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
