Other thermometers

Electrical thermometer

Response time varies from 0.1sec to 15sec.

Smaller probes have smaller heat capacity, thus faster response time.

Resistance thermometer

Principle

Electrical resistance of a metal increases linearly with temperature increase.

Example

• One that is based on platinum wire resistor.
• Often with Wheatstone bridge circuit to increase sensitive
• Platinum = 0.4 ohm per degree Celcius change
• Other metals used include copper and nickel

Advantage

• High accuracy

Disadvantage

• Price
• Large size
• Slow response

Thermistor

A solid state thermometer which is composed of fused oxides of certain heavy metal (e.g. cobalt).

Principle

• Resistance of (most) heavy metal oxide falls exponentially as the temperature rises.
• Often with Wheatstone bridge circuit to increase sensitive

Example

• Nickel(Ni), Cobalt (Co), Iron (Fe), Zinc (Zn), ?Manganese
• Used in Swan-Ganz catheter
• Used in thermodilution measurement of cardiac output

NB: Some metal oxides have positive temperature coefficient (resistance increases with temperature increase)

Advantage

• Cheaper than platinum-based thermometer
• Small size
• Fast response (0.2 sec)
• Continuous reading

Disadvantage

• Calibration is liable to change after severe change of temperature, e.g. heat sterilization.
• Hysteresis, changes with age
• Non-linear response

Thermocouple

Principle

Seebeck effect - at any junction of two dissimilar metals, a small voltage is produced, the magnitude of which depends on the temperature of the junction

46 microvolts per degree Celcius change

Example

Metals such as copper and constantan are used

(Constantan is an alloy of copper and nickel.)

Advantage

• Small size
  * Can be used in the form of a needle.
• Fast response
  * Low heat capacity due to size

Disadvantage

• small voltage output
• temperature at cold junction (i.e. the "control" end of the loop) needs to be kept constant

Others

Infrared ear and tympanic membrane thermometer

Principle

• Objects emit electromagnetic radiation over a range of wavelengths
• At body temperature, infrared radiation is the primary radiation

Example

Pyroelectric sensor

• Made of a type of ceramic crystalline material in which the electric charges on the molecules are slightly separated (i.e. polarization).
• Polarization changes with temperature, and this is detected using the material as a capacitor

Thermopile sensor

• Using many thermocouples connected in parallel.
• Allows continuous readings

Advantage

• Response time less than 5 seconds.
• No contact necessary
• Non-invasive

Disadvantage

• Intermittent readings only
• Inability to detect the focal point of the sensor