Temperature measurement Flashcards
what body sites can be used to measure temperature? and compare them.
tympanic - best measure of cerebral temp. fast but ear wax can reduce accuracy.
oesophageal (lower third)- good measure of core temp , more accurate
bladder - good measure of core temp but affected by flow and invasive
rectum - few degrees higher than core temp due to insulation and fermentation from bacteria. slight delay too.
skin - peripheral temp measurment
gold standard = pulmonary artery flotation catheter with thermistor . v. accurate but v. invasive.
what is the difference between core and peripheral temp?
core temp is a measurement of internal temp
peripheral temp is at the skin peripheries
usually a difference of 1-2 degrees
Give an overview of the methods used to measure temp..
These can be divided into electrical or non electrical
electrical:
* resistance thermometer
* thermistor
* thermocouple
* infrared tympanic membrane thermometer
non electrical
* bimetallic strip dial thermometer
* bourdon gauge dial thermometer
* liquid expansion thermometer
* liquid crystal thermometer
describe how a liquid expansion thermometer works?
As temperature increases it causes a liquid to expand.
A consists of a bulb containing liquid of either alcohol or mercury in continuation with a transparent glass column which has a calibrated scale along it.
as the temp increases, the liquid rises up the tube and settles at a value along this scale.
what are the pros and cons of a liquid expansion thermometer?
pros - simple, portable, not electronic
cons = risks associated with mercury poisoning, slow response , risk of glass breaking and causing injury. alcohol inaccurate at high temp, mercury innacurate at low temp
how does a bourdon gauge thermometer work?
based on principle that gas will expand with increasing temp - charles law
bulb with a spiral hollow tube containing gas
as the temp increases, the gas expands and the spiral unwinds
the spiral is connected to levers and a pointer which will move along a calibrated scale to read the temp
what are the pros and cons of a bourdon gauge thermometer?
easy to use, portable, non electric, continous measurements
slow response time
can demonstrate drift with time and hence needs recalibrating or becomes less accurate (changes to compliance of levers)
how does a thermistor work?
thermistors consist of metal oxides and act as semiconductors
these usually have a negative temp coeffient such that as temp increases, their resistance decreases with an exponential relationship
pros and cons of thermistors?
pros - very sensitive, cheap, quick response time ,very small so can be incorporated into catheters as small e.g PAFC
cons - more likely to have calibration error due to non-linear scale, prone to hysteresis and drift.
how does a thermocouple work?
Uses principle of seebeck effect
2 metals are used and at their junction a voltage between these is produced and measured. this will vary with temp
one metal is kept at a constant temp and the other is used as the measurement part.
usually copper and constanan
amplitude of voltage change is proportional to temp change - positive linear relationship
pros and cons of a thermocouple?
pros - fast, cheap, small
cons - voltage small and needs amplifying, reference needs to be kept constant - both of these can result in errors.
how does a bimetallic strip dial thermometer work?
2 dissimilar metals are used in a coil
each has a different thermal expansion coefficient
as temp changes they will expand to varying degrees causing the spiral to uncoil.
connected to levers and a pointer on a calibrated scale.
what are liquid crystal / chemical thermometers?
based on thermochromism - optical properties and colour of crystals change with temp
applied to forehead
cheap
however insensitive
pros and cons of bimetallic strip dial thermometer
cheap and robust
continous measurement
accuracy reduces with time due to changes in compliance - needs recalibration.
slow response time.
how does a resistance thermometer work?
uses a metal and the fact that the resistance of metals increaases with increasing temp in a linear relationship.
usually platinum wire
circuit include battery, ammeter and platinum wire + sometimes wheatstone bridge to improve accuracy.
draw a graph comparing thermocouple, resistance thermometer and thermistor
