16A19: Exam Report

Describe the methods of temperature measurement.

36% of candidates passed this question.

A good answer included a definition of temperature and a classification of the methods of measuring temperature such as electrical, non-electrical and infrared. There followed a brief description of the physical principles of thermistors, thermocouples and resistance  thermometers; mercury and alcohol thermometers, bimetallic strips; and of infrared methods.

Candidates who did well reproduced the content of the chapter on temperature measurement in the recommended text book. Candidates who were not familiar with this material attempted to answer the question by falling back on clinical experience of measuring temperature in different sites or  occasionally referring to concepts of thermoregulation. Neither approach gained credit.
Some candidates interpreted “methods” incorrectly as “site of measurement” so scored poorly.

R2i / 16A19: Describe the methods of temperature measurement

Definitions

Temperature = Property of a substance, measure of average kinetic E of the substance.  Determines which way heat travels

Thermometer = an instrument that measures the temperature of a system in a quantitative way by detecting changes in thermometric properties of substances. 

Non Electrical

Liquid Expansion

Principle

Volumetric expansion/contraction of mercury/alcohol with Δ temp

Mechanism

Glass capillary tube

Connected to liquid (mercury/alcohol) reservoir

Heating reservoir causes expansion of liquid

Liquid moves up the tube alongside a calibrated scale

Temp read

PROS

Easy to read/use

Accurate for measuring body temp

CONS

2-3mins for complete thermal equilibrium

Glass can break

Mercury toxic

Alcohol

-115 → 80 C

low temps

less toxic

less durable, evaporates easily

Principle

Mechanism

PROS

CONS

Mercury

-38 → 357C

high temps

highly toxic

durable does not easily evaporate

Used in labs to measure temps of chemical reactions

Principle

Mechanism

PROS

CONS

Gas Expansion

Principle

Charles’ Law:  at constant  pressure, gas expands when heated

Mechanism

Bulb filled with a fixed volume of gas

Attached to a mercury manometer (wh measures pressure)

As temperature of gas increases so does its pressure

Pressure of system is then measured with aneroid gauge (ie Bourdon Guage)– to correlated with temp change

PROS

Used to calibrate thermometers

Sensitive and accurate over wide range of temps

CONS

Slow response time

Bulky

Bimetalic Strips

Principle

Metals have different coefficients of thermal expansion

Mechanism

2 dissimilar metals are bound together in a coil

In the middle of the coil is a pointer

As the temperature increases/decreases, the coil loosens/tightens and the pointer moves over the scale

NB coil loosens bc one metal will expand more than the other

PROS

Cheap

CONS

Slow response time

Limited accuracy

Electrical

Resistance Thermometers

Principle

Resistance of a metal increases linerarly with its temperature

Mechanism

Fixed voltage passed through a metal wire

Current that flows is measured

Resistance calculated using Ohm’s Law V = IR

A Wheatsone Bridge can be incorporated to improve accuracy

PROS

Can measure temp change as small as 0.0001C!

CONS

Slow response time

Bulky

Seldom used in clinical practice

Thermistor

Principle

Resistance of a metal oxide falls exponentially with increasing temp

Mechanism

A metal oxide is inserted into equipment (ie PAC)

The electrical resistance of metal oxide falls exponentially as temp increases

Wheatstone bridge is added to improve accuracy

R derived from Ohm’s Law

PROS

Small

Robust

Can be incorporated into many things (nasopharyngeal temp probe, PAC)

Fast response time

CONS

Require calibration

Drift

Thermocouples

Principle

Seekbeck Effect: 

When two dissimilar metals are joined together a small potential difference occurs at the point of contact

Size of PD is temp dependant

Mechanism

2 dissimilar metals are joined

Voltage at the junction of two metals shows a potential difference – which depends on the temperature and so temp is measured

The temperature in the system is kept constant it is only the temp in the thermocouple that changes

PROS

Incoorporated into IV fluid warmers

Fast response time

Cheap

Small

CONS

Requires more complicated electronics

Circuit must be maintained at constant temp, only the thermocouple temp may change

Infrared

Infrared Ear and Tympanic Membrane

Principle

Wavelength and intensity of infrared radiation an object gives out varies with its temp

Mechanism

Small probe with a detector is inserted into ext auditory meatus

Detector receives infrared radiation from tympanic memb

Infrared signal converted into electrical signal

PROS

Fast – measures core temp accurately in 3 secs

CONS

Non-continuous readings

Probe must be aimed accurately at TM, ie wax can affect accuracy