25B18: Exam Report

  1. Define the following: (10% of marks)

    1. Heat
    2. Temperature
    3. Specific heat capacity

  2. Outline the principles underlying the different methods of temperature measurement using

    the following headings:

    1. Electrical methods (60% of marks)
    2. Non-electrical methods (30% of marks)

Your answer should include examples of each method and their advantages and disadvantages
of use.

40% of candidates passed this question.

  1. For a measurement question  the correct  units for any definitions  are  required.
  2. For this section candidates were expected to provide information  on the following:

Electrical thermometers

  • Resistance  thermometers;  thermistor  and  thermocouple
  • Infra-red thermometers.

Nonelectrical thermometers include

  • Liquid,  dial  and crystalline  principles.

The expected  information to score  well  included  an  outline of  each  of the  underlying principles and  how  it  is  used  to  measure  temperature,  an  example  of  a  type  of  thermometer,  and advantages and disadvantages of each method.

R1i / R2i / 25B18: Define heat, temperature, electrical methods and non electrical methods

A

i) Heat

  • Heat describes the transfer of thermal energy of one body to another body
  • It IS a form of E
  • It CAN do work
  • Heat is also described as the number of atoms in a substance x how much energy each atom possesses
  • SI unit: Joules; 1 Calorie = 4.184 kJ

ii) Temperature

  • Temperature is the average kinetic energy of molecules in a substance.
  • It is a physical property of a substance
  • It is not a form of E
  • It does NOT do work
  • SI unit: Kelvin, K.
    • K = Celsius + 273

iii) Specific heat capacity

  • Specific heat capacity describes how much energy (J) must be applied to a substance to raise its temperature by 1 degree Kelvin without a change in its state.

B

i) Electrical Methods

  • Resistance Thermometers
    • Electrical resistance of a metal increases linearly with temperature between 0 and 100 degrees Celsius.
    • A fixed voltage is passed across a metal wire
    • The current that flows is measured and the resistance is calculated using Ohm’s law
    • A Wheatstone bridge is added to improve accuracy
    • The temperature of the wire is determined.
    • PROS:
      • Extremely accurate (measure changes as small as 0.0001 degrees Celsius)
    • CONS:
      • Slow response time
      • BulkySeldom used in clinical practice
      • Seldom used in clinical practice
Graph of resistance increasing with temperature.
  • Thermistors
    • Similar principle to resistance thermometers but a metal oxide is used not metal
    • Electrical resistance of metal oxide falls exponentially as temperature increases
    • A Wheatstone bridge is used to improve accuracy
    • PROS:
      • Small
      • Robust
      • Incorporated in nasopharyngeal temperature probes and PACs
      • Fast response time
    • CONS:
      • Requires calibration
      • Output subject to drift
Graph: Resistance decreases as temperature increases.
  • Thermocouples
    • When two conductors made from dissimilar materials are joined together, a potential difference (Voltage) is produced at the point of contact.
    • The magnitude of the potential difference depends on temperature
    • This is the Seebeck effect.
    • By measuring the voltage at the junction of the two metals, temperature is determined.
    • Other electrical connections to the thermocouple are required to complete a circuit.
    • This connection must be held at a constant temperature.
    • Thus, any variation in measured voltage through the circuit must be due to the temperature in the thermocouple.
    • PROS:
      • Fast response time
      • Cheap
      • Used in IV fluid warmers and some body temperature probes
    • CONS:
      • Requires more complicated electronics (kept at a constant temperature)
      • Measured signal is small and requires amplification
  • Infrared
    • Wavelength and intensity of IR radiation emitted by an object varies with temp.
    • Pyroelectric effect = generation of a potential difference in the molecules of some substances when they are exposed to IR at certain frequencies.
    • The potential difference generated is proportional to the change in temperature.
    • Ceramic crystals with pyroelectric properties are used in IR tympanic thermometers.
    • The voltage generated across the crystals in response to IR radiation from the tympanic membrane is used to determine temperature. 
  • PROS:
    • Very fast response time.
  • CONS:
    • Inaccurate if not directed to tympanic membrane or if wax in the auditory canal.
    • Non-continuous readings.

ii) Non-Electrical Methods

  • Liquid Expansion Thermometry
    • Volumetric expansion or contraction of a liquid (mercury or alcohol) in response to change in temperature.
    • These liquids display linear expansion over the temperatures of interest
    • As the liquid warms, it expands and rises from a reservoir in a glass column alongside a calibration scale
    • Temperature is read
    • PROS:
      • Easy to read
      • Simple
      • Accurate in measuring body temperature
    • CONS:
      • Slow response time
      • Glass can break → injury or toxicity (if mercury)
      • Inaccurate at very low temperatures (mercury freezes at -39 degrees C) and very high temperatures (alcohol boils at 78.5 degrees C
Comparison of alcohol and mercury thermometers.

Key Differences:

  • Safety & Toxicity: Alcohol thermometers are safer as they are non-toxic, while mercury is hazardous and environmentally dangerous if broken.
  • Temperature Range: Mercury operates well at high temperatures (< 357°C), while alcohol is superior for very low temperatures (down to -115°C) but limited to low-range high temperatures (often up to 78°C).
  • Sensitivity & Expansion: Alcohol expands about 6 times more than mercury, making it more sensitive to small temperature changes.
  • Accuracy & Behavior: Mercury does not wet glass (does not stick), resulting in more accurate, instant readings. Alcohol sticks to glass, which can lead to inaccuracies.
  • Appearance: Mercury is silvery-white and opaque, while alcohol is typically dyed red or blue to be visible.


When to use which:

  • Mercury: Laboratory experiments, high-temperature industrial applications.
  • Alcohol: Medical, household, and weather (low-temperature) applications.
  • Gas Expansion
    • Volume of gas increases as temperature increases (Charles’ Law)
    • If a volume of a system is fixed, the pressure of the gas will increase as temperature increases (3rd perfect gas law)
    • The pressure can be measured using an aneroid gauge calibrated for temperature
    • PROS:
      • Sensitive
      • Accurate
    • CONS:
      • Slow response time
      • Large and bulky
  • Bimetallic Strips
    • 2 different metals joined together with different coefficients of thermal expansion, wound into a coil.
    • As T increases, one metal expands more than the other. Coil loosens
    • As T decreases, coil tightens
    • At the centre of the coil is a pointer which moves across a calibrated dial as the coil tightens or loosens.
    • Temperature is read
    • PROS:
      • Cheap
    • CONS:
      • Limited accuracy
      • Slow response time

SOURCES:

  • Kam and Power, 4th Edition
  • Equipment in Anaesthesia and Critical Care (Aston, Rivers, Dharmadasa). Best resource ever for all equipment related content

 

Author: Alex Fagarasan