Therm – Measurement

Master answer

Temperature is the tendency of a body to transfer heat energy to another body, and is measured in degrees.

It is distinct from heat, which is the kinetic energy content of a body, and is measured in Joules. The two are related by the specific heat capacity, which describes how much energy (J) must be applied to a body to raise its temperature from 14°C to 15°C, without a change in state.

Measurement of Temperature

Temperature is measured by a number of methods:

1. Liquid Expansion Thermometry
2. Electrical methods:
   • Resistance thermometry
   • Thermistor
   • Thermocouple

1. Liquid Expansion Thermometry

This is used in mercury thermometers. These consist of:

• A graduated evacuated capillary of negligible volume, attached to
• A mercury reservoir, of much greater volume, separated by
• A constriction ring
  Prevents travel of mercury up the capillary by gravity.

Mechanism:

• When heated, the kinetic energy of the mercury increases and it expands, forcing it up the capillary.
• As the thermal expansion coefficient for all liquids is very small, the capillary must be of a very small volume to create a useable device.
• The speed that this occurs is related to the time-constant of the system
  This is typically 30 seconds. Measurement therefore takes ~4 time-constants, or 2 minutes.

Pros

• Easy to use
• Accurate
• Reusable
• Sterilisable
• Cheap

Cons

• Slow response: Only accurate once it has reached thermal equilibrium.
• Glass can break: May cause release of mercury or alcohol.
• Inaccurate at:
  • Low temperatures with mercury
    Freezes at -38.8°C.
  • High temperatures with alcohol
    Boils at 78.5°C.

2. Electrical

Electrical methods include:

• Resistance thermometer
  Platinum wire increases electrical resistance with increasing temperature.
  • Therefore the voltage drop across the wire will correspond to the temperature of the wire
  • Change in resistance is linear across the temperature range
  • However, these are expensive.
• Thermistor
  Metal (e.g. SiO₂) semiconductor which changes its resistance in a predictably non-linear fashion (run-away exponent) with temperature.
  • Can be manufactured so that change is linear over the clinical range
  • Much cheaper than wire resistance methods
  • The degree of voltage drop is usually very small, however this can be amplified using a wheatstone bridge
• Thermocouple
  At the junction of two dissimilar metals, a potential difference will be produced proportional to their temperature. This is known as the Seebeck effect.
  • Non-linear (wash in exponent)
  • Degrade over time

JC 2019

Answer by JC