1999A05 Differentiate between the terms ‘heat’ and ‘temperature’. Explain briefly the principles of a mercury
thermometer and a thermistor, indicating their advantages and disadvantages.

Heat

·  Form of energy

·  Transferred from higher temp object to lower temp object in thermal contact

·  Units Joule = kg.m².s^-2

Temperature

·  Measure of the average kinetic energy of the particles within a substance per degree of freedom of those particles

·  Unit Kelvin (where 0K = -273.15°C = absolute zero)

Specific heat capacity

·  Describes relationship between heat and temperature for a substance

·  = amount of heat needing to be added to 1g substance to increase temp by 1K

·  Units J/K

Features

·  Glass vessel

·  Inner cavity: reservoir continuous with a capillary tube via an angled constriction

·  Cavity is evacuated except for mercury liquid and vapour

·  Graduations on the outside of the capillary tube, established by calibration

·  Number on the side is the time constant

How it works

·  ↑ temp -> expansion of fluid -> rise of fluid level up the tube

·  Fluid level increase ∝ temperature x (ratio size of reservoir : diameter of tube)

Pros

·  Cheap

·  Simple

·  Accurate

·  Short time constant = fast (equilibration within 2-3 minutes)

·  Can be chemically sterilized

·  No repeat calibration required

Cons

·  Mercury toxicity if breaks

·  Risk of cross-contamination

·  Unsuitable for low temperature (solidifies at -39°C)

·  Can’t be electronically processed

·  Not for body cavities

·  Not continuous

Features

·  Bead of metal oxide semi-conductor (e.g. manganese, iron, cobalt, nickel, copper)

·  +/- incorporated into a Wheatstone bridge to improve accuracy

How it works

·  ↑ temperature -> ↓ resistance

·  Non-linear relationship overall

·  Near linear within physiological range

Pros

·  Continuous readout

·  Rapid response

·  Small, robust (e.g. in pulmonary artery catheter)

·  Easy electronic processing

·  Requires very little current hence prevention of microshock (<10mA)

Cons

·  Non-linear relationship between rise in temp and fall in resistance

·  ↓accuracy with time

·  +/-↓accuracy with sterilization