Clinical Thermometer Types

A) Direct Reading

i) Liquid expansion

§ Measures change in volume of a liquid eg: mercury

ii) Bimetallic

§ Measures change in length of metal as in two strips of metal with different coefficients of expansion, fastened together, with the strip bending when heated

iii) Chemical

§ Disposable, use once only

iv) Change in volume of a gas

§ Constant pressure gas thermometer

v) Change in pressure of a gas

§ Constant volume gas thermometer

B)Indirect Reading

i) Resistance wire

§ resistance of certain metal wires increases as temperature increases eg: platinum resistance thermometer

ii) Thermistor

§ semiconductor made from fused oxides of heavy metals

§ resistance varies markedly with temperature over range of body temperature: able to detect very small temperature changes

§ The bead is very small: so respond rapidly

§ Ideally suited to PA catheter

§ Have been incorporated into endoradiosondes (radio pills) which are swallowed and then transmit the deep body temperature of mobile patients

iii) Thermocouple

§ Generates an electromotive force via two dissimilar metals joined in an electrical circuit