Next: THERMODILUTION CARDIAC OUTPUT COMPUTER

THERMOCOUPLE

Typical Number in Hospital:

Cost Bands:

References: 9

These work by the Seebeck effect, which is a combination of the Peltier effect by which a small voltage exists at the junction of two unlike metals, and a second effect credited to Lord Kelvin which produces a small voltage along a conductor in a temperature gradient. Both effects are proportional to the temperatures involved. The total voltage produced in a circuit including a number of thermocouples is zero as there is no temperature difference around the loop. Thus two thermocouples are normally employed. One is maintained at a reference temperature (e.g. the freezing point of water) and the other acts as the thermometer.

The sensitivities of common thermocouples range from 6.5 to 80 [u]V/[d]C with accuracies from 0.25% to 1%. Several thermocouples can be arranged in series to form a thermopile to increase the sensitivity. The advantages of thermocouples are their fast response (down to 1 ms), small size (down to 12 [u]m diameter), ease of fabrication and long-term stability. Their disadvantages are small output voltage, low sensitivity, and need for reference temperature. Small thermocouples can be inserted into catheters and hypodermic needles. Some clinical electronic thermometers employ thermocouples.