Mercury thermometer

Keypoints

• Capillary tube and reservoir
• Evacuated, with Hg inside
• Angled constriction prevents Hg column from returning to reservoir until shaken
• Reservoir large enough to provide sensitivity, small enough to minimize thermal inertia
• Adv: Inexpensive, easy to use
• Disadv: rigid, fragile, toxic, slow response, intermittent reading

Principle

Volume of mercury changes with temperature

Structure

Mercury (Hg)

Mercury is used because

• Rapid heat conductor
• Linear expansion
• Low freezing point (-40C), high boiling point (357C)
• No capillary force between mercury and tube
  * Mercury does not wet the tube
  * Surface tension doesn't draw Hg up the tube

Capillary tube and reservoir

A uniform evacuated glass capillary tube, connected to a mercury reservoir.

• Capillary tube is small (to increase sensitivity)
• Uniform (so scale can be linear)
• Evacuated
• Capillary tube is at the focal point of a parabola mirror, thus view of the mercury column is magnified

Reservoir

Size of the mercury reservoir is a compromised between

• large enough to provide adequate sensitivity
• small enough to minimise thermal inertia

Constriction

Angled constriction at the bottom of the capillary tube just above the reservoir.

• splits the mercury column after it reached its maximum reading
• thus preventing the mercury above from contracting into the bulb until the thermometer is shaken

Pros and Cons

Advantage

• Inexpensive
• Readily available
• Easy to use
• Accurate enough for clinical use
• Able to be chemically sterilised
• Available with a measuring range suitable for clinical use
• Response time is slow but adequate for many clinical use

Disadvantage

• Slow response time - 2 to 3 minutes for complete equilbrium
• Need contact to get conduction (no remote recording possible)
• Readings cannot be processed and automatically recorded or displayed
• Cannot be made very small
• Intermittent readings only
• Fragile, toxic, rigid
  * cannot be used in some orifices
  * risk of injury to patients
• Risk of cross infection