for monitoring weather, cooking, medical diagnosis, and scientific experiments.

Use / Function

• Meteorology: Measuring air and soil temperature to predict weather patterns.
• Medicine: Monitoring body temperature to detect fever and illness.
• Cooking: Ensuring food is cooked to safe temperatures.
• Industry: Monitoring processes that require specific thermal conditions.
• Scale: Personal to Industrial.

Operating principle

Most traditional thermometers rely on the principle of thermal expansion.

1. Expansion: A liquid (like mercury or alcohol) or a solid (bimetallic strip) expands when heated and contracts when cooled.
2. Constraint: The material is constrained in a narrow tube or fixed configuration, so the expansion translates into visible movement along a scale.
3. Readout: The level of the liquid or the position of the needle corresponds to a specific temperature value based on calibration.

How to create it

Creating a basic liquid-in-glass thermometer:

1. Tube Preparation: A glass tube with a very fine bore (capillary) is required. A bulb is blown at one end.
2. Filling: The bulb and part of the tube are filled with a liquid (alcohol dyed red is common and safer than mercury).
3. Sealing: The liquid is heated to expand it to the top of the tube to expel air, and then the top is sealed shut.
4. Calibration:
   • Place in melting ice: Mark this point as 0°C (32°F).
   • Place in boiling water (at sea level): Mark this point as 100°C (212°F).
   • Divide the distance between these points into equal degrees.

• Technical level: Intermediate (requires glassblowing skills).

Materials needed

• Essential:
  • Glass tubing: Uniform bore is critical for accuracy.
  • Expansion liquid: Alcohol (ethanol) or Mercury (toxic, use with caution).
• Tools: Heat source (torch) for glassworking, scale marking tools.
• Substitutes:
  • Bimetallic strip: Two different metals bonded together that curl when heated (used in dial thermometers).
  • Galileo thermometer: Uses glass spheres of different densities in a liquid.

Variants and improvements

• Liquid-in-glass: The classic design.
• Bimetallic: Robust, used in ovens and outdoor dials.
• Thermocouple: Modern, uses electrical voltage generated by dissimilar metals (Seebeck effect).
• Infrared: Measures thermal radiation from a distance (non-contact).

Limits and risks

• Range: Alcohol boils at ~78°C, so it cannot measure high temperatures (use mercury or bimetallic for that). Mercury freezes at -39°C.
• Fragility: Glass thermometers break easily.
• Toxicity: Mercury is highly toxic if the thermometer breaks.
• Calibration: Needs to be checked periodically; air pressure affects boiling point calibration.